"''llilllb :;!'.'. I.,,' A I I I I I Marine Biological Laboratory library Woods Hole, Mass. Presented by Dr. Wm. Amberson I I I I I I m a a a m a HUMAN PHYSIOLOGY BY I'ltoK. LFIGI JAJCIANI TKANSLATKI) Fli( >M THE ITALIAN Wl III A I'KI.I'Ari; i!\- PROF. .1. X. LANGLKV, F.1J.S. In ."i vols. Illustrated. 8vo. \'ol. I. Circulaliiin .-ind lirspiiMlioii. ]Ss. net. Vol. II. lutorniil Secretion Digestion Kxrrution Tlie Skin. 18s. net. Vol. III. Muscular and Nervous Systems. 18s.net. Vol. IV. The Sense Organs. Vol. V. Metabolism Temperature Reproduction, etc. [In the Press. LONDON: MACMILLAN AND CO., LTD. HUMAN PHYSIOLOGY MAI '.Ml I. LAX AND CO., LIMITI h LONDON IKI.MI-.AV < AUTTTA MAUIIAS MELBOURNE THE MACMILLAN ro.MI'ANY NEW YoliK l:n-ToN < MM AGO HAI.I.As SAN HiANCI-i i. THE MACMILLAN CO. OF CANADA, LTD. TORONTO HUMAN P H Y S I L O G Y BY PROFESSOR LUIGI LUCIANI DIRECTOR OF THC PHYSIOLOGICAL [NSTITUTE OF I'lIK I'.OVAI. rxiVKKsnv <>F ROME TRANSLATED BY FRANCES A. WELBY WITH A PREFACE BY J. N. L ANGLE Y, F.R.S. I'llol KSSOK nr 1'IIYSIOLOGY IN THE UXIVF.RSITY OF CAMBRII:F. IN FIVE VOLUMES VOL. IV EDITED BY GORDON M. HOLMES, THE SENSE ORGANS MACMILLAN AND CO., LIMITED ST. MARTIN'S STIIEET, LONDON 1917 COPYRIGHT PUBLISHERS' NOTE VOLUME V., completing the work, and dealing with Metabolism, Temperature, Reproduction, Stages of Life, Death, and Eaces of Mankind, is now in the press, and will appear in due course. June 1917. CONTENTS CHAPTER I PAGE CUTANEOUS SENSIBILITY ...... 1 1. Difference between modality a.nd quality o sensations. Johannes Miiller's law of specific energies. 2. Ditl'erent intensities of sensations. Weber's law and Fechner's law. :_!. Transformation of sensations into perceptions : philosopliical theories. 4. Four modalities of cutaneous sensation, according to Blix, Goldscheider, and v. Frey. 5. Cutaneous nerve-endings for sensations of pressure, pain, cold, and heat. 6. Phy- siological analysis of thermal sensations (heat and cold). 7. Touch and pressure sensations. 8. Capacity for localising cutaneous sensations. 9. Pain sensations. Bibliography. CHAPTER II SENSIBILITY OF THE INTERNAL ORGANS .... f>7 1. Classification of internal sensations. 2. Common sensation of the body or cociiiii'xthi'siit. 3. Pain in the internal organs and tissues. 4. Alimentary needs (hunger and thirst). 5. Sexual desire. 6. The muscular sense ; sensibility of muscles, tendons, and joints. 7. Inner- vation sense in the centres of voluntary movement. 8. Active tactile perceptions and their components. 9. The subconscious sense of muscular tone and its variations in reference to the functions of the labyrinth. Bibliography. CHAPTER III THE SENSI: OF TASTE ..... 126 1. Taste-buds the peripheral organs of the sense of taste. 2. Taste area mapped out by the physiological method of adequate stimuli. 3. Qualities of taste. 1. Meehanies ol' taste. 5. Correlation bel \\cen the chemical and physical constitution of sapid substances and the inti-n.-.it v vii viii PHYSIOLOGY PACK ami quality of the excited tastes. 6. Inadequate stimuli ; the so-called electrical taste. 7. Pathological alterations of taste-sense by disease or poisons. S. Specific energies of the nerves of taste. Bibliography. CHAPTER IV THE SENSE OF SMELL . . . . . .160 1. Peripheral organs ami nerves of smell. 2. External mechanism of olfactory function. '!. Kxcitatioii of smell by odorous substances in the form of gas or of aqueous solutions. Electrical excitation of smell. 4. Chemical and physical proper! ies of odorous substances. 5. Classi- fication of odours. 6. Determination of olfactory acuity (olfactometry and odorimetry). 7. Specific energy of the olfactory apparatus deduced from the phenomena of partial siiiin and partial olfactory fatigue. 8. Corrections and compensations of odours. 9. Physiological and p>ychical value of olfactory sensations. Bibliography. CHAPTER V THE SENSE OK HKAHIXG . . . . .1!)! 1. The organ of hearing. 2. Functions of the external ear. 3. Functions of the tympanic apparatus tympanic membrane and chain of ossicles). 4. Functions of internal ear muscles (tensor tympani and stapedius). 5. Functions of tympanic cavity, Eustachian tube and fenestra rotunda (cochleae). 6. Structure of organ of Corti, and dis- similar vibratory properties of the rods, basilar membrane, and tectorial membrane. 7. Compound tones, noises, simple tones, and differences of pitch and strength. 8. Limits of the perceptive capacity for tones, and faculty of discriminating between dilferent tones. 9. Timbre or quality of simple and compound tones. 10. Acoustic phenomena perceived on the simultaneous production of several tones. 11. Theory of perception of simple and compound tones. 12. Consonance and dis- sonance of tones ; musical chords. 13. Rising and falling phases of auditory sensation ; auditory fatigue. Entotic and subjective auditory sensations and hallucinations. 14. Binaural audition and localisation of sounds. Bibliography. CHAPTER VI DIOPTRIC MECHANISM OF THE EYE .... 266 1. General anatomy of eyeball. 2. Formation of retinal images ; underlying optical principles. 3. Optic constants of the eye. 4. Static refraction of the eye (emmetropia and ametropia). 5. Refraction of the eye ; mechanism and innervation of accommodation. 6. Far point and near point of clear vision ; range and speed of accommoda- tion. 7. Normal imperfections in the dioptric apparatus of the eye. CONTENTS ix PAOE 8. Dioptric importance of the iris. '.). Mechanism and innervation of pupil in accommodation : theory i>r pupil-rellexrs. 10. Absorption ami relleetion uf light in the eye ; ophthalmoscopy a ml skiascopy. Biblio- graphy. CHAPTER VII I!I:TINAL KXCITATION AND VISUAL STIMULATION . . . 330 1. Histological structure of retina. 2. Direct and indirect vision L blind spot, retinal elements tlie receptors of light-stimuli). 3. Ob- jective phenomena of retinal stimulation (visual purple, migration of pigment, contraction of epithelial cells and inner segment of cones, electromotive phenomena). 4. Colour vision; effects produced by different kinds of luminous radiation ; limits of their visibility. 5. Visual acuity ; phases of visual sensations ; positive and negative after- images. 6. Retinal adaptation to light and darkness ; sensibility of central and peripheral regions of retina to day vision and twilight vision (photopia and scotopia). 7. Achromatic and chromatic percep- tions in relation to intensity of light stimulus and retinal adaptation to light and darkness. 8. Duplicity theory of functions of rods and cones. 9. Colour mixtures ; complementary colours. 10. Colour contrast ; successive and simultaneous. 11. Theories of achromatic and chromatic vision. 12. Colour blindness ; partial and total. Bibliography. CHAPTER VIII OCULAR MOVEMENTS AND VISUAL PERCEPTIONS . . . 388 1. Articulation of eyeball in its socket ; its external muscles ; its movements and possible positions. 2. Isolated* and associated move- ments of its muscles. 3. The innervation and co-ordination of the eye- movements. 4. Simple binocular vision and the horopter. 5. Di- plopia. 6. Conflict between the visual images of both eyes and the phenomena of binocular contrast. 7. Spatial perception in monocular and binocular vision. 8. Stereoscopic binocular vision ; the stereo- scope. 9. Psycho-physical processes on which visual perceptions and representations depend ; relativity of our judgments of size, distance and form ; optical illusions and visual hallucinations. 10. Protective apparatus of the eye. 11. Origin of the aqueous humour. Bibliography. CHAPTER IX I'.sYCHO-l'HYSlCAL PlIKXOMKKA OK CONSCIOUSNESS AMI) SLEEP . 437 1. The range of mental life includes uii'-onscious as well as conscious processes. 2. States of complete and incomplete consciousness. 3. Subconscious activity : its great importance in relation to conduct and x PHYSIOLOGY genius. 4. Development ami integration of the mind a function of (In; subconscious, ba>ed on psycho-physical processes as distinct from ron- M-ious processes. ">. Disintegrations of personality (double conscious- ness, secondary personality, alternating personality). (5. Physiology of sleep. 7. Theories of sleep. 8. Psychology of sleep. 9. Dreams. 10. Telepathic phenomena. Vitalism and materialism. Bibliography. INDEX OF SUBJECTS . .491 INDEX OF AUTHORS f,o7 ERRATA Page 18, table, side-bracket, for " niccodennal " read " mesodermal." ,, 39, line 6, for "stiloid " read "slyloid." ,, 63, bottom line, for " nreto-genital " read " uro-genital." ,, 81, lines 15 and 17, for " Aronson " read "Aronsohn." ,, 82, Figs. '11 and 'JS, for "clitoris of female " read "human clitoris." ,, 84, Fig. -".I. for " female clitoris " read "human clitoris." 89, line 23, for " Hanson " read "Hensen." 90, line 12, for " Oley " read "Gley." ,, 130, line 7, for "Panaschi" read "Panasci." ,, 13(.i, line 4, for " Kastel" read "Rustle." ,, 146, line 10 from below, for " XaBr, " read." NaBr." ,, 147, line 15 from below, for " Cu " read " Mn." ,, 162, line 11 from below, for "hairs" read "cilia." ,, 170, line 4 I'roin below, for "Rnmsberg" read "Rnmberg." ,, 175, line IS from below} for "sulphate" read "sulphide." ,, 175, line 17 from below, for "phosphate'' read "phosphide." ., 1S5, line 9 from below, for " hypochloride " read " hypochlorite. " ,, 197, line 13 from below, for " Rnss " read " Riiss.'' ,, 203, line 19 from below, for "place" read "plane." ., 223, line 8, for " Cla Ini " read " Chladni." ,, 257, line 12 from below, for " Malte " read "Matte." 267, Fig. 106, for "A. E. Schafer " read " E. A. Schafer." ,, 269, lines 16 and 23, for " Brnch " read " Briicke." .. 298, line 12 from below, for "notable" read "not able." ,, 314, line 23, for " paracenthesis " read " paracentesis." ,, 336, Fig. 161, for "G. N. Golding-Bird " read " C. H. Golding-Bird. 351, line 20 from below, for " '0001 " read " O'OOl." ,, 396, bottom line, for "non-parallel" read " a-symmetric." ,, 398, line 7, for " Pauigrossi " read " Pauegrossi." ,, 410, Fig. 195, for "staircase" read "ladder." ,, 420, line 12 from below, for " Bemessi " read "Benussi." ,, 430, line 19 from below, for "covered" read "coloured." ,, 460, bottom line, for " ouiric " read "oneiric." CHAPTER I CUTANEOUS SENSIBILITY s. 1. Difference between modality and quality of sensations. Johannes M tiller's law of specific energies. 2. Different intensities of sensations. Weber's law and Fechner's law. 3. Transformation of sensations into perceptions : philosophical theories. 4. Four modalities of cutaneous sensation, according to Blix. Goldseheider, and v. Frey. 5. Cutaneous nerve -endings for sensations of pressure, pain, cold, and heat. 6. Physiological analysis of thermal sensations (heat and cold). 7. Touch and pressure sensations. 8. Capacity for localising cutaneous sensations. 9. Pain sensations. Bibliography. MOVEMENT and Sensation arc the two extremes of the processes of animal life by which the organism is brought into direct relation with the outer world. Movements are always objective in character and can be studied directly by external observation. Sensations are invariably subjective, and can only be directly analysed by introspection, and indirectly inferred from the expressional movements which are their external concomitants. It follows that the physiology of sensation in man is the necessary starting-point for the comparative physiology of sensation in animals ; and the intimate observation of our own consciousness is the only available basis for judging the psychical activities of animals or of other men. All organs of the body that are supplied with afferent nerves continually send information of their functional state to the i-rntral nervous system, and exert a reflex controlling influence along the efferent nerves without passing the threshold of consciousness. At other times they send to the centres messages which are not entirely subconscious, but emerge vaguely and indefinitely in consciousness as a more or less decided sense of \vdl-being or the reverse. Or again, the messages from the different organs to the centres may definitely cross the threshold of consciousness and give rise to distinct sensations which differ in quality and intensity. The complete excitation or functional activity of a sense is always a psycho -physical phenomenon that is a physiological VOL. iv i B PHYSIOLOGY CHAP. fact intimately associated with special states of consciousness. But very few of the impressions that reach us from the outer world, or from our own body, enter completely into c< msciousness, because the attention can only be focussed upon a small part of the impressions received. Sensations are distinguished, according to the most funda- mental difference in the psycho - physical phenomena which constitute them, as internal and external. Internal sensations tell us of the changes within our body and psychical personality; external sensations bring us news of the outer world, or the changes occurring therein. Internal sensations are always vague, indefinite, and often indefinable in character, even when \\v are fully aware of them; often, however, they operate unconsciously and modify our mentality, without being distinctly pen-rived. Such are the sensations of pain, hunger, thirst, nausea, fatigue, sexual desire, etc. The name coenaesthesia (from KOIPOS, common, i'o-0/jju!' sensation, the thermal sense must be subdivided into a heat sen-e and a cold sense. The psychological classification, founded on the dissimilar nature of the sensations, is evidently the most analytical and, therefore, the most rational to employ in defining and distinguishing the sense-organs. It is important to notice that two kinds of dissimilarity can be distinguished in the comparative study of sensations. Helm- holtz (1879) made a distinction between differences in modality and simple differences of quality. Sensations of different modality are so fundamentally dissimilar that transition from one to the other is not possible ; no degree of similarity, nor even a simple relation of intensity, can be established between them. No one, for instance, can say whether a given musical tone resembles more closely the colour red, or a bitter taste, or the scent of musk ; nor decide whether the light of a caudle is stronger or weaker than the sensation evoked by a certain solution of sugar, a given musical note, a sensation of pressure or temperature in the skin. If, on the other hand, we compare the sensations appreciable within each modality, we can indeed recognise qualitative differences; but these are not so profound as to make impossible a reciprocal transition from one to the other, or a comparison and judgment of their greater or less similarity, greater or less intensity. Two separate auditory sensations may be qualitatively distinguished by their difference of pitch ; it is also possible to judge which of them is the stronger. The colours of the spectrum not only present a gradual transition from one to the other, but we can also 'appreciate their greater or less resemblance or their relative brightness. The differences between the modalities of sensation observed on examining the higher sense-organs of vision and hearing, both in their mutual relations and in the relations between each of them and the lower sense-organs, could not well be more profound and striking. But this conspicuous disparity does not appear on comparing the sensations that arise from the less well-developed sense-organs. Tick (1879) first pointed out that the sensations of smell, taste, touch, temperature, and pain are modalities not so different in themselves that a gradual transition from one to the other is impossible. Thus, between the sensation of pricking produced by pepper on the tongue and that produced by a solution of table salt, the former being a tactile and the latter a gustatory sensation, a gradual transition is possible by means of a series of salt solutions and pepper extracts of increasing strength. In this case, therefore, the difference in modality assumes the character of differences in quality, between which a gradual transition is possible, as between the colours of the solar spectrum. 6 PHYSIOLOGY CHAP. This in mi way invalidates the distinction between modality and quality of sensation; it merely emphasises the fact that in the higher sensrs the differentiation in the modality of the sensations is far more pronounced and striking than in the less developed sensrs. The different modalities of the sensations do not depend on differences in the external stimuli which excite them, but on the specific nature of the different senses. Johannes Mliller (1840) published a masterly development of this theory, and brought out its full importance alike in physiology and psychology. It is usually known as the "Law of specific energy of the senxcx" (vol. iii. p. 262), and was summed up by M tiller in the following general propositions : (a) "No kind of sensation can be produced by external causes which cannot be equally excited in the absenee <>l external causes by intrinsic changes in our nerves." Purely internal causes may give rise to sensations of cold, heat, pain, pleasure, which are normally evoked by external stimuli acting on the skin. Certain olfactory and gustatory sensations are termed subjective, because they arise in the absence of any substance capable of arousing smell or taste. Auditory sensations may be due to internal or external causes : buzzing and subjective noises in the ear are common at the beginning of feverish disorders. Visual sensations light, darkness, and colours may occur without extrinsic causes. When the excita- bility of the optic nerve is exaggerated, subjective sensations of light and colour arise even with the eyes shut and in total darkness. Independently of transmission of any stimulus from the peripheral organs the nerve-centres may be thrown into activity by direct internal excitation. Under physiological conditions this happens in rf/v////.s, under pathological conditions in hallucination*. The outer world can therefore make no impression on us which purely internal causes are unable to arouse. (&) "The same internal or external cause evokes different sensations through the different senses, according to their nature or their specific sensibility." Hyperaemia or congestion of the sense-organs is an internal cause which produces specific effects on the different senses, as buzzing in the ear, flashes of light in the eye, pain in the sensory nerves of the skin or viscera, etc. The electrical current is a classical means of showing that the same external cause may produce sensations of dissimilar modality when it acts on different senses. If applied to the eye the galvanic current evokes luminous sensations, to the nose smell, to the tongue taste, to the skin sensations of pressure, warmth, cold, or pain, according to the nerve-organs encountered at the different parts to which it is directed. i CUTANEOUS SENSIBILITY 7 (<) "The specific sensations of each sensory nervr can IK; evoked by different internal and external stimuli." . . . "Sensa- tion is not the transmission to consciousness of a quality <>r state of an external body but of the quality, or the state of a sensory nerve as produced by an extrinsic cause, and these qualities (litter iii the different sensory nerves." Many attempts have been made, both by the predecessors and by the successors of Johannes Miiller, to explain the capacity of the different sensory nerves for receiving certain impressions, by ascribing to them a specific excitability to certain stimuli. This hypothesis is inadequate to explain the facts. We have seen that each sensory organ has an " adequate stimulus," that is, is specifically predisposed to become excited by a given stimulus. But this does not prevent its being excited also by other stimuli which we have termed "inadequate." Mechanical or electrical stimulation of the chorda tympani of man at the point at which it passes through the tympanic cavity excites sensations of taste at the tip of the tongue. The electrical current is not an adequate stimulus of any sense-organ ; there is no special sense- organ for this physical agent, as, e.g., the eye reacts to light, or the ear to sound. Yet electricity is capable of exciting every sense-organ, and evokes different sensations in each. We are therefore compelled "with Aristotle to attribute to each sensory nerve distinct energies, which are its vital qualities, just as contractility is the vital property of muscle. The sensation of sound is thus due to the specific energy of the auditory nerve, light and colour to that of the optic nerve, etc." (Miiller). When a certain number of air -vibrations impinge upon the auditory organ, they produce a sensation of sound ; when ether vibrations of a certain wave-length fall on the visual organ, a sensation of light results ; but sound and light as sensations are not comparable with the vibrations of the air or ether. The vi me vibrations of a tuning-fork that produce a note in the ear excite a sensation of vibration in the skin ; the same ether waves streaming from a lamp produce light through the eye and a sensation of warmth on the skin. In order to obtain sensations n| sound or light not only the vibratory movement of the air or ether, but also the presence of an auditory or visual organ, is indispensable. "Without the living ear there would be no sound in the world, but only vibrations. Without the living eye there would be no brightness, no colour, no night, only the oscillations of the imponderable matter of light, or the absence of them" (Miiller). How does the excitation of the sensory nerves arouse the different conscious sensations in the brain? Of what character is the active state of the sense-organs which generates in us the different modalities of sensation? In every age philosophers 8 PHYSIOLOGY CHAP. have sought to answer this question, but no reply is possible from the standpoint of experimental science. This is one of the transcendental problems to which Du Bois - Reymond replies ignoramus et ignorabimus. But the same answer had already been given by his master Johannes Mliller. " The nature of this state of the nerves whereby they see light, hear sound, the nature of sound as a property of the auditory nerve, of light as a property of the optic nerve, of taste, smell and touch, remain eternally unknown like the final causes in natural philosophy." The modern philosophical principle of the relativity of all knowledge acquired through the senses is a direct consequence of Miiller's law, that our sensations depend upon the innate qualities of our senses, and do not reproduce the phenomena of the outer world. (c?) " We do not know whether the different energies of the sensory nerves are intrinsic in them or in the parts of the brain and cord to which they run, but it is certain that the central portions of the corresponding sensory paths within the brain are capable of exciting the corresponding sensations, independently of the nerve-conductors." This conclusion leaves the question undecided whether the specific energies of the senses depend upon a property inherent in the respective sensory nerves or upon their central terminal organ. As we have already seen (iii. p. 262), this question is still unsolved, though the theory Johannes Miiller himself preferred receives most support, viz. the identity of nervous function, on which the nerves are regarded merely as indifferent conductors to the centres of the excitations that arise in the peripheral organs. The specific excitability of the several senses to given stimuli is due to the differentiation of the protoplasm, which is in relation with the nerve-endings of the peripheral sense-organ ; the specifically distinct sensations that arise in consciousness during excitation are due to the dissimilar nature of the central organs ; the sensory nerves that unite the peripheral organs with the central sense-organs are uniform conductors which are identical both in their internal structure and in their function. Hering, nevertheless, maintains the contrary hypothesis, and extends the concept of specific energy not only to the central cells but also to their processes, i.e. to the whole neurone. It is very difficult to determine the limits of the law of the specific energy of sensory nerves. The question is whether not modality only, but also the qualitative differences that occur within one and the same modality of sensation, depend on specific energies of the neurones that build up the sensory organ, or whether they can be explained on the assumption that the individual fibres of a sensory nerve are capable of serving different forms of excitation or activity. This question will be discussed in relation to each of the several senses. i CUTANEOUS SENSIBILITY 9 II. It is only within certain limits of intensity that external agents are effective stimuli. The minimal strength which is necessary to produce a sensation is known as the liminal intensity, or threshold stint ///its. The least perceptible increase of stimula- tion beyond this value is termed the liminal difference, or threshold <>/' difference. Every increment of stimulus up to a certain maximal limit produces an increase of sensation. The maximal sensation is obtained with a comparatively low strength of stimulus. Every increment of stimulus above that point not only fails to increase the sensation, but actually induces fatigue or exhaustion of the peripheral sense-organ, which is the more rapid and complete in proportion as the stimulus is excessive. The judgment we are able to form as to the intensity of a given sensation and the quantitative relation between the stimulus and the sensation is necessarily only approximate. We cannot state how much stronger or weaker one sensation is than another ; we can only say whether a sensation is stronger or weaker than, or equal to, another. Speaking generally, it may be said that sensation increases within certain limits with the strength of stimulus, but not proportionately to it ; doubling or trebling the stimulus does not double or treble the intensity of the sensation. Common observa- tion shows, in fact, that one and the same stimulus is perceived more, or less, or not at all, according to the conditions under which it takes effect. In the silence of night we perceive the ticking of a watch, while in the noise of day we scarcely hear the voice of any one speaking to us, and the clatter of the railway may prevent us from hearing our own voice. This means that the least stimulus can be perceived when the pre-existent sensation is feeble, and that a much stronger stimulus is required when the organ is excited by a previous strong stimulation. It is therefore obvious that intensity of sensation does not increase proportion- ately to strength of stimulus, but much more slowly. In order to determine the exact quantitative relation between stimulus and sensation it would be necessary to measure the intensity of both by the same methods. And as any such direct measurement of sensation is impossible, the only attempt we can make at solving the problem is to determine the threshold of difference, i.e. how much the strength of stimulus must be increased in order to obtain a perceptible increase in the intensity of the sensation. E. H. Weber (1831) first attempted this estimation. While testing the power of discrimination in musculo-cutaneous sensi- bility he met with a surprisingly simple result : the increase of stimulus necessary to produce an appreciable increase in sensation bears a constant ratio to the total stimulus, i.e. is always the same fraction of the total intensity of the stimulus. Thus to appreciate the minimal increase of a weight held in the hand, it is always 10 THYSIOLOGY CHAP. necessary to add the same fraction of the weight (average -*--, according to Weber), whatever its absolute value whether in ounces, pounds, grammes, or kilogrammes. Later observations by a number of investigators have proved that, within certain limits, Weber's law is approximately valid for all the different modalities of sensation, provided stimuli of medium strength are employed. On the other hand, there are more or less marked exceptions to the law when the stimulus is too strong or too weak. Generally speaking, Weber's law expresses a fact of great empirical importance, but has no claim to be a method of absolute mc/ixurement of sensation, or of exact deter- mination of the ratio between sensation and stimulus. The same cannot be said for the so-called "psycho-physical law" which Fechner (I860) formulated as a larger generalisa- tion from Weber's law. According to Fechner, if the increase of the sensation is proportional to the increase of the stimulus divided by the absolute intensity of the excitation, the sensations will stand in the same ratio to the stimuli as do logarithms to their numbers. Let S be the sensation, 11 the stimulus, C the constant/ represented by the liminal difference, and Fechner's "formula of psycho -physical measurement" is obtained : S= C log If, i.e. sensation is proportional to the logarithm of the stimulus. Fechner's theoretical interpretation of Weber's law is open to serious objections. Fechner assumes that the value of the liminal difference remains the same at all points of the scale (/SA = constant), while experiment shows that Weber's law only holds good within certain limits, and that the value of S& alters at the extremes of the strength of stimulus. Fechner further assumes that the smallest appreciable increase of a sensation represents its unit of magnitude, and that all sensations result from different sums of such units, which is a purely arbitrary interpretation of Weber's law, supported neither from introspective investigation nor from physiological observation. It is one thing to state with Weber that the relation between the appreciable increase of a stimulus and its absolute magnitude is constant within certain limits and quite another to say with Fechuer that every appreciable increment of stimulus invariably excites a sensation of the same value, and that these sensations together summate into a complex whole. The idea of giving a numerical measure of sensations is, according to William James, purely and simply a mathematical speculation upon eventual possibilities, which has. never found any practical application. The psycho- physical law will always remain a fossil in the history of psychology. III. Up to this point we have discussed sensations, and their different modalities, qualities, and intensities. But psychologists i CUTANEOUS SENSIBILITY 11 mean liy the term " sensation " the simplest and indivisible state of consciousness, l>y which \ve appreciate any alteration, c.tj. light, colour, a sound, a taste, etc., without associating with it any relation tu internal or external causes. 1'urc and simple sensations, as such, exist only in the new-liorn, in whom the sensory centres are incompletely developed. In adults, sensations are converted by a psychical process into perceptions, which are a complex of co-ordinated elementary sensations, by which we not only perceive the changes in our state of consciousness but are able to interpret and to objectify them. A simple tactile sensation, for instance, is inevitably connected with an external body coming into contact with the skin; a sensation of bitterness with the presence of something in the mouth; a sensation of sound or colour with the presence of a sounding or a coloured body in the outer world, and more or less remote from us. Each of our sensory perceptions, though composed of a complex of elementary sensations which are more or less distinct from each other, nevertheless presents itself as a kind of unit in our consciousness. In the physiology of the senses it is often no easy task to distinguish in apparently simple sense-perceptions the elementary sensations of which they are composed. The objectifying of perceptions, by which we refer the changes in our senses to external causes acting on them, is a fundamental characteristic common to all perception. The tendency to project our perceptions externally varies in the different senses. It is strongest in the higher senses of vision and hearing. Common visual and auditory perceptions appear unmistakably as properties attaching to external objects, more or less remote from us, apart from any appreciable sensation of change in our visual or auditory organs. The perceptions of the lower senses, touch, temperature, taste, and smell, have less tendency to projection. Tactile per- ceptions are, as a rule, projected to the place where the object that excites the cutaneous sense-organ is situated, and we are clearly able to distinguish the sensation of the external object that comes into contact with the skin from the change in the sensory surface. In the sensation produced by a warm body we may be uncertain whether we feel the heat of our skin or of the external object. So too in sensations of taste or smell, it is doubtful whether we arc most aware of the changes in the tongue and nasal mucous membrane, or of the presence of the sapid or odorous substance. More important, however, than the greater or less degree to which normal sensory perceptions are projected beyond us, or to the peripheral sense-organs, is the fact that both subjective and hallucinatory perceptions, and also the effects of experimental or pathological stimulation of the sensory nerve-trunks, are pro- jected externally : we refer them not to the place at which they are really excited, but to that to which we are accustomed to 12 PHYSIOLOGY CHAP. refer the corresponding normal peripheral excitations, as was shown in the chapter on the general physiology of the nervous system (Vol. III. p. 201). This leads us to grave philosophical questions which can only be briefly touched on. How does the objectitication of sensory perceptions come about ? How are we able to distinguish the outer world from ourselves ? Since we do not actually feel external objects, but only the Hianurs \\liidi these effect by means of the sensory nerves and sense-organs in the sensory centres which changes are quite different from the external objects why are we convinced that our senses are not deceiving us ? These problems are as important as they are hard to solve, and the interpretations given to them by psychologists and physiologists differ widely. In all ages the theory that the whole of our sensations and our fundamental notions of the external world are but illusions and phantasms of the mind has had many followers. Its most extreme form is the absolute phenomenalism of Hume. This obviously does not solve the question as to the origin of percep- tions and ideas, nor does it explain the common belief in the reality of the external world. Kant's critical idealism was a reaction from this theory. The phenomena of the outrr world have nothing in common with our sensations. We can know nothing about the true nature of the external world : the only things we can know directly are the states and phenomena of our consciousness. We can only conceive of the external world by the aid of physical hypotheses and speculations such as the undulatory theory, the atomistic hypothesis, the mechanical theory of heat, etc. Perceptions and ideas depend essentially upon congenital predispositions of the senses and the brain, and on original or innate properties of the mind. In opposition to this critical nativistic idealism is the sensory empiricism which assumes that ideas are the result of observation and education of the senses. Locke, Condillac, John Stuart Mill deny the existence of a priori ideas. Everything comes from experience or activity of the senses : the soul deprived of any experience is a tabula rasa. Sensations are merely simple signs representative of external objects, different indeed from them, but always interpreted in the same way, from which we always deduce the existence and properties of external objects by the aid of previous observations. Helmholtz, who partially accepted this theory, recognised its inadequacy to explain the facts, and assumed with Schopenhauer that all our perceptions and ideas presuppose the a-priority of the causal concept without which we cannot look upon objects as the extrinsic cause of our sensations. This theory was further i CUTANEOUS SENSIBILITY 13 developed by Herbart and \Vmnlt, the lirsl of whom specially brought out the importance of association of the various sensations and perceptions, while the second laid stress on the unconscious /'a toning processes. The iiew-born infant only possesses internal sensations, such as hunger, satiety, etc. Its visual, auditory, tactile and other sensations are only perceived as changes of its own being, and are not referred to the causes by which they are produced, nor projected externally. By degrees, however, it begins to notice various objects and accommodate its eyes for distant vision. Simultaneously the child moves its limbs and begins to exercise its cutaneous and muscular senses. Tactile sensations are at first perceived as internal sensations, as obstacles to movement ; but the eye perceives the movement of the hand, and the coincidence of visual and tactile sensations soon leads, by an unconscious process of reasoning, to the conviction that the object perceived by both senses is one and the same. Apart from the association of the two senses, touch alone is sufficient by un- conscious judgment to teach the babe to distinguish its own body from the outside world. When the hand comes in contact with another sensitive point of the skin, it receives a double sensation ; when, on the contrary, it touches an extraneous object, it is aware of one sensation only. For the adequate discussion of these and other problems the student must turn to text-books of psychology. Here we must confine ourselves to saying that the transformation of sensations into perceptions is still a wholly mysterious process, even if it can reasonably be said- to depend on and be favoured by the combined activity of all the senses. IV. The whole surface of the skin and the visible parts of the mucous membrane have important sensory functions which have long been grouped together under the common denomination of " tactile sensation," without regard to analysis of the different qualities of sensation. For this reason, perhaps, the study of these functions remained stationary for a long time, down to the last decades of the nineteenth century, when a conspicuous advance was made. Fechlin (1691) was the first who insisted on the anatomo- physiological distinction between tactile and thermal sensibility (caloris et frigoris sensus). Erasmus Darwin (1794), in his famous Zobnomia, proposed the same distinction, and adduced as evidence the case of a patient suffering from abolition of tactile sensibility, in whom the appreciation of warmth was normal. But this attempt to distinguish between the different cutaneous sensations was neglected until E. H. Weber (1834) undertook the systematic study of the physiology of cutaneous sensibility, and, after prolonged original and methodical research, obtained 14 PHYSIOLOGY CHAP. valuable, results which still constitute an important part of our knowledge of this subject. A new era in the physiology of the cutaneous senses was reached by the discovery of heat, coll, and pressure sjiots by Blix (1882), conlirmnl by Gold- scheider (1883) and Donald- si m (1885). Another marked development of the physio- logy of the cutaneous senses was tlic work of v. Frey (1894-97), which showed that in addition to the above there also existed in the skin a fourth sense-organ con- stituted by pain spots. The work of Herzen (1886) and < Joldscheider (1898) on the paralysis pro- duced by compression of the nerves of a limb also lent support to the theory that there are specifically distinct nerves and organs of sensa- tion in the skin: sensibility to cold and to pressure are mi >re strongly depressed and disappear UK ire rapidly than sensibility to heat and pain. Ponzo (1909) showed that stovaine by its peripheral action produces local anaes- thesia to stimuli of touch, pain, and cold, while sensi- bility to heat stimuli is re- tained. In this respect the work of Stranskys (1899) on the reappearance of sensi- bility in portions of skin grafted for surgical purposes is of great importance. It proves that tactile or pres- sure sensibility appears first, while sensibility to pain and to temperature develop later in the transplanted portions of skin. It is still uncertain whether in addition to the four modalities of cutaneous sensation, viz. the sensations of contact or pressure, of cold, of warmth, and of pain, other independent qualities -of sensation should be admitted, such as itching, tickling, sexual FIG. 1. Thermo - aesthesiometer <>r Ven-ss, si-en in section. The instrument consists of a hollow metal cylinder 4 cm. in diameter, divided internally by a metal plate (a) into two unequal parts, into mi' "I which is inserted the tube fur inflow, into the other that for outflow of the hot or cold water. At >* the cylinder becomes conical. At c the terminal part ' is screwed on, to carry the exciting surface c, which is applied without pressure on the skin. The end of a thermometer hk, to measure the tem- perature of the circulating water, is passed thrnii.uh the cork which closes the top of the apparatus. The exciting surface may be varied by usin;. the alternative pieces el, e-. CUTANEOUS SENSIBILITY 15 l>le;ismv, etc., or whether these should lie considered as special modifications of the senses of touch and pain. A clear idea of the form, arrangement, and number of the different sensory points on the skin may be obtained by briefly reviewing the experiments on which the discoveries of I'.lix, Goldscheider, and v. Frey were founded. Tin 1 simplest apparatus will serve for investigating heat and spdts. Small metal rods with blunt ends, that can be dipped into cold or hot water, would be suitable, except that thev have to be changed so frequently, and that it is impossible to be certain that they always act on the skin at uniform i em] -.era t uiv. The contrivance of Blix, which consists, in a small hollow metal cylinder, through which Hows a constant stream of water at uniform temperature, is more reliable. Alrut/. and Kiesow made various alterations in this apparatus, MI that it can be used for different purposes. The most perfect is the tliei mo-aesthesiometer of Yeress (Fig. 1), which is used for mapping out the thermal sensibility of small cutaneous areas of -2 or 6 mm. The end of the apparatus can be. unscrewed and readily replaced by surfaces of different sixes, or by a blunt point, when required for the investigation of heat spots. For pressure points the simplest and easiest method is that of v. Frey with the so-called exploring hairs or bristles. Hairs of varying thickness (horse-hair, woman's hair) are fixed to the end of a rod, the length of which varies from 1 to 4 cm. Fig. '2 gives the latest form of v. Frey's hair-aesthesiometer. The anterior graduated half of the metal cannula rims backwards and forwards, so that more or less of the hair is covered. If the point of the hair is placed on, and vertically pressed against, the scale-pan of the balance, the amount of pressure necessary to bend it lightly can be determined; this, of course, increases or diminishes according as the length of the hair is less or greater. The millimetre scale of the instrument serves for the empirical graduation of the degree of pressure required to bend the hair according to the length of the exposed portion. The same aesthesiometer may be used to determine pain points if the exposed portion of the hair is so short that it will bend only at a pressure sufficient to evoke a sensation of pricking. If a moderately cool metal point is brought into contact with the skin, without pressure, the sensation of cold is evoked only at circumscribed spots, distant 1-2 mm. from each other. These are the cold spots of Blix. If the metal point used for exploring the skin is much cooled, a sensation of cold can also be obtained from other surrounding areas of the skin; but it is always less intense, proving that it depends on transmission of the stimulus to the true cold spots. If the skin is tested with a hot metal point, sensitive spots are found which react in the same way by sensations of warmth. These are the he/ if *]>ots of Blix. Exploration of the skin with gentle tactile stimuli, as by hairs, gives Blix' pressure sftots. Finally, the same method will detect v. Frey's pain spots. V Of V. Frey. Ex- planation in text. 16 PHYSIOLOGY CHAP These numerous sensitive points for cold, heat, pressure, and pain are not superposed, but are distributed over different parts of the skin. Fig. 3 shows that Blix' points for cold, heat, and pressure are not really spots, but that the sensation spreads round them as though due to a sort of irradiation of the stimulus, so that the sensory points resemble small placques. These sensory points are notequidistant norregularly distributed, and consequently ' I r* w ' .Is' FIG. 3. Distribution of specific sensory spots on skin of the dorsal surface of the left thumb. (Blix.) Cold spots coloured green, warm spots reil, pressure spots black. Fie. 4. Distribution of thermal spots on palmar surface of left forearm. (KiHMnv.) Cold spots marked green, warm spots red. there are insensitive areas of skin of varying extension between them. The cold points are much more numerous than the heat points, and the pain points (not shown in Fig. 3) more numerous again than the points for contact or pressure. The number of points is much greater according to Goldscheider than to Blix. Kiesow's accurate researches show that the data of the latter are more reliable : he proved that the cold spots of Blix may be analysed into groups of individual cold points. Fig. 4 FIG. 5. Distribution of cold and tactile spots on dorsal surface of left wrist. (Kiesow.) Cold spots marked green, tactile spots black. The left-hand figure only contains cold spots, the right-hand cold spots and tactile spots in the same area. gives the distribution of the thermal points according to Kiesow on the palmar side of the left forearm, Fig. 5 the distribution of cold points and tactile points on the dorsal side of the left wrist. Kiesow further found that in regions provided with hairs the cold spots invariably lie near the tactile hair spots but do not coincide with them. He concludes that the vicinity of cold spots to the hair is in relation with the so-called " goose-skin " produced by the contraction of the pilo-motor muscles ; it is presumably due to a reflex arc. Sommer continued these studies and found in 1 sq. cm. of adult skin 6-23 cold spots and 0-3 heat spots; on an i CUTANEOUS SKXSIBILITV 17 average, therefore, ll'-l.". cold spots anil 1-2 heal spots per sij. em. 1'dix round that in the hair-clad regions oi' the skin, which he estimates at about 95 per cent of the whole, the pressure points coincide with the hair papillae; other pressure points that ran he detected here and there where there are no hairs probably correspond to rudimentary hair papillae. Hut the tactile surfaces proper, where the touch spots are closely arranged, are found in the regions that have no hairs -particularly the tips of the lingers, palm of the hand and sole of the foot, red part of the lips, tip of the tongue, etc. The number of pressure points, according to v. Frey, averages 25 to each sq. cm., except on the head. The number of pain points has not yet been estimated. On the back of the hand v. Frey found 100-200 in every sq. cm. Once the position of the sensory spots on any part of the skin is fixed by means of fast colours, it is easy not only to identify them at any time, but also to verify on them Miiller's law of the specific energies by showing that they react by the same form of sensation (warmth, cold, pressure or pain) when excited not only by adequate but also by inadequate stimuli. Sensations of cold, e.g., are obtained by exciting the corresponding spots not only with a cold point but also with a mechanical or electrical stimulus, or with a point heated to 45 v. Frey's j)ttr.i-ie(tl sensation of cold. The legitimate conclusion from these results is that the skin is provided with at least four distinct sets of sensory nerves, for the sensations of cold, warmth, contact or pressure, and pain ; that these nerves terminate within the skin in special peripheral sense- organs ; and, lastly, that the sensitive points of the cutaneous surface correspond to these sense-organs in the layers below them. V. Before attempting to solve the question whether four different organs or terminal corpuscles correspond with the four forms of cutaneous sensation, we must refer to the latest morpho- logical work on the nerve-endings in the skin. The sensory nerve-til ires that innervate the skin form a deep nerve-plexus in the subcutaneous panniculus adiposus. Most of the fibres of this plexus run towards the surface of the skin, and after crossing the reticular layer of the cutis reach the sul (papillary layer, where they form a second plexus less rich in fibres, the so-called superfieial cutaneous nerve-plexus. A deep vascular network corresponds to the deep nerve-plexus; a super- ficial vascular network to the surface plexus. Fibres are given off by the deep plexus which terminate after a short course in special corpuscles or peripheral sense-organs situated in the panniculus adiposus. From the superficial nerve- plexus still more numerous fibres branch off to end in special corpuscles in the different layers of the cutis --the reticular, VOL. IV C 18 PHYSIOLOGY CHAP. subpapillary and papillary layers. Some nerve-endings even reach the rete mucosum of the epidermis, more exactly the stratum germiuativum or layer of cylindrical, cells and the prickle or polyhedral cells, where they end not in complex corpuscles but in simple swellings or bull is. The following table from Ruffiui, adopted also by Crevatin and Dogiel, indicates the topography of the different nerve-endings present in the various layers of human skin : 81 Stratum corneum Stratum lucidum Rete mucosum /Stratum granulosum I . f Layers without nerves. I Layer of prickle cells . 1 Layers of longer nerves. (.Stratum germinativuin . J Hederiform expansions. t. - * i* B o _ -T o 8 09 /Basement or supporting membrane. Papillary layer .... Subpapillary layer . Reticular layer . . . . Layer of panniculus adiposus . corpuscles. Dogiel's corpuscles. "j Euffini's papillary endings. iQolgi-Mazzoni corpuscles. (" Meissner's corpuscles. ! Dogiel's arboriform terminations. iGolgi-Maz/oni corpuscles. Dogiel's arboriform terminations. rPacini's corpuscles. I Golgi-Max/.oni corpuscles. Ruttini's organs. ^Dogiel's arboriform terminations. As shown in this table, the most superficial nerve-endings of the skin lie in the two deepest layers of the rete mucosum or Malpighiau layer. Langerhans (1868) first saw that certain nerve -fibres, after losing their myt-lin sheath, penetrate the epidermis to form a network with loose meshes, and then spread in independent and varicose branches through the epithelium, to the outer limit of the layer of prickle cells, where they terminate in bulbs (Fig. 6). Phylogene tie-ally, these represent the oldest form of nerve-endings in the vertebrate epidermis. The so-called hederiform nerve-endings lie in the Malpighiau layer close to the sweat-glands. The nerve-fibres of which they are formed come from the superficial plexus of the skin. Near the epithelium they lose the rnyelin sheath, and divide into branches, which spread and twist between the prickle cells and terminate according to the latest work of Dogiel in baskets or nets (Fig. 7). Frequently, but not always, a cell of peculiar appearance is found within the basket, which Eanvier and Dogiel i ('I'TANKOUS SKXSLI'.lU'n I!) believe to In- sensory in clianidcr, like those found in the olfactory // sw. M /" x o--. | I HI . i 11 I ) ii I I! . ; . ; * \ - . . k 1 H ':M/A ^ f will A.^M ; Fii.. ti. Section through rpiderinis of human liaml. (Kanvi'T.) //., Horny layer: consistin.L; ni . Mijinrticial horny scales ; sn:, swollen lioniy cells ; n.L, stratum lucidnm. J/., reti- mucosum or Malpi^hiaii layer : consisting of;'., prickle cells, and <., elongated cells forming a single stratum iM'.-ir the corimn ; n., part of a plexus of nerve-libres in the superficial layer of the cut is vera ; from this jilt-xus varicose nerve-fibrils can be traced between the cells of the Malpighian layer. and gustatory organs. Phylogenetically, the hederiform endings Ki<;. 7. Iio-iel's small intia-epithelial baskns, .seen in profile. These consist nf ramilicutions of varicose myelinated nerve-fibrils, which become expanded in contact with the prickle < with terminal liaskets rejiresenL the latest form of nerve-ending in the vertebrate epid>'i -mis : they occur only in mammals. 20 PHYSIOLOGY CHAP. Meissner's corpuscles, which lie in the papillae, and sub- papillary layer of the corium, were discovered in 1852 by Meissner and R. Wagner in the cutaneous papillae of the hands and feet. Their structure is complex and very variable, so that, according to Ruffini, each corpuscle requires special description. They are found in man and the ape, but have not been recorded in other mammals. Usually they are oval or rather elongated. One, two, or more medullated fibres run to the corpuscle and penetrate its interior after winding round it once or twice, lose the myelin sheath and the sheath of Schwann, and then form a spiral coil with a number of more or less irregular con- volutions. The branches of the axis- cylinder which make the spiral are often very varicose, and have one or two ter- minal enlargements (Fig. 8). The non- nervous tissues of the corpuscle consist in an external capsule of lamellated connective tissue, and a homogeneous, finely granulated interior, which is prob- ably formed of fibrillary connective tissue, with a number of nuclei. Many varieties of Meissner's cor- puscles are known. Those last de- scril icd 1 >y Dogiel represent a transitional form between the typical nerve-endings of Meissner, which are collected in a corpuscle enclosed in a capsule, and the Flc: . 8.-Meissner'a corpnsch- in a non - typical nerve - endings, which do papilla from the skin of theliand, nofc f orm rea l CO niU8CleS, bllt remain stained with sold chloride. Highly . -n ina-niiir,i. (Hanvi-r.) . n, two free within the papilla, without a sur- nerve-libres, passing to the cor- j i ii n ^ l-usde: a, , t.Tininal varicose rOlllldlllg Capsule I these Were first de- ramilications of axis-cylinder c r . r jl )Pf l V) V Tfnffini C\ SQ9\ ill HIP within the corpuscle. ( l Q y z / papillae that contain no Meissner's cor- puscles, under the name of papillary bulbs. Dogiel's corpuscles consist of two parts : one closed, lying at the base of the papilla, the other open towards its apex. The former differs in no respect from the typical Meissner's corpuscle, the latter resembles one of the many forms of free nerve - endings described by Ruffini, Sfameni, and others (Fig. 9). Special corpuscles were described by Golgi (1880) in peri- tendinous connective tissue and the external perimysium of human muscle. These were thoroughly investigated by Mazzoni (1891), and are therefore known as the Golgi-Mazzoni corpuscles. Ruffini (1894) discovered that they are also present in subcutaneous con- nective tissue, as well as in the subpapillary and papillary layers. Their external form and dimensions vary ; they consist of a lamel- lated capsule and an internal core of fibrillary connective tissue CUTANEOl'S SENSIBILITY 21 with many nuclei. Two <>r more branches of .1 nn \ r - lihre penetrate tlu i core, uml there lose their sheath and hemme attenuated. Tlie pale fihivs divide and subdivide into a large number of branches, which do not form twisted convolutions, but run a tortuous course to the end of the core. The branched fibres for lln 1 most part present numerous varicosil ies of different shapes Fn.. 9. Dowel's (-"ipuM-le. '(, Varicose lilire, passing to the corpuscle; b, b, closed portion of corpuscle, corresponding to the base of tin- papilln ; c, free part, corresponding to die apex of tlie papilla, foniiKil of non-myelinated varicose fibres. and sizes (Fig. 10). In others the varicosities are scanty, and the appearance of the terminations is totally different (Fig. 11). In others again, according to Crevatin and Dogiel, one or more delicate non-medullated fibres also enter the corpuscle, where they ramify and form a slender plexus at the periphery of the core, and also penetrate inside and mingle with the ramifications of the myelinated fibres (Fig. 12). In the subcutaneous fatty tissue there are two other charac- teristic forms of corpuscles besides the Golgi - Mazzoni bodies: 22 PHYSIOLOGY CHAP. Pacini's and lluffiui's corpuscles. The former, already discovered by Vater, were described in de-tail by Pacini (1840), who saw them FII:. 10. Two Col.ui-MM/zoni corpuscles connected with a single bifurcated nerve-fibre. (Ruflini.) The ramilied tilii-es within tlie corpuscles pivsi-nt numerous vark-osities, varying in size and appearance. adhering to the branches of the nerves that run in the fat under the skin of the palm of the hand and sole of the foot, as small oval -=-' -" Fio. 11. Variety of Golgi-JIazzoni corpuscle, distinct from the preceding because the noil- myelinated nerve-fibre forms a characteristic interlacement in the core. (Crevatin.) bulbs, quite visible to the naked eye (Fig. 13). They are too well known to require further description. As is well shown in Fig. 14, the Paciuian corpuscle consists of a capsule of finely lamel- (TTANKOl'S SEN'S I T, I LIT V 23 lated iissMc and a central core penetrated by tin 1 medullated lihre, which rims through it diivei |<> the end, where id l>ranehe,s and rinls iii an enlargement. I'.etween the largest 1'ariniaii corpuscles, that are plainly visible to tlu 1 naked eye, and those of Golgi-Mazzoni, which can only be detected with the microscope, there is an uninterrupted series of intermediate or transitional forms. One very rare variety of 1'aeinian corpuscle found in subcutaneous tissue consists of small spherical corpuscles with an inner core which is also spherical, and nerve-endings represented by a cluster of hulbs (Fig. 15). Fi<;. IL'. Ciolgi-.Mazzniii corpuscle. (Cre- vatin.) Besides tli>- inycliuated nerve- tilu-e, a line non - myelinated iiln>' penetrates into the corpuscle and t'unns a network in tin- cap-iiili-, as described by Tiniofeew. Km. 13. Nerve of midillc lin.ui'r with I'acinian corpuscles. Natural si/e. (llenli- and Kiilliki-r.) The corpuscles which Buffi ni discovered in 1891 have in coiunion with Pacini's that they are found in approximately equal niunhers in subcutaneous cellular tissue, and like the Paciuian bodies are of very variable dimensions. They are cylindrical and -] 'indie-shaped. A capsule consisting of a few thin lamellae closely applied together can be, distinguished from a supporting bundle of h'brillary connective tissue and elastic fibres, between which the nerve-fibres penetrate and expand in the form of a Qon-myelinated ramification. Sometimes the nerve-fibres enter laterally (Kig. 16); at other times they enter at oiie end of the spindle (Fig. 17). Itullini's corpuscles also present many variations. The 24 PHYSIOLOGY CHAP. cutaneous nerve-plates 'of Crevatin, and the arboriferous termina- tions of Dogiel, have been described under this name, but differ in certain very important morphological characters. .ai/ -11 FIG. 14. Pacinian body, from cat's mesentery. Magnified. (Ranvier.) n, Peduncle, with nerve- fibre enclosed in sheath of Henle passing to the corpuscle ; n', m, its continuation after loss of sheath ; a, branched nerve-ending at the distal end of the core ; ;/, lines separating the tunics of the corpuscle ; /, channel through the tunics traversed by the nerve-fibre ; c, external tunics of corpuscle. All these end-bulbs are found more abundantly on the parts of the skin that are free of hairs, particularly such as habitually serve as tactile surfaces. Over the whole of the rest of the skin (TTANKors SKNSII'.IUTY 25 \\here there an- hairs, represent ing, according l,u v. l-'n y, ;il>uiit <)f) per cent of tin- total cutaneous area, the various corpuscles we have referred to are not aliment, but become less tVe(|uenfc, and are further apart in proportion as cutaneous sensi- bility in its different forms is less acute. To compensate for this the hairy parts of the skin contain a specially important form of nerve- ending, which is absent in other regions this is the nerve-plexus, which can be seen round the hair follicles beneath the mouth of the sebaceous glands. Arnstein (187(5) with the gold chloride method first successfully demon- strated the nerve - endings around ordinary hairs. He saw that after reaching the hair- follicle the medullated fibres lose their medul- lary sheath, divide, and give rise to a series of annular and longitudinal fibrils. The latter Fio. 15. Hare variety of Paciniiin corpuscle. (Huflini ;iiid Sfaiiieni.) c.t. PlG. 10. Kull'mi's corpnscli', showing iici-ve-libi>-> i-iili-rin.u from the si.le. (Kntlini.) '<.--., blood c:i i>i Maries ; n.v., IKTVI- Hidings ; ('., <-ii]>.>ul'- : c.t., nmnrrtive tissni-. are highly varicose and more external ; they rise along the hyaline layer towards the surface of the skin, and terminate in wide disc- 26 PHYSIOLOGY CHAI>. like enlargements (Fig. 18). The nerves of human hair have not Pio. 17. Ruffini's corpuscle, in which the liln.'s ]>i'iietiat'' into one end of the spindle, (ttullini. C., eaiisulc ; //.. -.h.'atii ..f iii-ulc; ../., sustentacular tissne ; .<-., yet been described and studied: but everything leads us to con- clude that they arc similar to those of the hairs of other mammals. As regards the specific sensory function of the several forms of cutaneous nerve-endings, it must be confessed that our knowledge has made little progress. The peripheral organs for appreciation of pressure are undoubtedly represented in all parts of the skin provided with hairs by the above-described nerve-plexus in the outer sheath of the hair-root. P>lix, and more recently v. Frey, have demonstrated that a pressure point corresponding with each hair lies near the point at which it emerges, on that side from which the hair follicle slopes. In regions that have no hairs it can be affirmed with great prob- ability that Meissner's corpuscles correspond to the pressure points. Fin. 18. Section through a hair and hair ,, ,. -,. J _-? sheath of cat mauiiilifd 100 times. Ifie results OI BllX and V. T&J in (Bohm.) V'-i >"erve plexus; A'., nerve; f *. flaTppw i<-K fh p n l r l vl ' pw nn wVn'fVi II., hair; t.i., tunica interna of root of laCTj gTt 6 WIT I \ 16W C 1 WHICH hair; t.e., tunica ext.-rna ://./., hyaline MeiSSHei's Corpuscles Were always Ijivsr ^ held to be tactile. Their superficial position in the skin corresponds to the sharp demarcation of tactile points, to their accessibility, un- like the nerve-plexus of the hairs, to electrical stimuli, and // ft . / e . It.l. CUTANEOUS SENSIBILITY to tin- I; i ft that appreciation nl' pressure is lust, in e.nta.nenii,- scars. Von Frey also suggested with much probability that the pain spots, which arc most abundant in the skin, an- served hy the IVi'i' terminations of tin- superficial nerve-plexus, which supply the epithelium of the Malpighian layer. It is possible that each pain spot corresponds not with a single nerve-ending but rather with a group of nerve-endings, otherwise the pain spots found by v. Krey in certain regions would have to be much more numerous and closer together. The fact that the cornea, which v. Frey found to be destitute of any specific sensibility except pain, is A B I-'IL. 10. Topography of areas sensitive to cold (A), and to warmth (13), on saw part of the anterior surface of the thiiji. (Goldscheider.) The black areas are highly sensitive to thermal stimuli ; the striated areas moderately so; the dotted areas very slightly ; the spaces left white an- not at all sensitive to such stimuli. provided with a nerve-plexus that has free infra-epithelial endings, as described by Cohnheim (1866), supports this conclusion. Similar nerve -endings have also been recently described in epithelium which is not ectodermal in origin, and in the interior of many tissues which increases the probability that they are related to pain sensibility, as this, when very slight, is allied to a sensation of tension or of simple contact, as Nagel(1895), in oppo- sition to v. Frey's view, observed in the cornea. It is far less easy to identify the peripheral organs that subserve the sensations of heat and cold. P>y elimination it may be said that Dogiel's corpuscles, Ruffini's papillary endings, and the Golgi-Mazzoni corpuscles are the organs for the sensation of cold, while Pacini's and Rutiini's corpuscles function, at least in the skin, as organs for the sensation nf heat. The fact that the PHYSIOLOGY CHAP. latter lie in the deepest layer of the skin agrees well with v. Frey's statement that the heat spots are the most difficult to determine and have a longer reaction time. On the other hand, it appears probable from an interesting observation by v. Frey that the sensation of cold is dependent on the end-bulbs described by Golgi and Mazzoni. The conjunctiva of the eye is insensitive to pressure and heat, while its sensitiveness to cold, on the con- trary, is very definite : Dogiel's observations show that the end- bulbs are abundant in the conjunctiva. VI. Although the sensations of heat and of cold represent two modalities which depend on distinct sense-organs they may conveniently be discussed together, as most of the observations on this subject gain in interest by comparison. Sensibility to cold and heat not merely includes the external cutaneous surface, but also extends to the skin of the auditory canal, and the mucous membrane of the nose, mouth, pharynx, and anus. The conjunctiva of the eye and external mucous membrane of the genital organs are insensitive to heat, but sensitive to cold. The rest of the mucous membrane, e.g. in stomach, intestine, etc., is totally destitute of any thermal sensi- bility as E. H. Weber showed in 1851. We saw that it is easy by means of punctiform stimulation to demonstrate that the two thermal senses are unequally distributed in the different cutaneous regions, and that cold spots are much more numerous than heat spots. Goldscheider, in order approximately to map out the distribu- tion of thermal sensibility, experimented on different cutaneous regions with thermo-aesthesiometers in the form of metal cylinders, 3-4 mm. in diameter. With this method it is possible to excite a greater or less number of thermal points by heat and cold. If the skin-surface investigated contains no thermal point, it has no thermal sensibility, and its thermal sensibility varies according as it contains many or few thermal points for heat or cold. It must be noted, however, that the degree of sensibility is not proportional to the number of excited sensory points, because the excitability of the latter has been experimentally proved to vary considerably : the presence of a few highly excitable points may make one area of the skin appear more sensitive than another which contains more thermal points that are less excitable. Goldscheider's method does not therefore determine the greater or less abundance of thermal points in different parts of the skin, but merely the mode in which these react to ordinary stimulation by heat and cold. Figs. 19 and 20 from Goldscheider's memoir illustrate the results obtained by this method. They show that the sensibility to heat is invariably less developed both in intensity and in extent than that to cold. According to Goldscheider there is no (TTANKOTS SKXSir.lUTY in which sensibility to warmth is more developed than I luil (<> cold. This holds good both lor the covered and 1'or the uncovered regions. Where the sensihility to heat is highly developed, that to cold still preponderates, both in intensity and in extent. There are as \ve ha\e said regions in which sensibility to cold is more or less acute while sensibility to \\armth is very low or entirely absent. The varying thermal sensibility in different cutaneous areas depends not only on the greater or less abundance of cutaneous nerves, but also on the varying thickness of epidermis that covers the nerve-endings, and also perhaps on the depth at which the nerve-endings themselves are situated. Previous to the discovery of the duality of thermal sensation Kn.. L'0. Topography of sensibility to cold (A), anil to heat (13), in saints part of palm of left hand. (Goldscheider.) Explanation in previous ligure. Weber and Nothnagel attempted to map out thermal sensibility by exploring certain regions of the skin with flasks of oil, or with the rounded ends of large keys previously cooled or heated. After the discovery of heat and cold spots, Goldscheider (1887) extended the research by using metal cylinders, at a temperature of 15 for cold and 4o c -49 for heat. More recently Veress (1902) has again investigated sensibility to heat on himself by means of bis thermo-aesthesiometer (Fig. 1, p. 14). Here we can only cite the most conclusive of his general results: (a) Sensibility to heat is not equal in the two halves of the body. On an average it is rather greater on the left than on the right. (6) The most mesial parts of the trunk are, generally speaking, less sensitive to heat than the lateral regions. (c) The trunk is. generally speaking, more sensitive to heat than the extremities. 30 PHYSIOLOGY CHAP. Sensibility to heat is not uniform in the extremities; some distant parts are mure sensitive than others more proximal. (e) The lateral surfaces of the extremities are less sensitive to heat than the mesial sides. To these conclusions we may add that in those cutaneous regions which are peculiarly adapted to tactile sensibility (as the hand in general, the tips of the fingers in particular) the thermal sensibility to both the cold and heat sense is less than in other regions. Parts that arc habitually covered are more sensitive to cold than exposed parts. This is not due entirely to habit, but principally to the fact that the covered parts contain a great many cold spots : for the same reason the skin of the face, though it is constantly exposed, is not less sensitive to cold than the covered parts of the skin. The terminal apparatus of the thermal nerves has in common with other nervous organs the property of being more strongly excited in proportion as the stimulation is more rapid. As the adequate stimulus consists in the addition or subtraction of heat at the thermal points, it may lie said that the excitation or reaction of the latter is more intense in proportion as the increment or decrement of heat occurs more rapidly. The strength of the sensation also depends partly upon the extent of cutaneous surface excited. A thermal stimulus dis- tributed over a large area of skin evokes a stronger sensation than a stimulus of the same strength acting on a smaller area. This is easily demonstrated by plunging one finger of one hand and the whole of the other hand into water ; or by dipping one finger into water at 40 C. and the other hand into water at 37 C. In both experiments the sensation of warmth is less in the finger than in the hand. Weber also noted that a stimulus which is purely thermal when applied to a small surface may become painful if it acts on a larger surface. One finger alone can be plunged into water at a temperature at which the immersion of the whole limb would be painful. The reaction time for sensations of cold and that for tactile sensations are equally short ; on the other hand, the reaction time for sensations of heat, as that for sensations of pain, is longer (Tanzi). According to Kiesow and Ponzo, the reaction time for heat is shortened if stimuli that penetrate the skin more readily than those employed by Tanzi are adopted, and if the specific points are excited directly. Nevertheless, it still remains longer than that for sensations of cold and contact. According to Kiesow's latest work, the reaction time to pain sensations is much shortened if sharp-pointed stimuli are used. From this it results that if one and the same cutaneous region is excited simultaneously with cold and hot stimuli, the sensation of cold i CUTANEOUS SENSIBILITY :;i precedes that iif heal. Further, the excitation of any spot l>y cold produces a. more lively sensation, that reaches its maximum more rapidly than excitation <>!' I lie same spot hy heat. Accord- ing to v. Frey this difference is not apparent on exciting the two thermal spots by electrical stimuli. From this he concluded that the nerve-organs of the warm spots lie in the deeper layers of the skin, and those of the cold spots in the more superficial layers. The physical properties of the thermal agents, again, have an influence on the effects of excitation. Stimuli may consist of solid, liquid, or gaseous bodies, and may act by conducting heat or by irradiation; they may be good or bad thermal conductors; their thermal capacity may lie large or small ; lastly, they may have a smooth or a rough surface. Thermal sensations are stronger according as the stimulating body is a good conductor of heat. Water at 25 C. is a stronger stimulus of cold than oil, and less strong than mercury at the same temperature. It is possible to arrange a graduated series of bodies with different thermal conductivities, but all of the same temperature, by which a series of thermal sensations of gradually increasing strength can be excited. This, however, applies only to intensity of sensation as evoked by the initial contact. With prolonged contact new relations are set up, due to variations in the thermal exchanges between the cutaneous surface and the external agent, so that a first impression of cold may be translated into a sensation of warmth. For instance, on dressing, or lying down in lied undressed, the first sensation is one of cold, followed quickly by the opposite sensation of warmth, which may be less or greater according to the nature and thickness of the clothing or bed-covering. Any body that serves as a thermal stimulus must, besides its power of conducting heat, also possess a certain minimal thermal equation in order to produce a sensation : the latter within certain limits may increase in intensity with an increasing thermal equation of the stimulating body. Thunberg has shown that various degrees of thermal excitation can be evoked in the skin by contact with bodies that have the same temperature but different thermal properties, for instance a series of silver or copper plates of various thicknesses. By means of these plates it is easy to determine the minimal degree of heat required to evoke a thermal sensation. The importance of the smoothness or roughness of the surface of the liody that is used as a thermal stimulus is easily under- stood, seeing that the conduction of heat, and hence the efficacy of stimulation, varies according as the points of contact between the skin and the conducting body are few or many. The essential conditions for the production of sensations of heat or cold must consist in the thermal changes that take place PHYSIOLOGY CHAP. in the skin. So lung as the temperature of any part of the skin remains constant between certain mean limits, there is no ex- citation; but as soon as the temperature of this region changes, either from external or internal reasons, thermal sensations at once arise. Normally a slow, continuous thermal current ilows through the skin from within, outwards. So long as the conditions of this current remain unchanged, the temperature of the nerve-organs remains the same; but if the current alters with a certain rapidity, there is a, sensation of warmth or cold in consequence of tin- rise or fall of temperature in the end -organs. According to Weber it is these changes in the temperature of the end-organs which constitute the adequate stimulus and the essential conditions of thermal sensation, no matter what caused the alteration of temperature. It almost seems, lie \\riies, as if we could detect the process of rise and fall in the temperature of our skin much better than the degree to which the temperature rises and falls. Since the discovery of specific organs for cold and heat, it has become possible to give a more exact definition to Weber's theory, by saying that the organs for cold are excited by fall of their temperature, and those tor heat by its rise. This theory gives a satisfactory explanation of many facts. We are aware of a sensation of cold both when the loss of heat through the skin increases, and when the peripheral blood-supply diminishes. We have a sensation of warmth both when the loss of heat by the skin is decreased in consequence of a rise in the temperature of the environment, and when the peripheral blood- supply increases. Accordingly, it is not the direction of the thermal heat current from within outwards, or from without inwards, nor the intensity of this current, which produces the thermal sensations, as assumed by Vierordt, but the changes in temperature at the thermal end-organ, no matter what process causes them. One fact, however, seems at first sight to contradict Weber's theory. If a metal at 3 C. is applied for some time to any part of the skin, for instance the forehead, and then removed, there will for some 20 seconds be a sensation of cold in that part instead of heat, as would be the case if the skin were growing warmer. Fechner and Vierordt also noted that it is possible to feel a prolonged sensation of heat or cold without any change in the temperature of the environment. . These facts led Hering to conclude that not only thermal changes, but the absolute degree of cutaneous temperature as well, may act as a stimulus of thermal end-organs. When the temperature of the environment remains fairly constant we are not as a rule aware of any thermal sensation, although the different parts of the skin may have a very different i CUTANEOUS SENSIBILITY 33 temperature, according as they are exposed or covered. Tin- temperature that produces no thermal sensation is not at any definite point of the thermometric scale, but, according to Leegaard, seldom ranges over more than 0-5 C. This indifferent temperature alters not only in the different regions of the skin, but also in the same region at various times. For instance, on passing from a room in which no thermal sensation is felt into one that is hotter or colder there is an immediate sensation of heat or cold. But if the difference in the temperature of the two rooms is not very great a new equilibrium will soon be set up so that no thermal sensation is perceptible. The surrounding temperature may therefore vary between considerable limits without producing any persistent thermal sensation. It might be supposed that this adaptability depends upon variations in the blood-supply to the skin, which to a certain extent protects the peripheral thermal end -organs from the oscillations of temperature in the environment. Thunberg, however, pointed out that it can be observed on a hand previously rendered bloodless. The adaptation therefore depends on an alteration of the excitability of the peripheral thermal end-organs, which causes a displacement of the level of the indifferent temperature or physiological zero-point (Hering) of thermal sensibility. Starting from this fact Hering maintains that any intrinsic temperature of the thermal organs above the physiological zero- point is perceived as heat, and any temperature below the zero-point as cold. The intensity of the sensation of heat or cold increases with the variation of the intrinsic temperature of the end -organ from the physiological zero. Any intrinsic temperature of the end -organ appreciated as heat causes an upward displacement of the zero-point : any temperature appreci- ated as cold, a downward displacement. All sensation of heat and cold ceases when, owing to the displacement of the zero- point, the latter coincides with the intrinsic temperature of the end-organ. The existence of two distinct senses for heat and cold is not fundamentally irreconcilable with this theory of Hering. It may be assumed that a rise in the cutaneous temperature acts only upon the organs of heat, and a fall upon the organs of cold. But the so-called paradoxical sensation of cold cannot be explained either by Weber's or by Bering's theory. If a metal point warmed to 45-50 C. is applied to a cold spot a sensation of cold is felt (Lehmann and v. Frey). The accuracy of this observation has been confirmed by many authors (Alrutz, Kiesow, Thunberg, Veress, Bader). All cold spots react by a sensation of cold when brought into contact with a warm point. When a thermo- aesthesiometer is applied over an extensive surface, cold spots are stimulated as well as heat spots, hut the sensation of warmth VOL. IV D 34 PHYSIOLOGY CHAP. predominates and masks the opposite sensation of cold. Thunberg, however, by choosing an appropriate form of stimulus succeeded in producing the two sensations separately, first cold and then heat, which is a strong argument that the nerve-organs for cold lie in a more superficial layer of the skin than those for heat. The paradoxical sensation of heat was first observed by Striimpell in anaesthesia produced by freezing, and was described under the name of "perverted thermal sensibility" : Lerda (1905) encountered it in certain small cicatrices of not too recent date ; Michael Sugar (1910) in a patient with syringomyelia and in a few cases of multiple sclerosis; Ponzo (1909) in areas of skin that had been artificially anaesthetised by means of subcutaneous injections of stovaine ; Fontana (1912) in patients suffering with large condyloniata. VII. Sensations of pressure, like sensations of heat and cold, are elementary, and cannot be split up into simpler components. Pressure sensations enable us to appreciate the surface contact of external objects independent of their temperature. Meissner believed it possible to distinguish sensations of simple contact from pressure sensations, as if they differed funda- mentally. But later observations showed that both are due to deformation of the cutaneous surface, and therefore represent different degrees of a single quality of sensation. When the contact is so light that it produces no pressure on the cutaneous surface, there is no sensation of any kind. The functional importance of the sense of contact or pressure lies in the fact that by its means we are able to perceive the slightest mechanical impact upon the surface of our bodies. Meissner's corpuscles and the nerve -rings that surround the sheath of the hairs are homologous organs that come into play during sensations of contact. They are capable of excitation by mechanical agents a thousand times weaker than those necessary for the direct excitation of the peripheral nerves (Tigerstedt). We have already pointed out that the heat and cold points do not coincide with the points for contact or pressure. We may now add that the regions of maximal sensibility to thermal stimuli differ from those of sensibility to tactile stimuli. An exact comparative determination of tactile or pressure sensibility in the different regions of the skin is very difficult. The pressure exerted by a gas or fluid is certainly the best means of exerting a uniform pressure upon every part of the curved surface of human skin. But we know from experience that such a pressure is not appreciable. We are quite unaware of the pressure exerted by the atmosphere upon the surface of the body as a whole, and of the hydrostatic pressure when the entire body is bathed in water. The physiological effect depends not only on the amount of pressure exerted on the skin, but also on i CUTANEOUS SENSIBILITY 35 the number of superficial cutaneous elements on which tin: pressure acts. The effect seems to be greater or more easily I'civcived in indirect relation to the area of skin compressed. Ynii 1'Yey's method of determining the tactile sensibility in different regions of the skin by hair* is based on this fundamental observation. With this method it is easy to prove that the parts most sensitive to mechanical agents are the tip of the tongue, the red part of the lips, and the ends of the fingers. Even in these parts there is a threshold of stimulation which must be crossed to evoke a sensation. This shows that the parts where the sense of pressure is most delicate do not coincide with those in which thermal sensibility is most developed, which is a valid argument in favour of the theory that the two forms of sensation arise in different end-organst We know nothing about the nature of the process by which the excitation takes place. Possibly it may consist in a discharge of energy caused by a chemical change within the end -organ, due to a displacement of fluid a change in the concentration of the dissolved substances which acts as a chemical stimulus. We know that continuous pressure of a certain intensity on a sensory nerve produces a continuous sensation, which is difficult to explain as a mechanical effect, because the work required to stimulate an exposed nerve is probably a thousand times greater than that sufficient to excite those nerves adequately when it acts on their terminal end-organs. We have consequently no reason to reject the hypothesis that mechanical stimuli can only excite the nerve indirectly, and that excitation is always due to alteration of the chemical structure or to osmotic pressure of the tissue fluids (v. Frey and Kiesow). At the same time we cannot exclude another simpler though more indefinite hypothesis, according to which excitation depends upon a purely physical process, by which the mechanical stimulus is changed into another form of energy, to which Meissner's corpuscles are far more sensitive. As regards the mode of action of a mechanical stimulus in producing sensations of contact and pressure, v. Frey and Kiesow (1899) showed that it is not compression in itself that determines the excitation, but the deformation of the skin surface, which produces an alteration in the pressure (Druckgefalle), and this again indirectly pioduces an active reaction of the terminal organ. This alteration is produced both by compressing the skin with a point or small surface, and by pulling on a small body or disc attached to the surface of the skin. In the first case the pressure is greatest at the compressed surface, and diminishes in the deeper p;irts and in the surrounding areas of the skin ; in the second the pressure is least in the part of the skin drawn up, and increases 36 PHYSIOLOGY CHAP. towards the deeper parts and in the surrounding areas (Fig. 21). Both the positive and the negative alterations give rise to sensa- tions of contact, and this with approximately equal strength of stimulus shows the same characteristics on compression and on traction. Accordingly the excitation of the sense-organ of pressure is due to alteration in the intrinsic pressure of the organ, and the intensity of the sensation depends on the amount and not on the direction of the alteration. Since it is not possible to measure exactly the alteration in pressure which, with different means of mechanical excitation of the skin, gives rise to the sensation of pressure, it follows that in onler to obtain even an approximate valuation of the effective threshold of stimulation we must take into account all the factors that may raise or depress it. The investigations of v. Frey and Kiesow show that the liminal value varies with the rate of .:::&' / ..-' / Kn.. -1. Diagram of deformation of a cutaneous area (shown in section) by compression with a weight (A), and by traction on a disc previously attached to the skin'(B). The continuous curved lines in A represent the positive change of pressure ; the dotted lines in B represent its negative change. The variations of pressure in the skin are maximal at the edges of the weight or disc, and become gradually less below the area compressed <>r puller! upon. stimulation and with the nature, size, and depth of the cutaneous deformation. As regards the manner in which the mechanical stimuli may at least be appreciated relatively if not measured exactly, they concluded that : (a) The limiual mechanical stimulus cannot be estimated by weight, because the effect of a given weight always depends on the area of the surface of contact. (6) When the surface of contact remains constant, a given weight produces a different effect on different parts of the skin, because the number and the sensibility of the nerve-endings excited varies in different cutaneous areas. It is consequently only possible to compare limiual estimations when the experi- ment is confined to the excitation of single nerve-endings, i.e. to single tactile spots, by means of v. Frey's hairs. (c) If the same tactile spot is stimulated by a weight which has a constant surface of contact, so as to produce near any such point a deformation constant in depth and surface, the effect of such an excitation varies with the rate at which the deformation i CUTANEOUS SENSIBILITY 37 takes place. A deformation rapidly produced lias more effect, HUM one produced slowly. It follows that the effect of the stimulus is not dependent on the mechanical work performed, because different amounts of mechanical work may produce identical -1'iisutions, and rice versa. (d) If on stimulating one and the same tactile spot the surface area of the stimulus is altered, then to obtain approximately the same effect the weight and rapidity of stimulation must be correspondingly altered. Hence the results obtained with different methods can only be compared when the increment of weight for the unit of time and surface, i.e. the rate of pressure, remains constant. The results which v. Frey and Kiesow obtained on exciting large and moderate cutaneous areas show that the threshold values of the weights do not increase in proportion with the increase of the surface deformed. This can only be explained on the theory that the excitation which causes a sensation of pressure depends on the. alteration in pressure produced within the skin, and that any pressure that is equal on all sides produces no effect at all. As the excited surface grows larger the fall of pressure in the skin becomes less. On the other hand, the smaller the stimulating surface, the more rapid will be the alteration in pressure ; an increase in surface pressure then becomes necessary to produce a change in pressure at the level of the nerve-organ adequate to excite it. On the strength of these investigations it is easy to explain the well-known experiment of Meissner. If the hand is dipped into a fluid water or mercury of the same temperature as the hand a pressure sensation is not felt over the whole surface of the submerged skin, but only at the boundary between the parts compressed and those not compressed. If, e.g., one finger is dipped into mercury at the same temperature as the finger, a sensation is felt of a ring compressing the finger. This sensation is referred to the level at which there is an alteration in the pressure, while there is no sensation over the whole surface that is exposed to a gradual and slowly increasing pressure, since the variation is so slight that it remains below the effective threshold of stimulation. Kiesow (1904), in a long series of patient and delicate re- searches by the most modern methods, attempted to estimate as accurately as possible how the sensibility to touch and pressure alters upon the different parts of the surface of the body. He investigated in two directions. He determined separately the number of the touch spots (or pressure spots of Blix and Gold- scheider) in the surface unit, and then the liminal stimulus for the touch spot, that is the mean value of the threshold, obtained by a series of separate observations in different cutaneous regions. These estimations were made by means of v. Frey's hairs. PHYSIOLOGY CHAP. The results of these researches were published by Kiesow in a number of tables, of which we can only cite the final conclusions. In these Kiesow compared the relative sensibility of the different cutaneous areas which he examined (arranged in order of increas- ing sensibility), according to the mean liminal values obtained and to the number of the touch spots per surface unit. Kiesow started from the region in which sensibility to contact or pressure is lowest, which he designates as 1, and on comparing all the other regions to this, obtained the following results for their mean liminal values : Bacfc, median line, level of 3rd doraa] vertebra 1-00 Abdomen, white line, midway between umbilicus and pubic symphysis . 1-06 Z^raa, median line, level of 5th intercostal space 1-24 Thorax, left axillary line, le\el ol' ."ilh intercostal space. . . . 1-33 Z7wra&, median line, level of 4th intercostal space .... 1-39 Thorax, left axillary line, midway between xiphoid and umbilicus . 1-79 Left patella, middle of 1-95 Left let/, in the middle of the anterior surface 1-99 BacA, median line, level of antero-superior iliac spine .... 2-23 Left ihiiih, anterior surface, about 1 cm. from edge of patella . . 2-31 2>ac&, median line, level of 7th cervical vertebra 2-72 ZTwrao;, median line, level of 2nd intercostal space .... 2-77 Left leg, calf ... 2-96 Left arm, middle of ilexor surface ........ 3-01 Left 7'T/V, stiloid process of ulna 3-05 Left elbow. 3-09 Le/t /orearm, upper part of flexor surface ...... 3-12 Left wrixt, dorsal ,-urface, middle line 3-26 Dor sum of left foot . . . . . . . . . . .3-38 Left wrist, radial surface 3-49 Left forearm, middle of flexor surface ....... 3-80 Jurist, 2-7 cm. aliove the joint 3-80 Left upper eyelid 7-16 Forehead (glabella) 7-54 For the tip of the tongue, red part of lips, and tips of fingers, Kiesow, starting from the same value of 1, and taking the minimal liminal values (not the mean, as above), obtained the following results : Finger-tips, of left hand . . 3 Edge of lower lip, middle part ........ 50 Tip of tongue ............ 60 Kiesow obtained the following number of touch spots in the surface unit (1 sq. cm.), starting from the region in which they are fewest ( = 1 ) : Ley, middle of anterior surface 1 -00 Calf. .... ... . 1-16 Left patella, middle 1-60 Left forearm, middle of flexor surface . . . . . . .1-85 Left arm, upper part of flexor surface ....... 2-00 Left elbow 2-43 i CUTANKOIS SENSIBILITY 39 Left thiiih, anterior surface, about I cm. from cd^i- of patella . 2-7 Bcwi, middle line, level of ant aup. iliac spine ... . :MH Ltf/ hn-i'i tnii, middle of llexor stirfaee ....... 3-22 77/nn/.'-, left axillary line between xiphoid process and umbilicus . . 3-2:") Z%oroo^ middle line, level of 2nd intercostal space .... 3-85 Le/% wrist, stiloid process of ulna ........ 4-10 '/'//"/././, middle of axillary line, level df fit h inteirusial space . . 4- If, C, middle line, level of 4th. intercostal space ..... 4-35 of left foot, middle ......... 4-7."! ft, middle line, level of 3rd dorsal vertebra ... . 4-75 Thorn. >\ middle line, level of f)th intercostal space. .... 4-95 Left wri.e developed and improved by practice. Czermak and Gartner found that the power of localisation is more highly developed in the blind than in normal people, and Volkrnann noted that the improvement takes place on both sides of the body, although the sense of touch is nearly always better ap- preciated by the right hand. One of the most striking proofs that tactile discrimination is improved by practice is that in compositors it is extraordinarily well developed in the finger- tips. In the highly mobile parts of the limbs, a few hours of practice are enough to increase tactile discrimination to a Fn:. 22. Ponzo's new aesthesiometer, substituted for Weber's compass, consists in ;t handle and two arms that can be adjusted by means of a screw at the end of the handle. The arms carry two brass clip.s with two points at their ends. To vary the quality of the tactile, thermal, or painful stimulus the blunt ivory points may be replaced! by blunt or sharp metal points. remarkable degree, almost to double it. In the immobile and more protected regions, on the contrary (e.g. the skin of the trunk where it is low), even prolonged exercises do not increase it perceptibly. It is certain that education of any area of the skin on one side increases sensibility, not only in the vicinity of that area, but also in the corresponding area of the opposite side. Many conditions alter the delicacy of tactile localisation. If a limb is raised so as to make it anaemic, or the veins are com- pressed till there is congestion or venous stasis, spatial sensi- bility is blunted. The same occurs when the attention is fatigued by unduly protracted tests (Alsberg), and by the action of cold (Goltz) ; after prolonged application of the anode of a galvanic current (Spanke) ; on passive distension of the skin (Czermak) ; by certain poisons atropine, daturine, morphine, strychnine, cannabine, alcohol, chloral hydrate, potassium bromide (Sichtent'els and others). 42 PHYSIOLOGY CHAP. According to Sherrington, cerebral cortical lesions in man disturb tactile localisation far more than any other form <>!' cutaneous sensibility ; the patient, in fact, may refer a touch on the hand to the forearm. Before discussing the results obtained by various experimenters on the sense of localisation in different regions, it is necessary to point out certain facts that must be remembered in using Weber's aesthesiometer. These are : (a) If the two ends of the compass are put down one after the other, instead of simultaneously, the two contacts will be appreciated at a less distance. (6) The same occurs if, in estimating the limiual distance at which the compass-ends are separately perceived, the alteration is made from greater to less distances between the points, instead of from less to greater. (c) If one of the ends is warmer or colder than the skin the two contacts will be perceived at a less distance than if both points are of the same temperature as the skin. (d) Bathing the skin witli indifferent fluids increases tactile discrimination, i.e. the discrimination is sharpened. () If the skin is gently stroked between the two ends of the compass, or electrified with weak currents, one end only will be detected, where both had previously been perceived. The following table gives the value in millimetres of the mean liminal distances for perception of the two points of the aesthesio- meter, obtained by Weber on a normal adult subject, and by Landois on an intelligent boy of 12 years old. Adult. Boy. Tip of tongue 1-1 1-1 Palmar surface of third phalanx ..... . -2-2 1-7 Red part of lips 4-5 3-9 Palmar surface of second phalanx 5 3-9 Palmar side of nrst phalanx ..... . 5 Dorsal side of third phalanx ...... . 6-8 4-:, Tip of nose ......... . 6-8 4-5 Ball of thiuul. . 7 ... Middle of palm . 8-9 ... Middle of dorsum and edge of tongue .... 9 6-8 Metacarpus of thumb 9 6-8 Plantar surface of third phalanx of big toe . . 11-3 6-8 Dorsal surface of second phalanx ..... . 11-3 9 Cheek . . 11-3 9 Eyelids . 11-3 9 Centre of hard palate ....... . 13-5 11-3 Palmar side of lower third forearm .... . 15-0 Anterior part of zygomatic region ..... . 15-8 11-3 Plantar side of metacarpus of big toe . . 15-8 9 Dorsal surface of first phalanx . . . 15-8 9 Dorsal head of metacarpus ...... . 18 13-5 Inner part of lips ........ . 20-3 13-:> Posterior part of zygomatic region .... . 2iMi 20-3 31-6 i CUTANEOUS SENSIBILITY 43 A.luli. Boy. Lower occipital region ....... -71 -J.-i-(> Dorsum of hand ... Chin \Vrte\ nt' head Knee-joint ......... Sacral and gluteal regions ...... Forearm and leg Dorsum of foot near toes Sternum ......... Neck, high up ........ Dorsal spine, lover thoracic and lumbar region . Middle of neck 33-8 >>.(> 3G-J 31-6 40-6 33- s 40-6 36-1 40-6 : J .'M 45-1 33-8 54-1 36-1 54-1 67-7 07-7 40-6 Middle of arm, thigh, back ...... Weber gave the name of tactile circle to the area within which the two points of the aesthesionieter are appreciated as a single point. If in any cutaneous area the localisation is equally developed in all directions, the circles are round, i.e. they approximate to the figure of a true geometrical circle ; but this is very seldom the case. More often, particularly in the extremities, they are oval, because tactile discrimination is, as we have seen, more developed in the transverse than in the longitudinal direction. For this reason Hermann prefers the term tactile fields to tactile circles. These tactile circles or fields have no fixed anatomical limits, and do not correspond to the peripheral distribution of a single nerve -fibre. If they did there would be a sudden transition from a single perception (when the two points were applied within one circle) to a double one (when the equidistant points were applied to two adjacent circles), which is not the case, since each point of the skin may bej taken as the centre of a circle. As, moreover, discriminative sensibility differs enormously in different regions of the skin, as shown by the above table, this assumption is obviously irreconcilable with such a varying peripheral distribution of the sensory cutaneous fibres in the different parts. Weber accordingly assumed that each tactile circle contains many nerve-endings, and that for the recognition of the two contacts it is necessary that there should be between the two excited nerve-endings a certain number of unexcited end-organs, which vary in different regions according to their congenital arrangement. This theory is obviously less an explana- tion than a simple statement of fact. It does not explain how the tactile fields can be diminished by practice. From the psychological point of view Lotze supposed that each nerve- fibre distributed to the skin or adjacent mucous membranes is provided in the brain with & local s'ujn of recognition of the place to which it is distributed in the periphery. In developing this idea Wundt concluded that on stimulation each cutaneous area transmits to the brain not n>\\ -n-SeOjUanl, Kunkc, Mtinsterberg). \YG have already seen that tin 1 iiKire recent work of v. Frey and Kiesow ia decidedly in favour of this view, and must now investigate the arguments on which it is founded. Knnko (18SO) was the first to call the attention of physio- logists to the interesting clinical observation that a dissociated t>!' Lumbricus, Normann lias proved that they cannot have the significance of expressions of pain, because the same reactions are seen in the segments with and without nerve ganglia. Loeb found that if a Planaria was divided in half, the anterior part continued to move quietly as though it had felt no pain. In Gammarus the stomach can be cut away during copulation without interrupting it. Bethe noticed that the aMonien can be cut off a honey-sucking bee without disturbing its occupation. The frog reacts violently to electrical stimulation of the sciatic nerve, but whether il feels much pain is doubtful, as the same reactions take place after decerebration. Herbivora are less sensitive to pain thai i earnivora. Veterinary surgeons know that horses continue to eat while undergoing an operation, and the rabbit eats directly after serious operations. These and other observations show that the development of sensibility to pain is parallel to the development of the intelligence. In human races sensibility to pain is more developed in proportion as they are more civilised ; in imbeciles, idiots, and dements it is very low. Pain, then, is a function of the intelligence, a psychical element superposed upon the subconscious protective reflexes. A painful sensation produced by a mechanical or thermal agent at the cutaneous periphery may from the teleological point of view be compared with the nauseating taste of a poison. Weber observed that the temperature which began to produce pain (48 C.) when applied to the skin was the same at which the nerve substance begins to alter. The teleological relation between the painful stimulation of certain afferent paths and certain instinctive reactions witnesses to the protective significance of pain. Nevertheless, it cannot be affirmed that pain is an infallible indication of menace to life. There may lie severe pain, as in neuralgia, with no manifest lesion of the tissues ; at other times there may be no pain although the tissues are fatally affected, as occurs with an invasion of pathogenic bacteria. This shows that in the world of living beings co-ordination of function to a given end takes place within definite limits, and that the sense-organs, like all the other organs, are adapted to function teleologically during normal relations with the environment, and not in ex- ceptional circumstances. Whether the sensations of tickling and itching are to be con- sidered as specific sensations in the same category as sensations of pressure and of pain, or merely as modifications of the latter, is still a matter of controversy. We must first examine the conditions which give rise to them. To arouse tickling in the cutaneous regions provided with hairs, it is only necessary to touch these parts lightly, e.g. by a feather. Even in the parts that have no hair the red of the lips, the nostrils, eyelids, and i CUTANEOUS SENSIBILITY 53 forehead the slightest touch will arouse tickling and ;i, desire tn scratch to remove the annoyance. In these parts it is not necessary to excite with light ;i,nd deliea.ie stimuli : coarse mechanical st imulat ion will arouse such ;i, tickling I hat it causes \ iolent retlex movements spreading to almost all the muscles, and uncontrollable by the will. The sensation of itching which accompanies different cutaneous diseases is normally produced by the sting of an insect, and may readily be aroused by the prick of a fine needle. But Jessner holds, on the contrary, that itching is a, paraesthesia, i.e. a morbid variety of cutaneous sensibility that is absent in normal individuals. There is no very marked difference between the sensations of tickling and itching: there are intermediate sensations, which may be regarded as mixed sensations, due to the simultaneous excitation of several sense-organs. According to Weber, tickling depends on a diffusion of the excitation, and on the persistence and increase of the sensation after the stimulation has ceased. Funke, too, regards tickling as a. secondary effect of sensations of contact, which only arises on applying weak stimuli. Goldscheider, who, as we saw, ascribed pain sensibility to the pressure end-organ, refers tickling also to a special mode of excitation of the same organ. Von Frey and Kiesow, on the contrary, hold the organs for pressure and pain to be distinct, and refer the sensation of tickling to the first, of itching to the second. With Quincke they regard tickling and itching not as primary, but as secondary sensations, caused by reflexes acting from the nerves of touch and pain upon the vaso- rnotor nerves. But on what does the peculiar feeling of these sensations depend ? Why do they arise with weak stimulation and disappear when the stimulus is strengthened ? These questions are unsolved. Alrutz has disputed the theory of v. Frey and Kiesow. He considers that tickling and itching are two varieties of a single modality of sensation, depending on special nerves other than those of pressure and pain. He states that the sensation of tickling is produced by excitation of cutaneous spots other than the touch and pain spots. He quotes a case of lead-poisoning described by Bean, in which there was analgesia, without disturb- ance of pressure sensibility, but with insensibility to tickling. In two other cases of circumscribed or diffuse analgesia he observed the same state, that is, persistence of tactile sensibility in parts insensitive to tickling. He further cites a case of hyperalgesia communicated by Goldscheider in which there was hyperaesthesia for sensations of tickling and itching. These must accordingly run parallel with the pain sense and not with tactile sensibility. It remains for further researches to decide which of these opposing theories is correct. 54 PHYSIOLOGY CHAP. EDITORIAL NOTE This chapter would not be complete for English readers at least without a reference to the investigations of Head and Rivers on the mechanism of peripheral sensibility. Their conclusions were drawn mainly from a study of the sensory changes produced by section of a small cutaneous nerve in Head's arm and the observation of the sensory phenomena that occurred during its regeneration, but they were supported by numerous clinical examinations made in conjunction with Sherren. In addition to those fibres concerned with cutaneous sensibility, they described a system that subserves deep sensibility (see Chapter II.) the end- organs of which respond to pressure either by sensations of contact or of pain if the pressure is excessive, and to the movements of joints, tendons, and muscles. The sensations of pressure evoked can be accurately localised, and the direction of the movement appreciated correctly, even though the over- lying skin is totally insensitive, but two compass points applied simultaneously over the part cannot be discriminated by this system alone. The nerves of tli is deep sensibility run mainly with the muscular nerves and are not destroyed if all the cutaneous fibres are cut. Head and Thompson, however, suggest that fibres belonging to the deep system also reach the skin. Cutaneous sensibility was divided into two separate systems ; the one called "protopathic " is capable of reacting to all painful cutaneous stimuli of every nature, and to the more extreme degrees of heat and cold, that is, to thermal stimuli above 40" C. and below 24 C., but the sensations produced are diffuse, unnaturally intense, and unaccompanied by a definite recognition of the locality of the spot stimulated. Through the second system, styled " epicritic" light contact and the inter- mediate degrees of heat and cold are appreciated, and on it, in addition, depends cutaneous localisation, the discrimination of the compass points, and the appreciation of si/r. The protopathic system is essentially one of punctate sensibility, as sensations of pain, heat, and cold can be excited only from the corresponding spots ; the sensation of warmth depends probably on nerve-endings that lie between the sparsely scattered heat spots, and the appreciation of coolness, as contrasted with cold, may be due to end-organs other than those of the cold spots. It is suggested that these three peripheral systems were developed at different phylogenetic periods. The two cutaneous systems, the so-called " epicritic " and the " protopathic,''' can be, according to Head, studied separ- ately on an area of skin after section of a sensory nerve, as the loss of the protopathic elements is usually less extensive than the epicritic, and during regeneration and recovery of function, since the " protopathic " sensibilities reappear first. On the glans penis, too, there is only "protopathic" sensation. These ingenious and elaborate observations have not been, however, verified or supported by authoritative independent investigations, and the researches of Trotter and Davies, especially, have thrown much doubt on them and on the accuracy of the conclusions drawn from them. These workers investigated the effects of the section of seven cutaneous nerves in themselves ; they found that this operation produces a central area of profound sensory loss, an intermediate zone of moderate extent surrounding this of partial loss, and a larger zone in which qualitative changes only can be detected. When regeneration sets in, the return of all sensory functions begins about the same time, but is irregular. They failed to discover that identity in the states of sensation in the intermediate zone of partial loss and in the central area during the progress of recovery, which is an essential i CUTANEOUS SENSIBILITY 55 point in Head's theory, and they could not confirm the siniiiltaiieoii- return of sensibility to toucn and to moderate degrees of temperature in die same areas, on which Head's hypothesis largely depends. BIBLIOGRAPHY For Johannes Miillcr's theory of the Specific Energies of the Sense Organs, the student may refer to the two following monographs, in which all the earliest literature is gathered up : GoLi'sriiK.iDKK. Die Lehre von den spe/.iliselien Energien der Sinnesnerven. Berlin, 1881. WEISSMAXX. Die Lehre von den spezih'scheu Sinnesenergien. Hamburg and Leipzig, 1895. The following are the most important monographs of the Physiology of the Cutaneous Senses : E. H. WEBEI:. Annotationes anat. et phys. Lipsiae, 1834. E. H. AVEBER. Wagner's Hand \\orterbueh d. Pliysiol. iii. Part 2, p. 481. Brunswick, 1846. MKIXSNKK. Beitr. z. Anat. u. Physiol. der Haut. Leipzig, 1853. 1 i ' UXER. Elemente der Psychophysik. Leipzig, I860. HERING. Sitzungsber. d. AVien. Ak. Ixxv., 1877. FINKE. Hermann's Handbuch d. Physiol. iii. Part 2, p. 289. Leipzig, 1880. BMX. Zeitschrift fiir Biologie, xx., xxi., 1844-85. GOLDSCHEIDER. Archiv fiir Anat. u. Physiol., Suppl. Vol., 1885. GOLDSCHEIDER. Uber den Schmerz. Berlin, 1894. GOLDSCHEIDER. Gcsammelten Abhandlungen. Leipzig, 1898. Vo\- FREY and KIESOW. Zeitschr. f. Psychol. u. Physiol. des Sinnesorg. xx., 1889. NAGEL. Pfliiger's Arch, lix., 1895. Vox FREY. Berichte d. k. sachs. Ges. d. AViss. xxiii., 1896. VON FREY. Abhandlungen derselben Gesellschaft, 1896. KIESOW. Arch. ital. de biol. xxxvi., 1901. KIESOW. AVundt's Philos. Stud, xix., 1902. Kir.sow. Zeitschr. fiir Psychologic, xxxv., 1904. KIESOW. Archiv fiir die gesamte Psj'chologie, x., 1907 ; xviii., 1910; xxii., 1911. ALRUTZ. Skandin. Arch. f. Physiol. xvii., 1905. ALRUTZ. Atti del V Congresso int. di Psicologia a Roma, 1906. THUXBERG. Nagel's Handbuch d. Physiol. iii. 647, 1906. (This is a complete monograph which reviews and quotes the whole literature bearing on this subject. ) M. Poxzo. "Uber die "\Virknng des Stovains auf die Organe des Geschmacks, der Hautempfindungen, des Geruchs und des Gehors etc." Archiv fiir die gesamte Psychologic, iii. and iv., 1909. M. SUGAR. " Thermoanaestesia cutis paradoxa." Orvosi Hetilap, No. 9, 1910. KIESOW and Poxzo. Archiv fiir die ges. Psychologic, xvi., 1910. Poxzo. Archiv f. die ges. Psychologic, xiv. , 1909 ; xvi., 1910. Poxzo. Memorie della R. Ace. delle Scienze di Torino, Series II. lx., 1909 ; Ixi., 1910. Poxzo. Arch. ital. de biol. lx. and Ixi., 1911. Poxzo. Atti della R. Ace. delle Scienze di Torino, Ixvi., 1911. A. Srur.Mi'ELL. Trattato di patologia speciale medica e terapia, ii. Part 2. Malattie del sistema nervoso, j>. ''>. G. LERDA. "Sur 1'evolution de la sensibilite dans les cicatrices, dans les auto- plasties et dans les grelfes." Arch. ital. de biol. xliv. Part 1, 190'). A. FOXTANY. Contribuzione allo studio delle sensilnlita nei condilomi acuminati. Communication to the International Congress of Dermatology and Siphilo- graphy, Rome, April 8-13, 1912. Cutaneous Nervous Apparatus ; see the latest histological memoir : RUFFIXI. Revue generale d'histologie, pub. Renaut et Regaud. Lyons, Paris, 1905. 56 PHYSIOLOGY CHAP, i Recent English Literature : HEAD and RIVERS. A Human Experiment in Nerve Division. Brain, 1908, xxxi. 323. TROTTER and DAVIES. Experimental Studies in the Innervation of the Skin. Journ. of Physiol., 1909, xxxviii. 134. TROTTER and DAVIES. The Peculiarities of Sensibility found in Cutaneous Areas supplied by Regenerating Nerves. Journ. f. Psychol. u. Neurol., 1913, xx. 102. BORING. Cutaneous Sensation after Nerve Division. Quart. Journ. of Experi- ment. Physiol., 1916, x. 1. MruRAT. A Qualitative Analysis of Tickling. Amoric. Journ. of Psycho]., 1909, xx. 386. CHAPTEE II SENSI1UL1TY OF THE INTERNAL OKCANS CONII:NTS. 1. Classification of internal sensations. 2. Common sensation of the body or coenaesthesia. ':>. Pain in tlie internal organs and tissues. 4. Ali- nn'iitarv needs (hunger and thirst). 5. Sexual desire. 6. The muscular sense : s, nubility of muscles, tendons, and joints. 7. Innervation sense in the centres of voluntary movement. 8. Active tactile perceptions and their components. 9. The siil'i-miM'ious sense of muscular tone and its variations in reference to the functions of the labyrinth. Bibliography. ALL internal organs and tissues provided with afferent nerves have a greater or less degree of sensibility. The sensations aroused from the peripheral terminations of these nerves are almost always independent of external stimuli, and depend as a rule upon the somatic conditions inherent in the organism. They are accord- ingly grouped together under the name of Internal or Bodily Sensations. While the specific sensations aroused by the action of the outer world are the basis from which our intellect is developed and perfected, the internal sensations do not normally give any clear indications of our internal world. Nevertheless they are of great importance from the psychological and philosophical point of view, as was well brought out by Cabanis at the beginning of the last century in his famous book Rapports du physique et du moral de I'homme. He showed that, even when they do not pass the threshold of consciousness, the internal sensations may send impressions to the brain which alter our psychical personality. On the other hand we know that they exercise reflexly, along ill-- efferent nerves, a controlling inlluence upon all the functions of the vegetative and animal life. I. The physiological study of the internal sensations of the nrvraiis has progressed very little because their indetinite char- acter usually makes a strict application of experimental methods impossible. Physiologists have been content to hand over the study of this category of phenomena, to clinicians, who have frequent opportunities of investi-nlim;- them in their patients, in whom they are often exaggerated and become more conspicuous, 57 58 PHYSIOLOGY CHAP. or are, on the other hand, suppressed so that the effects of deficiency can be studied. Few physiologists have attempted to classify them according to rational criteria. Magendie seems to have been the first who divided the internal sensations into four groups, on a physiological basis : (a) The first come into play when it is desirable that the organs should function. This group comprises the wants, desires, and instinctive appetites that originate from a too protracted abstinence. Such are hunger, thirst, desire to micturate or to del'aecate, sexual desire, etc. (i) The second appear during the activity of the organs. They are often obscure or quite subconscious sensations; but maybe urgent, as the sensations felt during the excretion of urine or faeces, and especially during ejaculation, which is the culminating point of sexual activity. Highly important among the sensations of this group, and one of the best studied, is that of tension, constriction, and effort felt during muscular activity, by which we judge of the range, speed, direction, and energy of our movements. (c) The third grmip includes the feelings that arise after protracted or energetic action of the organs. Such is the sense of fatigue that succeeds after too pr< (longed or excessive activity of the muscles, drowsiness after long waking, the feeling of exhaustion and languor after sexual indulgence, of satiety after a full meal, etc. (d) The fourth group includes the innumerable internal sensa- tions associated with illness, which range from a vague general sense of discomfort to more or less acute and diffuse pain. In this group we may include the shivering which ushers in attacks of fever, the heaviness and burning in the head which is more or less characteristic of febrile processes, the vertigo often present in attacks of nervous illness, the nausea that precedes vomiting, the so-called " visceral hallucinations," etc. However ingenious this classification may be it is incomplete. It omits two other groups of bodily feelings, which are no less important in their effects although vague and indefinite in character, so that it is doubtful whether they normally cross the threshold of consciousness. These are : (e) The common sensation of well-being or coenaesthesia con- comitant with the state of perfect health, which is expressed in adolescence by a more or less accentuated exuberance of movement. (/) The obscure feeling by which we become aware of the position of our body and its individual parts (head, trunk, limbs) ; and the equally obscure sense of equilibration and orientation of the body in respect of the external world, in so far as these can be independent of the active state of the muscles and the specific external senses. ii SENSIBILITY OF THE INTERNAL ORGANS 59 M;iny of the bodily feelings t hus classified escape physiological analysis owing to their \ague and obscure character. Of others we know little, and (hat little claims no special mention here, either lieeauseit falls within the domain of common observation, or because it comes into the special department of neuropathology and psychiatry. \\'e must here confine ourselves to the more important physiological points that have heen cleared up. II. Henle gave the name of "common sensation" (Gemeingefulil or ooxn-xf/ii'xitf} to "the sum, the confused chaos of the sensations \vhieh are incessantly transmitted to the hrain 1'rom all the parts of the body." Normally we have no clear and distinct conscious- ness of the functions of the internal organs and tissues, hut we undoubtedly have a dull and ohscure knowledge of them, similar to that of the sensations that provoke and accompany the respiratory movements. We have in short an incessant awareness of our hotly, which Condillac termed the "fundamental sense of existence," and which is the Link hetween psychical and physio- logical life. In the state of equilibrium that constitutes perfect health, this feeling is continuous, uniform, and always equal, so that it remains at the threshold of consciousness, and is prevented from becoming a distinct sensation with special characters and specific Localisation. But when it reaches a certain intensity it is perceived as a vague sense of general well-being or the reverse. The former, known to clinicians as euphoria, is the expression of an exaltation of the physiological functions of the organs, tin- latter of their disorder, transmitted to consciousness by the cerebro-spiual sensory nerves, or by the afferent nerves of the sympathetic system. " It is probable," Foster writes, " that sensory impulses, not of t In- character of pain, are continually, or from time to time, passing upwards from the abdominal viscera to the central nervous system. These do not affect our consciousness in such a distinct manner as to enable us to examine them psychologically in the same way that we are able to examine special sensations such as those of sight, or even sensations of pain; they are even less well-defined than those of the muscular sense ; nevertheless they do enter, though obscurely, into our consciousness, so that we become aware of any great change in them." A striking proof of the real existence of common sensation is seen in the fact that in certain morbid cases it may be wholly or partially suppressed. In some forms of mental disturbance, in certain cases of anaesthesia or partial paralysis, the patients have no sensation in one part of the body (>.//. in one limb, the stomach, the brain, etc.), or from some cerebral disease sensation in one part is abnormal r.y. the patient fancies he has a glass or wooden arm. More rarely there is a total abolition of coenaest hesia. It is said that the, obstetrician Baudelocque in the last days of his life lost 60 PHYSIOLOGY CHAP. consciousness of his own body. Probably the same phenomenon takes place in insane subjects who speak of themselves in the third person. All the alterations and perversions of organic sensibility, of which Kibot gave a brilliant analysis in his Maladies de la personnaliU, come into this group of phenomena, since coenaesthesia is the physical basis of individual personality. Can the sensibility of the internal organs be so extended and intensified that the work of the organs of vegetative life, which is normally carried on unconsciously, may reach 'consciousness ? Is the threshold of consciousness at a fixed and constant level, or does it oscillate, and can it under certain extraordinary or abnormal conditions drop so much that its range is correspondingly increased and widened ? This question is as important as it is delicate. Keliable authors who have concerned themselves with hypnosis and other similar states (Beaunis, Liebeault, and others) affirm that many persons have in the hypnotic state a more or less clear sense of the organic changes and of normal or morbid states that occur within the organs of vegetative life. It is well authenticated, according to Beaunis, that during hypnotic sleep and even in the somnambulist waking state, all the functions of vegetative life can be modified by suggestion the pulse rate can be altered, redness and persistent congestion can be produced in certain regions of the skin, cutaneous haemorrhage can lie induced, the menstrual flow can be diminished, increased, or regulated, the different secre- tions (tears, sweat, milk, urine, intestinal juices) can be excited or arrested, uterine contractions similar to those of parturition can be produced, the temperature of the skin raised, and lastly, blisters formed in the skin. These surprising phenomena show that the brain is able under certain conditions to transmit a centrifugal effect even to the organs of vegetative life, and to affect their activities as it does the muscles of animal life, and implies the existence of a centripetal current from these organs to the brain, by which it may receive a more or less distinct sensation of the processes going on in the organs. In order, says Liebeault, to explain the suggestive action of thought on the tissues as a whole during somnambulism, it is necessary to admit that the brain which transmits orders to the glands, blood-vessels, etc., is aware of the sensations that come from them. Apart from his spiritualist convictions, the posthumous work of F. W. Myers on " Human Personality " contains a fund of in- controvertible facts which have not yet been analysed by the physiologist. Some of these observations show that the threshold of consciousness is not fixed and invariable, but may alter con- siderably, spontaneously or artificially, in different states of the nervous system occurring in individuals who are specially pre- disposed or trained by special education. III. From the practical point of view the commonest and ii SKNsmiUTY OK THE CNTERNAL ORGANS (il most important modality l exaltation Mini perversion <>r the sensibility iif tin- internal organs ;unl tissues is cerla.inly repre- sented liy fxii/i, in its various forms MIM! varieties. In tiir last, chapter we examined pain as one ui' the distinct modalities <>!' cutaneous sensibility, having nerves and aerve- endings different I'roni those ol' the other sense-organs of I he skin. But unlike the senses of pressure and of cold and heat, the sense of pain is not specific, hut belongs to I he group of internal senses that give rise to sensations that, a, re incapable of transformation into perceptions. It further differs from the other cutaneous sensibilities in certain important characters: it is not excited by special tti/i'ijiti/f,' stimuli, but can be amused by any kind of stimu- lation (mechanical, thermal, electrical, chemical) that is capable of acting tin the uerve-iibres along their course; it has incompar- ably longer periods of latent- excitation and after-excitation ; the pain impulses have a greater capacity of summation so that the sensation is rendered continuous ; and lastly, they have a greater tendency to spread in every direct ion and have no precise local signs, excepting the cutaneous pain spots which according to Ponzo's recent work can be localised as exactly as the touch spots. The sensitiveness of the skin to pain is only a more evolved and perfected form of the common sensibility proper to all internal tissues that possess afferent nerves. This theory is by no means new. The earliest physiologists distinguished pain, either cutaneous or of the internal tissues and organs, from the specific- sensations, and referred it to the group of crude sensations- hunger, thirst, nausea, fatigue, etc. But, since on the ground of v. Frey's work, the existence of special nerves and nerve-endings for pain, constituting one of the cutaneous senses, is now admitted, it must be asked whether these are capable only of reacting by pain, sensations to every kind of stimulus, or whether (like the afferent nerves of the internal organs and tissues) they can also react by obscure and non-painful sensations, i.e. can they transmit subconscious sensations to the centres on normal weak stimulation, and sensations of greater or less pain, which is more or less conscious, on abnormal excessive stimulation ? All the evidence is in favour of this last supposition. We have seen that pain is not a primordial form of sensibility, but that, in the animal series, it develops along with the develop- ment of intelligence, and is psychically superposed on the protective subconscious reflexes, the better to protect the individual from the injurious action of the outer world. As Foster pointed out, " It may happen to a man to suffer pain in a particular region or tissue of the body once only in the course of his life -time, or possibly not even once ; nay, we may suppose that in this or that region or tissue pain is felt once 'only in one individual among a large number of persons.' 1 In such a case, if there really 62 PHYSIOLOGY CHAP. were special organs destined exclusively for sensations of pain, we should be " driven to conclude that such ... a mechanism of pain has been preserved intact but unused through whole generations in order that it may once in a while come into use, which is in the highest degree improbable. This difficulty disappears if we suppose that the constantly smouldering embers of common sensibility may be at any moment fanned into the flame of pain." So that, if we assume with v. Frey that there are numerous pain spots in the skin with corresponding nerves and end-organs, this does not mean that there is a specific apparatus exclusively intended to serve pain sensibility ; it is the same that subserves the common sensibility of the internal organs and tissues, and normally transmits subconscious excitations only. Granting this to be reasonable, it does not therefore exempt us from examining whether the afferent nerves of the internal organs have normally, like those of the external tegument, the capacity for arousing pain, when artificially stimulated with excessively strong stimuli. This question is very important from a practical point of view. The physiologists of the seventeenth and eighteenth centuries were much occupied in testing the sensibility of the internal parts to pain in animals, and many important surgical observations were also made on man previous to the introduction of ether and chloroform narcosis. The introduction in recent times of the method of local anaesthesia, specially by cocaine for major operations, has opened a new era in the study of this subject, making it possible to test the sensibility of the different tissues. But the results are at present contradictory or uncertain. The general results obtained from the whole of these observa- tions, new and old, may be summed up as follows : (a) Only the tissues provided with nerves are sensitive to pain stimuli : the epidermis, the horny tissues in general, the cartilages and fibro-cartilages are totally insensitive, because they have no nerves. (&) The organs, tissues, and internal membranes innervated by the sensory roots of the nerves of the cerebro-spinal axis are more or less sensitive to painful stimulation. (c) The organs and internal tissues innervated exclusively by the nerve-fibres of the sympathetic system are little sensitive to pain stimuli under normal anatomical and functional conditions, but in a state of inflammation they may acquire an exquisite sensibility to pain. There are no exceptions nor comments for the first proposition ; the second and third, on the contrary, must be examined. The connective tissues, ligaments, tendons, and aponeuroses have, under normal conditions, an indefinite sensibility to pain. The periosteum is very painful, as shown on scraping the bones in certain surgical operations ; but bone itself, particularly the n SENSH'.ILITV OK TIIK 1NTKKXAI, oitGANS compact substance, is insensitive, as proved in anipiilat ions wi chloroform. The pain sensibility of bone-marrow under phy.-i"- lo-ical ronditions is doubtful. The muscles in the normal stale are bill little sensiti\c to pain. During amputations without, anaesthetics they gi\e no pain. Strong compression gives rise to a specific dull pain ; intense faradisation is very painful. This sensitiveness to pain is not due to excitation of the cutaneous nerves, because Duchenne ol >served it with direct electrical stimulation of the pectoral is major muscle exposed during excision of the breast. The feeling of muscular fatigue presents every gradation from a simple sense of heaviness to acute pain, which may last 24-48 hours, and is accentuated on the slightest pressure. But in this case the state of the muscle is evidently altered, owing probably to the accumulation of fatigue products, which act as an irritant poison. Similar abnormal conditions underlie the muscular and articular pains of a rheumatic and gouty character. On the other hand, the sharp pain that accompanies the cramp caused by violent and involuntary contracture of the muscles is transitory. It has been attributed to the compression of the cutaneous sensory nerves that traverse the muscles, but this is a fallacy, because in that case, in accordance with the law of peripheral projection, the pain would be perceived in the skin and not in the contractured muscle. Serous membranes in general, as the peritoneum, pleura, cerebral and spinal dura mater, and the synovium, are believed to be sensitive to pain even under normal conditions, and when inflamed become much more so. The pain sensibility of the mucous membrane of the digestive tract is generally very acute near its junction with the skin (oral and pharyngeal cavities), but it diminishes in the oesophagus. The painful sensation of choking produced when an alimentary bolus that is too large or too hard sticks near the cardiac aperture of the stomach is not due solely to the sensibility of the mucous membrane, but rather to the cramp that compresses the nerve fibres that surround the canal. The pain sensibility of the stomach is moderately acute, that of the intestine low, but it increases again in the rectum and at the anal orifice. Puncture, section, cauterisation (as shown by experiments on rabbits and dogs, and surgical operations in man), do not produce true sensations of pain in any part of the intestinal canal under normal conditions. But in a pathological state, the intestine may become the seat of severe pains, such as those of colic. The mucous membrane of the respiratory apparatus is sensitive to pain in the nasal and laryugeal tracts, but insensitive through- out the bronchial ramifications. The mucous membrane of the ureto-genital system is very 64 PHYSIOLOGY < HAP. sensitive along the urethra.! canal, particularly in the prostatic or membranous part ; that of the bladder, ou the contrary, has little sensibility. Even large calculi may remain imperceived for some time until inflammation sets in. The vulva is sensitive, but the vagina, cervix of the uterus, and the uterus itself are only moderately sensitive. As long as they are normal they can be cut or cauterised without producing pain. Tain in these parts undoubtedly depends mi compression, or traction of the sensory nerves that lie in the depths of the tissue, or in the uterine appendages and the vaginal canal. The excretory ducts of the glands are usually very sensitive to distension. The intense pain of hepatic and nephritic colic is well known. The heart, arteries, and veins are insensitive to pain in the normal state. The same may lie said of the hepatic parenchyma, spleen, pancreas, kidneys, and lymphatic glands. The genital glands, the testicles, the ovaries and their appendages are, on the contrary, highly sensitive. ('(impression of these parts causes acute pain, and may even induce syncope. From all these facts it is clear that the internal tissues and organs have as a rule a lower sensibility to pain than the surface of the body; and that the deep organs innervated by the sympathetic normally feel little pain, but they have a very high latent pain sensibility which may become apparent under abnormal conditions, particularly in inflammation. Lennander (1902-4), on the basis of a new series of clinical observations, opposed this hypothesis, and maintained that the difference of sensibility shown by the internal tissues, according as they are in normal or pathological states, is to a large extent apparent only. According to his observations, the pains that can be produced in the abdominal cavity must be referred to the parts innervated by the lumbar and sacral nerves, particularly those to the parietal peritoneum. This is sensitive under both normal and abnormal conditions, especially to mechanical stimuli (traction, dilatation) ; while the whole of the intraperitoueal viscera and the visceral peritoneum which covers them are, on the contrary, incapable of initiating pain either in the normal or the pathological state. When these viscera are diseased, the pains do not indicate exaggeration of their normal obscure sensibility ; they remain insensitive, but transmit the irritation to the sensitive parietal peritoneum, either by an exaggerated peristalsis, or by meteorism or abnormal distension of the intestinal canal, by the traction due to inflammatory adhesions, or lastly by the produc- tion of toxiues or irritative chemical products. The hyperalgesia of the parietal peritoneum caused by these products fully explains the fact that in acute abdominal diseases the weakest stimuli may provoke very intense pain. ii SKXSIBILITY OF T1FK ENTEENAL OIMIAXS (if, According to Lennander, the same holds for the thoraeie ; IM d cranial cavities. The lungs and visceral plc-ura are insensitive, ;ind the pains felt in tin- ehest in certain illnesses are caused by the transmission of excitations to the parietal pleura, which is sensitive under normal conditions also. The hra in is insensitive, and the pains in the head so frequently felt are due to transmission of excitation to the dura mater. Generally speaking, Lennander holds it probable that all the organs innervated by the sympathetic alone, and by the branches of the vagus after the separation of the recurrent nerve, are insensitive to pain, not merely in the healthy but also in the inflammatory state. This statement does not seem to be justifiable, at any rate not in such a general form. How can we deny the sensitiveness to pain of the bile duct, the ureter, and the intestinal tract in cases of gall stones, of hernia, and of other forms of obstruction of the digestive canal ? On the other hand, the work which Ducceschi carried out in our laboratory " On the nerves of the stomach " (1905) shows clearly that mechanical, thermal, and electrical stimuli applied to the outer surface of the stomach of normal dogs and cats cause obviously painful reactions (general agitation, disturbed respiration, cries, characteristic movements of the tail, similar to those made by the cat when it is hurt by stimulation of the cutaneous sensory nerves). The reactions are seen even after section of both vagi, or both splanchnics ; they only cease when both have been cut. " It is interesting," writes Ducceschi, " to note that the stomach in certain cases seems to become more sensitive in proportion to the time that has elapsed since its exposure. Simultaneously with the increase of sensibility in the stomach, cutaneous sensibility declines. At the end of the experi- ment, after about two hours, a slight tap on the wall of the stomach causes strong general reactions, while pinching the ear, paw, or the skin of the abdomen does not cause even ,the slightest reaction. There is evidently shock of the peripheral sensory apparatus, accompanied by gastric hyperaesthesia." From Lennaiider's latest communications it appears probable that the mucous membrane not Only of the rectum, vagina, and uterus, but also of the ovary, oviduct, and ligameuta lata are insensitive to pain. All these parts can, he says, be operated on without pain to the patient, provided there is no traction of the connective tissue by which they are united to the walls of the pelvis and the parietal peritoneum. Probably the testicles and epididymis too contain no nerves of pain, though the parietal fold of the tunica vaginalis is highly sensitive. We must reserve our opinion on these theories also. In opposition to and parallel with the clinical observations of Lennander, the clinical theory of referral/ /nn'n has recently VOL. IV F 66 PHYSIOLOGY CHAT. assumed great physiological importance. Laiujv was the tirst who considered the pains and cutaneous hyperaesthesia that accompany certain diseases of the internal organs that are little sensitive, or insensitive, to be reflex. Since the work of Eoss (1888), of Mackenzie (1892), and specially the more detailed Km. 25. Diagram of zones and areas of hyperalgesia, after the clinical researches of Head. Explanation on p. 68. observations of Head (1898), the theory of referred pain has acquired great interest, and in view of the modern segmental theory is of marked theoretical importance. According to Head, certain morbid conditions of the internal organs are capable of provoking painful sensations, but these are almost always falsely located, i.e. in a part other than that affected. In the diseased organ, too, abnormal sensations II SENSIBILITY OF THK INTERNAL ORGANS are often It-It, but these are m>i so much true pain as ;m obscure feeling of lit-avincss or strain, while true, sharp, stabl)ing pain is projected to the surface of the body. According to Head, this false localisation is an effect of the low sensibility to pain of the internal organs and tissues, and of the connection between these l-'i';. 2i;.l Marram of zones and areas of hypi-i-d-i'M.-!, after the clinical researches Explanation on p. 68. and the nerve-centres of the much more sensitive external tissues. Head's law of the localisation of pain runs as follows : " When a painful stimulus is applied to a part of low sensi- bility in close central connection with a part of much greater sensibility, the pain produced is felt in the part of higher sensi- bility rather than in the part of lower sensibility to which the stimulus was actually applied." * 1 Brain, 1893, vol. xvi. p. 127. 68 PHYSIOLOGY CHAP. The internal organs air, generally speaking, less sensitive than the skin ; their afferent nerves are, according to Head, in close relation with the centres of the cutaneous sensory nerves of the same spinal segment. The same theory applies, according to Head, to the cutaneous hyperalgesias observed in visceral affections. When abnormal excitations from a diseased internal organ reach the cord by way of the afferent nerves the excitability of the spinal segment becomes exaggerated, so that when another cutaneous excitation of low intensity reaches the same segment it provokes pain, whereas under normal conditions it arouses merely a sensation of contact. These views \vere strengthened by Head's mapping out of the hyperalgesic zones that are observed in different affections of the viscera. Each diseased organ produces hyperalgesic zones which are characteristic in form and localisation. According to Head the zones of herpes zoster coincide with those present in hyper- algesia. Head's hyperalgesic zone corresponds not with the areas to which the cutaneous nerves are peripherally distributed, but with those supplied by the dorsal roots. As shown in Figs. 25 and 26, these do not overlap, while the cutaneous metameres or dermatomes of the dorsal roots, on the contrary, according to Sherriugton, do overlap to a large extent. But Sherrington's later work showed the overlapping to be different for different qualities of sensation ; it is much more extensive for tactile sensa- tion, much narrower for pain sensation. We saw (Vol. III. p. 306) that the work of Winkler and van Eynberk on the central area of dermatomes has thrown new light on this subject. And it is probable that Head's hyperalgesic zones represent segmental zones of Sherrington's pain areas, or of the central areas of the dermatomes of Winkler and van Rynberk. Head's clinical investigations have such great practical importance that it is desirable to reproduce the following diagram and table, which sum up his results. Figs. 25 and 26 show the segmental cutaneous areas of the trunk, extremities, and head. The form and extent of these were arrived at : (a) by mapping out the areas in a number of cases of cutaneous hyper- aesthesia with coincident visceral affections ; (b) from the topography of the eruptions in 52 cases of herpes zoster ; (c) by mapping out the analgesic areas in organic diseases of the spinal cord and roots. The 8 cervical segments are indicated by Cl, 02... C8; the 12 dorsal or thoracic segments by Dl, D2...D12 ; the 5 lumbar segments by Ll, L2...L5 ; and the 4 sacral segments by Sac. 1, Sac. 2... Sac. 4. The areas of the head are indicated as follows : N = nasal or rostral area ; FN = fronto-nasal area ; MO = medio-orbital area ; FT = fronto-temporal area ; T = temporal area ; V = vertical area ; P = parietal area ; O = occipital area ; = naso-labial area; Max. maxillary area; Man. = mandibular area; II SENSIBILITY OF THE INTERNAL OKGANS 60 M Miriu.i] nva ; LS = superior laryngea-] ,-uva ; LT = inferior laryngeal area; T0= hyoi'l aiva. 'I'll.' t'nl lo\\ ill!,' lalik' slmws tllr ivlatimis lictwccll lilt- rillaiiron- :IM-;I- ,ind tin- iutrrinl ii tin- Trunk and l.imlis. Eearl . 03, 04 - D-J I >s . Lungs . Stnm;lrli I nil >tinr Rectum Liver . 03, C4-D4 !)! . . . . . . . . D7 - D9 . . . . .... D9-D12 . . . .... Sac. 2 - Sac. 4 . . . 03, 04 - D7 - D10 . . . G.-ill Madder . . D8 - 1)9 . . . . Kidney and urethra Dll-Ll . . . . Bladder (murous membrane and inrk) Sac. 3 - Sac. 4 . . Detrusor vesicae . D11-L2 Prostate . . DlO - D12 Sac. 1 Sac. 3. Kpididymis . Dll - D12 Testicle . . DlO Ovary . . . DlO Ovarian appendix Dll - Ll . . . . Uterus . . DlO -LI Neck of uterus .... Sac. 2 Sac. 4. Mammae . . . . D4 - D5 . . . Spleen (from Signorelli) D6 . . . . Area in the Head. I Ventricles and aorta, N, FN, MO. I-T. I Auricle's .... FT, T, V, I' . . . . N, FN, MO, FT, T, V, P. . . . . FN, MO, T, V, I' V, \',0 . . . . FN, MO, T, V, 1', ... T, V . . O O IV. Of internal sensations summed up under the generic name of " desires," that for food is certainly one of the most im- portant from the teleological point of view, because it is directed to satisfy ino- unc of the conditions indispensable to life the supply of nourishment. In its milder stages this desire is not unpleasant and is even an agreeable feeling, commonly known as "appetite"; \vlieii more insistent it becomes painful and oppressive and is known as "hunger." In most of the higher animals and man appetite and hunger are rhythmical sensations, which do not occur until a certain time after the meal, according to the habits of the individual. In man they are generally felt 5-6 hours after the morning meal, 12 hours after the evening meal. "Regularity of meals," said Beaunis, "causes the sensations of hunger to recur with the precision of clockwork." Change of habit in the hours of meals is able to modify the rhythm of hunger: if the meal is delayed L-2 hours the appearance of hunger is delayed by a corresponding t ime. The degree of hunger varies conspicuously in different in- dividuals, and in relation to age, to the rate of metabolism in different constitutions, in different seasons, different professions, and so on. Generally speaking, hunger is an unpleasant sensation at the level of the epigastric region, which disappears and is replaced by 70 PHYSIOLOGY CHAP. a pleasant sensation as the stomach becomes filled by food. But if the satisfaction of the want is delayed the unpleasant sensation increases, and spreads from the epigastrium to the surrounding parts, until sensations of constriction, cramp, and strain are produced throughout the abdominal walls, and located specially in the stomach, oesophagus, pharynx, floor of the mouth, soft palate, parotid region, masticatory muscles, temples, and epicranial apoueurosis, where it assumes the diffuse form of dull headache. In other words, hunger is a complex sensation in which all the organs that function during alimentation and digestion participate more or less distinctly. The discomfort in the epigastric region arising from the cardiac orifice or from the whole stomach is the fundamental feeling in hunger, to which are subsequently added the accessory feelings that come from other parts of the digestive system as a whole and the associated organs and tissues. All conditions that stimulate general metabolism and increase the loss from the organism such as muscular exercise, coLl air, mountain climate, sra air, convalescence after fever, or the early stages of growth accentuate the sensations of hunger, which under these conditions becomes proportional to the need of compensation, restoration of the loss, and development of the tissues. All conditions that retard metabolism and diminish loss produce the opposite effect : such are the summer season, sedentary habits, complete muscular inactivity, old age, narcotics (opium, tobacco, cocaine, alcohol). Under abnormal conditions hunger may reach a morbid level, clinically known as luliiiiln. In this case hunger sets in again 1-2 hours after a meal, and if not satisfied may rapidly produce intense pains in the stomach, dimness of vision, agitation, delirium, fainting. All these symptoms subside after a full meal. There are also morbid perversions of the sense of hunger, in which appetite for things that are not eatable and are even dis- gusting, such as earth, clay, ash, coal, straw, hair, and excrement, may be developed. In most acute febrile diseases, although there is tissue-waste and progressive emaciation, there is also loss of appetite (anorexia?), and hunger may be replaced by repugnance to food of even the most delicate and tempting kinds, which is due not to abolition but to perversion of the sense of taste. In smokers the acquired need to smoke disappears along with the sense of hunger. Anorexia is not uncommon in hysterical patients, and in predisposed subjects it may be suggested during hypnosis. In the insane sitiophobia (repulsion to food) is frequent, but is then due to delusions and not to absence of the sensation of hunger. Even in sane people an intense moral emotion, e.g. bad news, drives away the feeling of hunger. If the attention is keenly ii SENSIBILITY OF THE INTERNAL ORGANS 71 attracted by an interesting book or intellectual preoccupation with some important problem, the sense of hunger disappears, and the hour of the meal may be forgotten. The intensity of hunger is not generally proportionate to its duration. It is important to distinguish between the hunger that accompanies forced inanition and that of voluntary fasting. In forced inanition hunger is present from the first in abnormal intensity, and is complicated later on by a peculiar delirium (hunger or starvation delirium) which in recorded cases of ship- wrecks assumed a terrible form of acute mania. In voluntary fasts, on the contrary, perhaps from auto-suggestion, the sensation of hunger may be tolerable in the first two clays of abstinence, and may decrease and entirely disappear after that. Succi, in one of his many fasts of thirty days which we investigated (Florence, 1889), required a narcotic to allay his hunger only in the first two days ; in the remaining twenty-eight he only ingested mineral waters, and showed no sign of suffering. The lawyer, Antonio Viterbi, to avoid the disgrace of execution, resolved to kill himself by starvation: he kept a diary of his fast, and wrote in the last seventeen days, during which he neither ate nor drank, that hunger only lasted one day, reappeared for one short hour on the fifth day, and then disappeared entirely. Thirst, on the contrary, was painful up to two days before death, when it also disappeared. On the eve of his death he wrote the following words : " I reach the term of my existence with the serenity of a just man. Hunger no longer torments me ; thirst has entirely ceased ; stomach and intestines are quiet ; my head is untroubled, my sight clear. The few remaining moments are flowing gently by like the current of a little stream in a delicious meadow. The lamp is going out for lack of oil." Thirst, too, is a complicated feeling, located in the first instance at the back of the mouth, whence it spreads and becomes general in proportion as it grows in intensity. It is a sensation of scorch- ing, dryness, and constriction of the throat which spreads over the whole buccal cavity, and is specially associated with a general hyperexcitability, with tachypnea and tachycardia as in fever, hot and fetid breath, and dry, burning skin. At its extreme thirst is more painful than hunger ; the craving and anguish the fate of Tantalus, which is the most appalling the human organism can endure may induce delirium, which soon brings deatli in its train. Thirst increases more rapidly than hunger with the duration of the fast, and becomes even more intense. But here, again, \\e must distinguish between forced and voluntary abstinence. As we said above, in Viterbi 's case thirst was painful and lasted much longer than hunger, but il , too, decreased, ami finally dis- appeared in the last two days of life. 72 PHYSIOLOGY CHAP. All causes that reduce the high percentage of water in the composition of the body are able to produce the sensation of thirst. The heat of the atmosphere which increases cutaneous and pulmonary perspiration, and muscular exercise which excites secretion of sweat, accentuate thirst. Hydropic effusions, diarrhoea, diabetic polyuria, haemorrhage, etc. promote the desire to drink and produce poly dips i. Ingestion of highly spiced or salt ^ I foods develops the sensation of thirst by subtracting water from the circulating tissue fluids. Adipsia or suppression of the sense of thirst is very rare. It is seen in certain serious fevers, and is a fatal symptom, presaging the final exhaustion of the nervous system. The physiological researches directed towards clearing up the "i-iuin of hunger and thirst have not led to aim very satisfactory results. It is a priori evident that the fundamental internal condition of these sensations must consist in the impoverishment of the circulating fluids by loss of water, which produces a corre- sponding impoverishment of the tissues. This can be shown experimentally. If artificially prepared nutrient .substances are introduced into the veins of a. fasting dog, it is possible, according to Schiff, not only to assuage hunger but also to nourish the animal. By means also of intravenous or intraperitoneal trans- fusion of den'brinated blood, hunger can be relieved in dogs, but the starvation deficit cannot be arn-stf-d (Luciani and Bufalini, 1882). In certain clinical eases in which ingestion of food by the stomach becomes impossible the pangs of hunger may be relieved by nutrient enemata. As regards thirst, Dupuytren caused dogs to run in the sun and then relieved their thirst by intravenous injections of slightly saline water. Schiff repeated this experiment successfully. But how is it, since they are determined by a general craving of the whole of the tissues of the body, that the sensations of hunger and thirst are localised in the h'rst place to definite regions of the digestive system? Are these sensations central or peripheral in origin ? Various physiological theories have been propounded in reply to these questions, all of which appear to us to be insufficient or erroneous. Let us see if it is not possible, on the basis of the facts above discussed, to construct a new theory of hunger and thirst better calculated to satisfy the requirements of scientific criticism. It is undeniable that hunger and thirst are at the outset true local sensations, and that it is only as they become intensified that they spread and assume the complex characters of general sensa- tions. This fact in no way contradicts the preceding observation that the fundamental quality of hunger and thirst, on which their teleological value as " desires " depends, is more or less ii SKNSI1ULITY OK THE INTERNAL ORGANS 73 diffused over the whole of the living tissue elements. In fact, it is conceivable that the sensory nerves of the- upper part, of the, digestive apparatus are peculiarly sensitive to the general effects of deprivation of food and drink in comparison with all other nerves of common sensibility. They are, so to speak, the advanced guard which transmits to the centres a warning of defective nutriment in the tissues by arousing the characteristic sensations of alimentary desires. An analogous fact may be observed in the cutaneous sensory nerves in regard to pain ; in these the lirninal stimulus that causes pain is normally much lower than that of the sensory nerves of the internal organs. They are the sentinels whose duty it is to defend the entire organism against injurious external agents (mechanical, thermal, and chemical), and to arouse appropriate protective reflexes. Hunger is therefore specially localised in the stomach for the simple reason that the sensory nerves to the mucous membrane of the latter are the most excitable to deprivation of food. Thirst is specially localised in the pharyngeal and buccal mucous membrane because the sensory nerves to these parts are peculiarly sensitive to lack of water in the circulating fluids of the body. What condition of the stomach constitutes the peripheral stimulus of the sensation of hunger ? It is not the state in which the stomach is empty, because all observations made on patients with a gastric fistula, beginning with the famous Canadian subject studied by Beaumont, show that hunger sets in some time after the stomach has been entirely emptied. Nor does the stimulus consist in exaggerated movements of the stomach, for these are much more active during gastric digestion, and cease almost entirely after the stomach has been emptied. Nor can it consist in excess of hydrochloric acid in the stomach, since it is well known that the contents of an empty stomach are slightly acid, or neutral, or sometimes alkaline. The most acceptable hypothesis is that of Beaumont, who attributes the sense of hunger to turgor of the gastric mucous menibrane, which increases after the stomach has been emptied, and is due, as Heidenhain showed, to the increased volume of the chief cells of the gastric glands (see Vol. II. Figs. 40, 41, pp. 120, 123). It is possible that the gastric turgescence excites the peripheral endings of the sensory nerves to the mucous menibrane ; but it seems to us more probable that the excitation depends on the chemical changes in the epithelial protoplasm. On our theory it is easy to account for the fact that the sensations of hunger only last for a couple of days in a prolonged voluntary fast, as was observed on Succi. In fact it is natural to suppose, that inanition, which attacks all the tissues, gradually reduces the turgor of the mucous membrane by diminishing the protoplasm of the epithelial cells that act as a 1 stimulus to hunger. 74 PHYSIOLOGY CHAP. So, too, it may be held that the peripheral stimulus for thirst consists in the dryiiess of the mucous memhraue of the mouth and pharynx, which causes physico-chemical changes in the epithelia, which again excite the terminations of the corresponding sensory nerves. By what paths arc the peripheral excitations of hunger trans- mitted to the centres ? It has been shown in numerous experi- ments on fasting animals by Sedillot, Schiff, Longet, and Beaunis that the sensation of hunger persists after section of the vagi in the neck and also below the diaphragm. Brachet (1834), how- ever, on starving a dog for 24 hours saw that section of the vagi, performed after ascertaining that the animal was ready to devour the food presented to it, ipso facto arrested the desire to eat. But he took no account of the depressing effects of pain, and did not note how long the inhibition lasted, nor when hunger set in again. We have recently attempted to repeat Brachet's experiment under more favourable conditions, si nee it is so far as we know- unique hi the whole literature of physiology. Two young dogs, each weighing 4500 grins., were kept fasting for 24 hours. We then, under chloroform, exposed and dissected out both vagi at the root of the neck, and passed an aseptic thread round them, so that the nerves could easily be drawn out and divided; the edges of the wound were then sewn l oget her. While waiting for the effects of the chloroform to wear off, and to increase hunger, the two dogs operated on were kept in a cage with a trough containing wain- only. After 48 hours' starvation for the one animal and 7'2 hours for the other, both vagi were cut, under cocaine, to avoid any pain. Previous to this operation both dogs were very hungry. When shown a bit of meat they eagerly tried to seize it, and snatch it from one's hand. Immediately after the nerves had been divided they ran about the room as vigorously as before ; but when meat was offered them, they rejected it, after sniffing and licking it. This condition of absolute loss of appetite began to pass off in the first dog (2 days' starvation) after 40 minutes, in the second (3 days' starvation) after 2 hours. On repeating the test in the succeeding hours, the appetite of both dogs was found to be increasing gradually, until it reached the stage of acute hunger, to judge from the avidity with winch the animals devoured meat and bones. These experiments, which complete the too long neglected work of Brachet, seem by their simplicity to be of no little value to the theory of the genesis of hunger. They show, not (as Brachet thought) that the sensory branches of the vagus are the only means of transmitting the excitations of hunger to the centres, but that they undoubtedly represent the most excitable paths for these impulses. They further prove indirectly that the ii SKXSir.ILITY OF THE INTERNAL O KUANS afferent lilircs of the sympathetic arc It'ss excitable in impulses. and only become active some time after the vn/_;i have been cut. or when on prolonged fasting hunger becomes mon- acute. The centres for hunger and thirst are certainly, even if not exclusively, localised iu the bulb and pons. This is proved by anencephalous human monsters, which, though they have no ffrebrum or cerebellum, utter cries a few hours after birth, make restless movements like normal new -born infants, and like the latter are only stilled when their mouth finds the nipple, which they suck with the same avidity. The renowned "brainless dog" of (Joltz also appeared to have sensations of hunger and thirst. At the usual hours for meals its movements were accelerated; it uttered impatient cries, raised itself and put its fore-paws on the bars of the cage. When a dish of milk and big pieces of meat were brought near its nose it lapped and chewed and swallowed with evident satisfaction, like a normal dog. Schii'f opposed to the theory of local peripheral origin of hunger and thirst the theory of their central origin. Starting from the fact that abstinence from food and drink alters the con- stitution of the blood, he held that this must directly excite the nervous centres. Local sensations of hunger and thirst are, he says, illusory effects of the state of the centres, according to the general law of the peripheral projection of sensation. Just as the patient has sensations of the amputated limb, so the hungering and thirsting subject feels in the stomach and throat the sensa- tions which really arise centrally. This hypothesis is fairly met by the fact that hunger can be diminished even by the introduction of non-nutritive matters into the stomach. In times of famine stones, chalk, and indigestible vegetable remains are often eaten. Thirst can be temporarily relieved e.g. in cases of atresia of the oesophagus, by taking a little water into the mouth. Do not these facts prove that such sensations have a local peripheral origin ? Even more than these exceptional facts, which are difficult of control, we have at hand, within the reach of every one to verify, a valid argument against Schiff's theory viz. that the sensation of hunger disappears rapidly on introducing food into the stomach long before it has been digested and absorbed, and therefore before the alteration in the blood, which Schii'f held to be the direct stimulus to the centres, can have been corrected. The same may be said of the i ni reduction of beverages and the sensation of thirst, the more so since we know how difficult and slow a process is the absorption of water in the stomach. To give any experimental basis to Schiff's theory, it would be necessary to prove that the nerve-centres were more excitable to stimuli than the peripheral nerve-endings. Any such attempt, 76 PHYSIOLOGY CHAP. however, is superfluous, seeing that the exact opposite is upheld by every physiologist ; the centres, that is certain parts or nuclei of grey matter, are totally inexcitable to direct stimuli, and have in other parts (as the so-called excitable area of the cerebral cortex) and other nuclei of grey matter, like the nerves along their course, a much higher liminal excitability than that of the peripheral endings of the afferent nerves. The theory of the central origin of hunger and thirst has thus no advantage over that of its local or peripheral origin, and has no such physiological foundation as would force us to regard it as a necessary complement or integration of the general theory of these sensations. One objection that seems serious at first sight might be made to the theory of the local origin of hunger. Patients who have successfully undergone almost total extirpation of the stomach do not lose the capacity for feeling hunger ; in fact they crave for nutriment in the shape of milk or other foods, preferably liquid, more frequently than normal individuals. This objection, how- ever, disappears when it is remembered that in this operation it is always necessary to leave a greater or less portion of the cardiac region, which probably contains the most sensitive part of the gastric mucous membrane ; and that in any case the sensory nerves of the stomach, while normally the most excitable to the stimulus of hunger, are not the only nerves capable of transmitting this impulse to the medulla oblongata. The afferent nerves of the intestinal tract are also capable of the same function, and become active when hunger is intense. Obviously they can convey to the centres the craving for food after an operation of gastrectomy. V. Just as the alimentary wants are ideologically co-ordinated with the preservation of the individual, so sexual desire is corre- lated with the preservation of the species. This desire is felt vaguely and indefinitely from early childhood ; it acquires increas- ingly definite and localised characteristics ; finally it becomes imperative when the genital organs suddenly arrive at maturity, that is at the epoch of puberty. The whole organism then under- goes a crisis ; the genital organs become the starting-point of new sensations, till then unknown, which more or less involve the whole nervous system, and are signalised by a pronounced altera- tion of the intellect, feelings, character, and tastes. Both in the male and in the female the commencement of sexual maturity is marked by a complex of organic and physio- logical characteristics in addition to the full development of the genital organs, such as the development of the larynx and change of voice (which becomes deeper, more sonorous, and expressional), the growth of the beard and other hairy appendages, the develop- ment of the breasts, appearance of menstruation, etc. ii SKXSir.ILlTY OK TIIK 1NTKKNAL ORGANS 77 hi must animals, oilier than man, the sexual desire a with puberty, and is only felt at crrLain seasons, the periods of "heat" or "rut" In men, on the contrary, and in the higher apes sexual desire is present at all seasons, from puberty 1 old age; in women it lasts till the climacteric, when the ovaries cease to t'unetioii, except in certain eases of retarded sexuality. In animals the female, after fertilisation, obstinately refuses to consort with the male; in the. human race and the higher ap< the female has no repugnance to sexual intercourse, even after impregnation. This distinction is not, however, absolute. In the domestic animals, in which the two sexes are continually in contact, the periods of sexual excitement- are more t'reipient, and there is, particularly in the male, a tendency to persistence of sexual desire, as in the higher apes and in man. On the other hand, close observation of the human species reveals a periodicity in erotic desire, particularly in women. The most interesting manifestations of sexual appetite in the higher animals are the. struggle of the male to possess the female, and the persistent courting of females in the period of heat to induce them to satisfy the male desire. The male is always the more active ; the female is passive, and at first repellent, and only gives way later, when the sexual want is well developed in her too, and the ovule is maturated. According to Darwin, all the gestures and expressive play of affection by which the male seeks to ingratiate himself with the female are directed by sexual desire ; but it may be held with Beaunis that they rather aim at increasing the sex impulse in the female, and accelerating the ripening of the ovule, since the love-drama may be observed even in the absence of rivals. Sex desire is the most powerful motive of human life. Differ- ences of individual temperament, of climate, of social surroundings, of moral and religious education give a different character to the manifestations of this appetite. The crude, brutal desire is nearly always mingled in man with a psychical element, which may attain the noblest heights of love, based not merely on physical attractions but also upon moral and intellectual worth. But if love purifies and ennobles the erotic impulse, it does not calm it, but increases its vigour and intensity by the introduction of psychical factors. When pushed to a morbid degree, sexual desire may assume the form of erotomania or nympho mania. The perversions of 363 instinct in different forms and degrees, and the still more frequent cases of sexual inversion, belong to psychiatry and forensic medicine. Here we must confine ourselves to considering the sex want or instinct from an exclusively physiological point of view, and must first determine its origin, that is the internal and external 78 PHYSIOLOGY CHAP. causes of the excitation which, it' transmitted to ihr centres, produces the consensus of pleasant and voluptuous sensations that finally lead to completion of the sexual act. The sex impulse is essentially connected with the presence of the male and female germinal elements, the spermato/oon and the ovule. This is the fundamental fact by which the indispensable internal conditions of sexual impulses are determined. Evidence for this is afforded by castration, which as a rule abolishes or checks sexual desire. To this rule there are undeniable excep- tions: the Mussulmans accordingly insist that the guardians of their harems shall undergo amputation of the penis as well as the testicles. The exceptional occurrence of erotic erection in castrated persons is probably due to the castration having been performed not in infancy but in advanced childhood or adolescence. Another interesting fact may be observed in eunuchs. Alt hough they lose the reproductive desire properly so-called, the voluptuous sensations of sexual affection are not wholly abolished, viz. such as are furnished by sight, hearing, the tactile and muscular sense, and the olfactory sense. Owing to these impulses they become enamoured of their charges, and are the more strict as gaolers in proportion as their passions are involved. We have thus sutlicient evidence that the internal conditions of the erotic excitation which arouses sex desire consist in the development and accumulation of the germinal elements in both sexes, but that the internal excitation is constantly associated with external stimulation from the peripheral organs of the special senses, which may persist even after castration. Animals exhibit practically the same phenomena. Experi- ments have been made un them to determine the relative import- ance of the respective senses in regard to sexual desire, and the results are to a large extent applicable to man also. In the first place there is the work of Lazzaro Spallanzani, who made a great number of experiments on reproduction, particularly on toads and frogs. He observed that during copula- tion these animals may be pricked, wounded, and mutilated in various ways without loosening the sexual clasp. The following experiment is particularly interesting: 1 "Finding two toads in copulation I separated them forcibly ; I cut off the thighs of the male and put it down near the female ; it then embraced her anew. I cut off the hands of a male toad and placed it near a female ; as we know, the males use their hands in copulation ; it seized the female with its bleeding stumps and did not release her till all the ova were fertilised. On cutting off the head of a male frog in the act of copulation, it did not let go of the female with its arms and hands ; it bathed the ova for an hour and three quarters with its seminal fluid, and nearly all of them 1 Quoted from the Genevan edition of 1876, by Senebier. ii SENSIBILITY OF TIFF, INTERNAL ORGANS 79 developed into tadpoles. . . :" Two interesting conclusions c,m be drawn 1'roiu these experiments : (") The sexual impulse in lo.-ids and frogs is more potent than the most painful sensations these animals ean undergo. (6) Removal of the most sensitive parts and of the whole brain, including of course the olfactory and visual organs, does noi. inhihit the sexual clasp nor interrupt it if already in progress. Goltz continued Spallanzani's experiments on spawning frogs, and tried in particular to solve three problems: Which part of the body of the female exerts the attractive force on the male that leads to copulation? By what sensory paths is the male attracted towards the female and led to copulate ? On what part of the nervous system does the persistent muscular contraction by which the male embraces the female depend, and by what paths is this centre excited '. On these points Goltz came to the following conclusions : () At the breeding season every part of the body of the female attracts the male. This was proved by a number of curious ex- periments in which the female was successively deprived of different organs (the ovaries, sense organs, the whole of the skin, etc.) without checking the impulse of the male to copulation. In fact the male will even embrace the dead female. (&) The male is attracted to the female from afar not by one sense, but by all the senses that can come into play. Goltz showed that all the sense organs successively can be removed from dif- ferent males, without their ceasing to copulate with the female. (c) The centre on which the clasp depends lies in the upper segment of the cord. The activity of this centre is excited by the mechanical cutaneous stimuli of pressure or friction. Goltz proved that the clasp persisted, not only after decapitation, as seen by Spallanzani, but even after transverse section of the cord between the third and fourth vertebra, or after both these operations. If after isolating the thoracic portion, including the three upper vertebrae and the whole thoracic girdle, from tin- rest of the body in a frog, the skin of the breast and flexor surface of the arms is stroked, the arms will clasp the finger of the operator in a firm clasp which grows stronger if the friction is repeated. If the breast is skinned, or the three dorsal roots which it contains are cut, this reflex spasm no longer takes place. Tarchanoff continued Goltz' experiments on the frog and succeeded in isolating the stimulus that produces sexual desire in the male; it is due to the tension in the seminal vesicles when the spermatic fluid collects there. While no other mutilation disturbs the copulating male, which persists, as we have seen, after removal of heart, lungs, and testicles, the moment the seminal vesicles are taken away, or merely opened and emptied, the clasp ceases at once, or does not occur if not already begun. On the 80 PHYSIOLOGY CHAP. other hand, mere dilatation of the vesicle with an indifferent lluid. such as milk, creates the sexual impulse artificially. Accordingly, in spawning, when the nerve-centres are highly excitable, the impulse that gives rise to sexual desire comes from the dilatation of the seminal vesicles, and is transmitted hy the sensory roots. This is the Fundamental factor that gives rise in the male to the desire to seek the female and to copulate with her. During copulation, the whole of the senses with their respective nerve-centres are active, and it is necessary to extirpate them all he fore the clasp can lie inhibited. No doubt much the same process takes place in the higher animals. Every one knows that in mammals, e.g. in dogs, the odour guides the male to lind the female, and increases the erection of the genital organs due to repletion of the spermatic vesicles; it is more particularly the o 'our of the secretion from the small glands of the mucous membrane of the vulva of the female that exerts powerful attraction on the male. The other senses are, however, actively involved in different degrees. As regards the special centres connected in mammals and in man with sexual desire, the cerebellum according to the theory of Gall, revived hy Lussana and of late years by Bunge is the centre of the reprodiiet i\ e instinct, of physical love or the erotic sense. This theory was effectively put out of court by our experi- ments on the total extirpation of the cerebellum in dogs. After this operation dogs have, like normal animals, their periods of sexual excitement and all the concomitant erotic phenomena: hitches have periods of heat, in which the whole mucous mem- brane of the 'jynital organs is congested and secretes a viscid, hloody lluid which excites the olfactory sense in the male, whose advances are received with evident pleasure by the female two, three, or even more suitors 1 icing accepted. On the other hand, Goltz' researches on the effect of successive ablations of the hemispheres proved that sexual desire is diminished with each successive mutilation. But he notes expressly that dogs with a small residue of cerebral cortex still exhiliit traces of sexual impulse, since they sniff at the genital organs of other dogs, even if only momentarily. The " brainless dog," on the contrary, never gave the slightest sign of sexual attraction during the eighteen months in which it was under observation. So that there can be no doubt that the centre which is particularly active before and during coitus lies in the fore-brain. But in which portion of it ? If it were credible, as some state, that excision of the olfactory lobes and nerves obliterates the sexual impulse in dogs, the question would be solved ; but as we have not controlled this assertion we cannot accept it unreservedly. Broadly speaking, the same facts can be observed in man as ii SENSIBILITY OF THE INTERNAL ORGANS 81 in ;uiiin;ils, although in different degrees, inasmuch as they are subordinated to the higher development of the intellect and the evolution of the aesthetic and moral sense. One further physiological problem must here be taken into consideration. Is the sense of pleasure, which is localised especi- ally in the mucous membrane of the internal genital organs of both sexes, a special modification of the sense of contact, or is it a special sense, served by speeific corpuscles or nerve-endings? Much morphological research has been directed to this subject, but no conclusive solution has at present been reached. Krause (1866-81) first studied the nerve-endings in the external genital organs of both sexes, and described special corpuscles in the form of end-bulbs, which he termed "genital corpuscles." Of the many other histologists who have studied this subject, Retzius (1876-90), Aronson (1886), Dogiel (1893), Timofeew (1891), and Sfameni (1904) deserve special mention. Retzius and Aronson, who investigated the skin of the glans penis, clitoris, and vagina of the rabbit, discovered large and small genital corpuscles. They found that the nerve -fibres to these parts divided into fine branches, which ended in knobs. Dogiel investigated the human genitals as well as those of animals. In addition to Krause's end-bulbs or spherical corpuscles, large and small, he also found Meissner's corpuscles. He further discovered that filaments ran out from Meissner's corpuscles, and terminated in oval cuneiform or pyriform swellings, in the midst of the cells of the deep layers of the epithelium ; by these con- tinuity is established between the nerve corpuscles and the epithelial cells. He, moreover, found a nerve network in the epithelium which also reached the more superficial layers, in the formation of which not only the myeliuated fibres, but . also the fibres non-myelinated from their origin, participated. Timofeew described a special capsulated nerve -ending in the male sexual organs of certain mammals. Two distinct kinds of nerve-fibres penetrate these one thick and medullated, which lose their myelin sheath as soon as they emerge from the capsular sheath, and then expand into the form of a band with dentellated edges, and terminate at the opposite pole of the ramified or simple, pointed or rounded corpuscle ; the other, much finer, which also lose their myelin sheaths, and terminate after branching repeatedly in delicate varicose fibrils which form a network. He continued the presence of Pacinian corpuscles on the external genital organs of both sexes, as already described by Schweiger- Seidel, Klein, Rauber, and others. Sfameni's more recent observations were made upon the genital organs of the cow, sheep, mare, ;iss, bitch, and woman. In all these species the differences in the nerve-endings are insignificant. In any one animal the different types of eorpuseles present an VOL. IV G 82 PHYSIOLOGY CHAP. endless series of transitional forms which pass imperceptibly from the typical Pacinian corpuscle to more elaborate and diffuse nerve-endings. There is accordingly no fundamental difference in their structure, and they can all be referred to the following uniform type: "A nerve-organ, provided with, or destitute of, a sheath of connective tissue, and consisting of one or more nerve- fibres, which after losing their medullary sheath (if myelinated) expand within and around a, granular and nucleated substance." Three points here deserve consideration : (a) Sfameni neglects the nerve -fibres which are distributed to the epithelium, because they are invisible by the gold chloride method of staining, which he adopted. The intra- papillary nerve-endings show certain small differences between one region ", ' /'..-: V **> \ w& ;---:\ A \ , . '' ..;& ' * >/ . * -_ *>- Fn;. 27. 1 !;. 28. FIGS. 27 and 28.- Papillae of clitoris of li-mali-. (Slaiiieni.) .c., 'granular ivticulum ; p.t., papillary tuft, the branches of which arc lust in the network. and another ; in the clitoris, e.g., there are more nerve elements, and also more elaborate forms of nerve-endings, than in the labia minores. In all cases the papillae are supplied by a double system of myelinated and non-myelinated fibres, which arise in the super- ficial plexus. Both kinds of fibres expand in the papilla, some- times in the irregular form of a granulated network (Fig. 27), often in a perfectly regular form which simulates a true end-bulb. In addition to these nerve -endings others may be found which resemble very simple Meissuer's corpuscles, as well as forms analogous to the papillary nerve plexus which Ruffini described in the finger-tips (Fig. 28). Within the network certain cellular ft >rinations are to be seen, some of which stain with gold chloride almost like the nerve-fibres (Fig. 29), and which can be seen in direct continuity with the nerve network. According to Sfameni, therefore, these must be true nerve-cells, and not connective tissue-cells, as some have asserted. II SENSIBILITY OF THE INTERNAL ORGANS 83 The reticular layer nf the cutis contains a great, variety of nerve-endings. The simplest and most superficial form is Krause's end-bulb, which Dogiel described minutely, first in the conjunctiva of the eye, and subsequently in the external genital organs of both sexes (Fig. 30). From these more elementary forms of corpuscles there is a gradual transition to other more complex forms, the so-called "genital corpuscles," which Sfameni, like Dogiel, holds to be compound Krause's bulbs (Figs. 31 and 32 show two of the many varieties). The name of genital corpuscles is morphologically a misnomer, because similar forms exist not ; ,/y , ^ \*~ 4 ' \. t . \V * ,V ;: -A ,-;'M^' ; - ".: V;.): -^--.v . '; ' :.;:-' i ! ' >..> ;**:'&: r-. : --V-x KjMv * .'; :/- -v . Section oblique to surface. (Sfameni.) ./, myelinated nerve-librc ; g.r., granular reticuluni ; cc, superficial cells of derma in relation with the granular reticuluin. only in the conjunctiva, but also in the joints. From the so-called genital corpuscles there is a further gradual transition to more elaborate corpuscles, more or less similar to those described by Golgi and Mazzoni, and by Ruftini in the finger-tips. Lastly, there are corpuscles which appear to be transitional forms between Golgi -Mazzoni corpuscles and Pacini's corpuscles. These are largely represented in the female genital organs. (c) There are comparatively few nerve-endings in the loose subcutaneous tissue. Ruffini's end-organs are present in various forms (Figs. 33 and 34), also Pacini's classical corpuscles (Fig. 14) and other related forms, such as the Golgi-Mazzoni corpuscles, which here are usually smaller than those shown in Figs. 10, 11, 12. 84 PHYSIOLOGY CHAP. ff.s. The general morphological conclusions of Sfameni from his own observations and those of Dogiel upon the genital organs are shown in his diagram (Fig. 35). Without pausing to discuss and analyse the hypothesis, by which, according to Sfameni, the different nerve corpuscles are to be regarded as small peripheral ganglia (analogous to the spinal ganglia), the function of which is to modify the nervous excitations that reach them by way of the true nerve-endings (the intra-dermal and intra-epithelial fibres), we will con- line ourselves to stating that ac- cording to Sfameni the whole of the nervous apparatus which he re- presents must l)e the substrate not only of the male and female genital organs, but of the organ of tactile sense in general. Consequently, the anatomists who follow Sfameni neglect all the physiological evidence, and arrive at a theory which is wholly contrary to that which physiologists have adopted from minute researches into the FIG. 30. Krause's club, from mucous mem- FIG. 31. Spherical genital corpuscle from female brane of vulva of bitch. (Sfameni.) ?(./., clitoris. (Sfameni.) n.f., myelinated nerve - myelinated nerve-fibre; g.s., granulated Bbre ; o., axonal network. substance ; c.s., connective sheath. different modalities of sensation at distinct parts of the body- surface. The skin, in which physiologists distinguish four dif- ferent senses, possesses, according to Sfameni, only one of the five senses recognised by physiology from all time, i.e. tactile sensi- bility. This enormous disparity proves the vast superiority of physiological methods of research over the anatomical methods of analysis of the sense-organs. The topography of the different kinds of sensibility in the human penis was studied by v. Frey. One of the most important facts which he discovered is that the gland of the penis has no ii -KXSIBILITY OK TIIK I NTKUNAL ORGANS 85 true loueh spots. On f\ciiin._r \\itli pointed merlm meal stimuli an except ion.Ulv high threshold ,,f excitation is found, eorre- c. . :: '. ^' s I % fill I 1 '!". 32. Compound genital corpuscle from lubi;i minores. (SfauiPni.) b.c., blooii a. axonul network ; c.s., connective tissue sheath. spending to that of pain, hut not to that of contact, which is normally much lower. At the root of the penis the touch spots become less Fi'.. 33. Cylindrical Rofflni's corpuscle, I'mni Inl'i.i niinnivs. (Sfampni.) ./., myelinated nei ve- f; e. '.*., elastic connect! V' sh'-iMi. sui nunidin^ ^nmulju sulislam .. frequent; they are more abundant as the border of the pre- puce is reached ; on the internal surface of the prepuce, which covers the gland, they diminish gradually and disappear. The 86 PHYSIOLOGY CHAP. frenulum is rich in touch spots. Mechanical excitations of the gland which move the whole penis can be transmitted to and perceived by the tactile points of the frenulum and prepuce ; but if the gland is pinched without moving the penis, it is seen that slight pressures are not noticed, and strong pressures produce pain. At the edge of the prepuce, on the contrary, and on the freuulum, a slight pinch does give a sensation of contact, and a stronger pinch causes pain. Corre- sponding results are obtained with far- adic stimulation. The pain produced in the gland is different in character from that felt in the prepuce and the skin in general: it is a tearing and cutting pain, which seems to arise deeper down. The thermal sense also presents cer- tain peculiarities in the human penis. The number of thermal points for heat and cold increases like the pressure points from the roots of the penis to the edge of the prepuce ; but in descending along the inner surface of the prepuce to the root of the gland thermal sensi- bility increases instead of diminishing. The corona is one of the regions in which thermal sensibility to cold is most in- tense (as in the tips of the mammae, the eyelids, lips, and tongue). Passing from the corona towards the mouth of the urethra, this .sensibility rapidly diminishes and disappears. The frenu- lum and meatus alone contain many cold points. FIG. 34.-Globuhir Rufflni's corpuscle, ^ ne CO ^ SpOtS Ot the glaild react to from labia minores. (stunp'iii.) adequate stimuli as well as to faradic n.f., myelinated nerve-tibre ; e.c.s., ,- T mi i i J_T i elastic sheath of connective tissue: Stimuli. J-hey aiSO ShOW the phenO- ^heUcTbr,'? 1 '" 1 menon of paradoxical excitation in a remarkably acute form. Stimulation of the cold spots with the end of a heated metal cone produces an indubitable sensation of cold, which increases as the temperature of the cone is raised above the mean temperature of the body. Above 50" C., however, a sensation of burning heat is associated with that of cold. Owing to the great number of cold spots in the corona, the contact of hot metal surfaces also produces a sensation of cold that is more intense within physiological limits in proportion as the temperature of the stimulating body is raised. It is only around the mouth of the urethra that the heat stimulus produces a sensation of warmth. Determination of II SENSIBILITY OF TIIK INTKKNAI, <> KUANS 87 lirat, spots is therefore rendered very difficult l>y the number of eold spots present. Apart from thermal and pain sensihility in the gland, ;md ]>ressure sensihility in the prepuce, frenulum, and skin of Flo. 35.- Diagram of terminal IRTVC apparatus in t'nnali- ^ruitals, corresponding, according to St'anicni, with the fiHl-oruans n\' tin/tile scnsiliility in .ncni-ral. The tactile oryan is supplied liy myliiiatiMl (f.n.,f.n.',j.n.'") and miii-iiiyi'liiiatril (/.., /..',/./..) fibres, Tin 1 liist lorin a net- work with lai-fjf branclirs and noilositii's (r. i'i'onil, a in-twork with wide meshes and line laniilical ions (r..<.). These Networks are found together in the epithelial and suli-epitlielial layers. The branches of the tirst (r.g.) come into contae-t with ditl'eient iated cellular elements (/./.): peri]ihei-ul sensory cells. Some fibres, both nne'inated and iKui-myelinated, do not run -In ect to the periphery to enter the corresponding network, but contribute on thr u ;l \ toth,- stiuetui'e of one or more end-corpuscles, which they leave us ultra- terminal or, better, ultra- corpuscular librils (/..,./..'). In each corpuscle there are two nerve-endings ; one, primary (t./i.), i ies from the large myelinated spinal libres (f.n., f.n."); the other, secondary (t.*.), from the sympathetic (Hues (/..', .//-.). penis, v. Frey found no sensory points reacting to any other quality of sensation. There is accordingly no foundation for the view by which some have sought to define a special sense on the outer surface of the genital organs for voluptuous pleasure. Excitation of the tactile points, and perhaps also of the pain points and thermal points of the penis, may certainly be associated with voluptuous sensations 88 PHYSIOLOGY CHAP. such as occur in other special regions of the skin and other mucous membranes. But this association is neither constant nor necessary, and probably depends on the summation of particular conditions of excitability in the sensory centres. VI. In the group of functional sensations, that is the sensa- tions that accompany the various functions of the internal organs, a peculiar importance, both from the physiological and from the psychological point of view, attaches to the sensations by which we become aware, directly or indirectly, of the state of the muscles, the modes and different degrees of their functional activity, and the changes, generally speaking, in the active and passive organs of the motor system. It is by the sum of the sensations arising from the motor system that we are able to control our movements, and to carry them out with the necessary precision. The character of these sensations is always vague and ill- defined. The slight degree of tension normally present in inactive muscle, on which muscular tone depends, certainly lies below the threshold of consciousness. We are able without hesitation to call up the exact degree of muscular contraction necessary to reach a given aim, e.g. to produce a musical note of a certain pitch. This means that we have, unconsciously, an exact notion of the degree of tension present in the vocal muscles previous to their voluntary contraction. The sensations of tension and resistance that accompany the state of contraction and the contractile movement of the muscles are obscurely perceived. With the eyes blindfolded or in total darkness, we are conscious of a certain position in which we place, say, an arm ; we are able to describe it, and even to imitate and reproduce it exactly with the other arm. We are aware of the changes of movement of the arm, whether these are made voluntarily or passively. When we lift a weight, or in an active movement meet with an obstacle or a resistance imposed by a body external to the limb, we are able to apprehend the degree of effort exerted in raising the object or removing it. The estimation we make of our sensations of tension and resistance, and of the force required to overcome them, are, as we shall see, the most important elements in the judgment we form of the weight of objects. These and other more obscure, less well defined, and poorly localised bodily sensations which accompany functional activity of the motor organs in animal life are collected by the majority of physiologists into one category known as the muscular sensations. This term is inappropriate, and must be rejected as erroneous if it is taken to mean that all the different varieties of sensations which it comprises originate from specific sensory elements con- tained in the muscles ; for we reason experimentally that most of ii SENSIBILITY OF THE INTERNAL ORGANS 89 tin-in In- in tin- tendons in- tendinous slu-atlis, Lin- lasem, ;nnl the surl'act-s of the joints. So that, tin- term '' muscular sensations" is justifiable only if we admit, with Sherrington, that it covera the sum c! the sensations which originate in the motor apparatus, that is, in the muscles and accessory organs of movement. Various theories of the nature and origin of these sensations have been put forward, which we will next examine one by one, this being the best way to discuss the whole subject and its significance. It is evident that the sensations that originate in the active state of the muscles are intimately mingled with the cutaneous sensations of contact or pressure. When a muscle contracts, the soft parts are displaced, so that the skin relaxes and forms folds in certain regions, while iu others it becomes tense. The relaxa- tion or tension of the skin, and the rapidity with which it occurs, are proportional to the extent, rapidity, and duration of the movement or contraction of the muscle. So that the sensations aroused by means of the tactile cutaneous nerves are able to inform us of the energy, speed, and duration of the muscular contraction. On the strength of this fact, and in view of the delicacy of cutaneous sensibility, certain physiologists, including M. . Schil'f (1855), Lotze, Hanseu, and Auber, assumed that in explaining the origin of the so-called muscular sensations it was unnecessary to recognise the existence of a specific peripheral sensory apparatus, with special afferent nerves, located in the organs of movement, inasmuch as they were adequately aroused by excitation of the tactile cutaneous nerves which inevitably takes place whenever the muscles become active. But even if we admit the more or less appreciable intervention of cutaneous sensation during the activity of the muscles, it can easily be demonstrated that this is not sufficient to explain the whole of the phenomena included under the term " muscular sensations." In the frog it is possible to suppress the whole of the sensations that are cutaneous in origin by removing the skin from the four legs without perceptibly affecting the regularity of the customary movements of walking, jumping, and swimming. This is Cl. Bernard's experiment, which proves that the regularity of the frog's movements is independent of any possible controlling action by the cutaneous sensations, making it highly probable that the controlling action depends fundamentally upon sensations trans- mitted directly from the muscle- or indirectly from the passive organs of the motor system. For the true solution of the question it is necessary to take into consideration the phenomena observed on man in cases of pathological alterations of cutaneous and muscular sensibility. 90 PHYSIOLOGY CHAP. Neuropathology presents many such cases. Anaesthesias ut' hysterical origin, those of traumatic neurosis, and those observed in syringomyelia are comparatively common. These patients sometimes retain the power of carrying out all movements with the affected limbs in a normal or almost normal manner, so long as their eyes are open. But when the eyes are shut they lose consciousness of the movements they are making, and are unable to describe the position actively or passively taken up by the anaesthetic limb. These cases are difficult to interpret. It may be thought that the anaesthesia is confined to the cutaneous nerves. and that this, on Schiff's theory, involves loss of muscular con- sciousness (Magnin, Oley, and others). But more probably the defect depends on abolition or suspension of both cutaneous and muscular sensibility (Beaunis and others). This is apparent from a case described by Striimpell (1902) of total paralysis of every kind of sensation in the forearm and right hand, with complete preservation of motility, in a patient who received a knife-wound in the cervical spine which probably cut through the grey matter of the dorsal horn and the lateral portion of the right dorsal column. The injury was followed by a com- plicated illness with widespread symptoms; but when the wound healed after about nine months, all the symptoms were confined to the right upper limit. With closed eyes the patient was unable to say whether the lingers of his hand were flexed or open, or to maintain it in any posture in which he was placed with his eyes open (Figs. 36 and 37). . Under the control of vision he was able to exert a strong pressure with his hand and to place the fingers in any position; with shut eyes he was incapable of carrying out any definite, complicated movement with that hand, or of extend- ing or flexing it at command. This is a clear demonstration that the abolition of superficial and deep sensibility in the hand and forearm renders the patient incapable, without the use of his eyes, of accurate sensation either of the position or of the active and passive movements of the hand or fingers. From our standpoint cases of well-authenticated dissociation of superficial and deep sensibility are more interesting. Clinical cases have been well described, in which the sensibility of the deep tissues was wholly or partly retained, while cutaneous sensi- bility was entirely abolished. In a hysteric described by Duchenne (Boulogne) there was total insensibility of the left upper limb (analgesia, anaesthesia, insensibility of muscles to electrical and mechanical stimuli), although the patient, even with closed eyes, was aware of the active and passive movements of the limb, could estimate the weight of objects placed in the hand, and did not let them drop, proving, according to Duchenue, that the sensibility of the articular tissues persisted. II SKXSIIUI.ITY OF TIIK I \TKK\A I, OEGANS 91 T\\i> patients described li\ Lindry exhibited diametrically opposite plieiinmemi of dissociation; tac.tile ;uid |>ain sensibility of the skin on one side were retained, while the sensibility of , !'!'.. 8* . Kn.s. 30 anil 37. Strttmpell's patient, who Mill'iTi'il limn romjih-ti- loss of snpi'i lirial ami ih'rp M-n>iliility nl' njjit ann. Wit.li ryi-s o]n'ii In' was able to plarr anil main lain hut h liamls in tin- same position; \\itli py.^ slml In- in\ nlunl ai il\ altered tin' pnsilnni of the- rii;ht ham! i in tlii- i the deep tissues was completely almlished. In these cases, as soon as vision was excluded, the patients lost consciousness of the position of their limits, and wen- no longer able to appreciate either active or passive movements. 92 PHYSIOLOGY CHAP. Not a few other similar cases have been described by neuro- pathologists ; but in the majority of them the loss of deep sensi- bility was incomplete, or was associated with a slight disturbance of superficial sensibility. In proof of the secondary importance of superficial sensibility as compared with that of the muscles and the deep tissues in general, we may refer to the experiment of Beaunis (1887) on the function of the laryngeal muscles. After anaesthetising the mucous membrane of the glottis of a tenor by the application of cocaine, which also made it pale owing to vascular constric- tion, he found that the intonation of the voice, that is, the exact formation of the separate musical notes, was not appreciably altered ; the purity and timbre of the sounds alone seemed some- what affected, which might be due to the altered blood-supply of the organ. The conclusion drawn by Beaunis seems satisfactory, that " muscular sensibility plays the leading part in the tension of the vocal cords by which accuracy of tone is determined, and the sensibility of the mucous membrane only intervenes, if at all, in a purely secondary manner." We have consequently sufficient evidence for assuming that the muscular sensations which depend on specific sense organs situated in the muscles, tendons, joints, and accessory organs of the motor system are independent of the sensations of the skin and adjacent mucous membrane. The founders of the theory of muscular sense as a sixth sense were Charles Bell (1832) and Panizza (1834) (see Vol. III. p. 467). They founded their entire theory on the phenomena of the dis- organised movements of the limbs obtained after section of the dorsal roots (root ataxy). E. H. Weber (1846) developed the theory of a sense by which we become aware of the degree of muscular effort necessary to overcome the resistance that opposes our movements, and gave it the name of sense of effort (Kra/tsinri}. He succeeded by ingenious experiments in demonstrating that we are able to appreciate the difference between two weights far more exactly by this sense than by tactile or pressure sensibility. By sensations of pressure alone, such as those produced by weights upon the fingers resting supinely upon a support, the difference in weights which are as 29 : 30 can be perceived. When the muscle sense is employed, as in raising with the fingers a pan on which the weights are placed, w 7 e are able to distinguish them when the values are in the ratio of 39 : 40. In this case (according to Weber) the lower threshold of difference does not depend on the association of tactile and muscular sensations, because in judging of the weights raised we entirely neglect the sensation of pressure of which we are aware in the hand that supports the scale-pan. In fact our judgment does not alter when we voluntarily increase II SENSIBILITY OF THE INTERNAL ORGANS 93 I his pressure beyond what is strictly necessary to sustain the weights. Anatomical proof that the muscles, tendons, and joints art- sensitive, o\\ing not only to the sensory nerves that traverse them to reach the skin, hut also to the lihres that terminate there, was given hy Reichert, Kolliker, and others. According to Kolliker tin- sensory nerve -fibres of muscles almost always run towards the surface and ends of the muscle, and terminate in the connective tissue, perimysium, and tendons, never in the sarcolemma of the, muscle-fibres. Rauber (1883) and Ciaccio Fie. ',',*.- Muililieil I'aeinian corpuscle of cat . (Rulfini.) !'[.. M. I'acinian cm piiM-l.' ,,{ r; t li!ijt. inoilili.-cl vi ;is to i i-.i>]iitili- t In- cluli-shaped corpuscles of Golgi-Mazzoni. (Rufflni.) (1889) first described in the muscle sheaths, tendinous sheaths. and joint capsules, nerve-endings resembling Pacinian corpuscles, of various forms and sizes, which differ slightly from those of the subcutaneous connective tissues. A more minute description of their conformation, topography, and relations was afterwards (1897) given hy Sherrington and liul'lini i'Kigs. 38,39). Kolliker (1862) and Kiihne 1863) discovered amung the ordinary muscle-fibres rharacleristio bundles containing a lew 94 PHYSIOLOGY CHAP. f.S.Stf. o. :. 40. Semi-diagrammatic representation of neuro-muscular spindle from adult cat to show complex nerve-endings. (Ruf- fini.) c., capsule; n.t., nerve -trunk; FP.6., Weissmann's bundle; H. (... nerve- fibres ending in the muscle bundles that surround the nemo - muscular spindle ; t.m.p., motor end-plates (after Cipollone) ; t.s.pr., primary sensory endings; t.s.sec., secondary sensory endings. muscle-fibres of embryonic appear- ance, invested by a sheath, similar to that of the Pacinian corpuscles, which assumed a spindle form at the point of entrance of the nerve, and were therefore called nerve- muscle spindles. According to A. Cattaneo and Kolliker they are usually found near the tendinous ends of the muscles ; but Sherring- ton and Ruffini found numbers of them also in the fleshy parts of the muscles. A minute anatomical description of the neuro-muscular spindles, and particularly of the different modes in which the nerves terminate in them, was first given by Ruffini (1892-98). Here we can only reproduce one of the most characteristic figures which he oil- served on the cat (Fig. 40). The sensory nature of the nerve- muscle spindles was recognised by Korschner (1888) and Ruffini (1892), and experimentally demon- strated by Sherriugton (1894). He saw that the myelinated nerve - fibivs of the spindles underwent no change after section of the ventral spinal roots, and concluded that tbry originated in the cells of the dorsal root - ganglia. Cipollone (1898) by other ingenious experi- ments showed that the fine medul- lated fibres of the spindles as well as the end-plates connected with them are motor fibres and endings, while the large medullated fibres and the plentiful primary and secondary endings of the fusiform swelling are sensory fibres and endings. In rabbits the first de- generate, the second remain per- fectly intact, when necrosis of the grey matter and root cells of the lumbar cord is produced by Sten- sou's method, while the cells of the II SENSIBILITY OF TIIK [NTEENAL OEGANS 95 licrves corresponding spinal ganglia and the peripheral remain intact. A third nerve end -organ was discovered by (iolgi (1880) in both man and the higher vertebrates in the transit ion.,1 between the tendons a.nd t he muscle - fibres, and is known as the mnsculo- tendinous organ or cor- puscle. Such organs were found by Marchi in the tendons ofthe eye-muscles ' 1SM1\ and were studied in closer anatomical detail by A. Cattaneo (1888). For the most part, they are fusiform, sometimes cylin- drical, bodies of different si/es with a smaller ten- dinous end turned towards the insert ion ofthe tendon, and a larger muscular end turned towards the belly of the muscle. We cannot enter into their structure, which is plainly shown in Figs. 41, 42, and 43. taken O ' ' from Cattaneo and Ruftiui. The sensory nerves and nerve-endings of the joints have been less studied ana- tomically, alt hough clinical experience has proved their extreme sensibility in cases of inflammation. The articular Cartilages FlQ . n . Two musculo- tendinous organs of rabbits, seein to he destitute of treated with silvei nitrate and osmic acid, enlarged aboul mo diameters. (A. Cattaneo.) d, i.ifmeatio,, nerves hilt these are point of a fibre thai inn, Avail's tw 'gans of Golgi ; ,1 i .. '., endothelium invslinu this m^an; Ii llenle's abundant in tlie ends 01 sheath, \\\\\<-\\ the nerve i<.se s on entering \\\<- cor- the liiim-si thp ripviii^tciiiii puscle; m., mnsele bundle unit.-,! with the small tones, ine pei iosw um, ,,. M ,i,,n ofthe org t o< articular ligaments, and synovia! capsules. llauber found more or less modified Pac-mian corpusides in the vicinity of nearly all the joints; it is not yet known whether Golgi's organs are also present there. It follows from the preceding discussion that the active and passive organs of movement are supplied with sensory tibivs that have three special end-organs, 1 he modified I'aciida,n corpusides. the musculo-teiidinoiis organs of Golgi, and the ncuro-miiscular 96 PHYSIOLOGY CHAP. spindles. Many ingenious deductions have been made from the structure or topography of these three organs with a view to determining their respective functions as organs of the muscular sense. The conclusions of Cipollone and of Sherrington seem to be of special importance. After Cipollone's demonstration of motor FIG. 42. ^.i^S^i^S Fio. 43. Ki'.s. 42 and 43. Twofmusculo-tendinous organs from cat, each containing two modified I'acinian (m-pusi-les. (Ruffini.) i.e., terminal nerve expansion belonging to musculo-tendinous organ; /'.i., modified Paciniau corpuscle ;. a.c., annular constriction (A. Cattaneo), or small strip of < ... connective tissue (Ciaccio). nerve-endings in the neuro-muscular spindles, there could be no doubt that the special muscle-fibres which they contain contract during the excitation transmitted by the motor nerves, and in contracting mechanically excite the ring - shaped and spiral sensory nerve -endings by which they are surrounded. The more or less appreciable sensations which they arouse in the central nervous system are proportional in intensity to the degree ii SENSIBILITY OF THK IN'TKKXAL OUGANS 97 in change of Turin of the spindles. In a \v<>nl, they function as ixn/oiiic dynamometers, by which the centres become a\\are : of the degree of active contraction of the muscle and perhaps also of the degree of passive traction to which it is subjected hy the action of the antagonist- muscles. This hypothesis seems to us simpler and more acceptable than that of other authors who maintain that the sensitive ends of the spindles are excited by the action-current developed in the muscle, or by the molecular chemical changes that take place in it during the contraction. While the sensory nerve-endings in the spindles are in direct. relation with the muscle tihres, and the stimulus to which they react is given by the form changes of the muscle, the sensory endings in the organs of Golgi are in relation with the tendinous fibres, and the mechanical stimulus to which they react is produced by the tension they are subjected to in consequence of the active state of the muscle. While the former function as isotonic dynamometers, the latter function as isometric dynamometers that is, they signal to the centres the tension changes, rather than the form changes of the muscle. For the due performance of this function it is unnecessary to predicate any extensibility of the tendinous organs of Golgi, as was assumed by Cipollone. In fact, if they were extensible their isometric signals would be incorrect. According to Marchi, the muscles of the eye-ball contain no other sensory organs capable of signalling the delicate antagonism of their functions, which proves that the musculo-teudinous organs must be able to send intelligence of the least traction exerted by these muscles. To explain this great sensibility of the organs of Golgi, Cipollone happily takes into consideration their curved form and oblique position in respect of the tendinous fascia, and the occasionally undulating course of the tendinous fibrils of which they consist. It is evident that, under these conditions, the tendinous organs of Golgi can be affected even by the weak traction exerted on them by the muscle; there is no real increase in their length, but an adjustment of the fibres that run curved, undulating or obliquely to the line of traction of the muscle. This adjustment or displacement may be accepted as a stimulus adequate to excite the nerve-endings mechanically. As regards the functional value of the modified Pacinian organs present in the motor system, particularly in the tendons, bones, and articular tissues in general, Sherrington correctly points out that they are by their position, and particularly from their structure, eminently suited to signal the different degrees of compression that occur during the, changes of relation of the articular surfaces, whether these are actively produced or passively imposed. Which of these three different sensory organs located in the VOL. IV II 98 PHYSIOLOGY CHAP. muscles, tendons, and articular tissues has the greatest physio- logical importance in arousing the so-called muscular sensations by which movements are controlled ? In this connection some interesting physiological and clinical observations may be cited. Certain early observations of Haller and Bichat, confirmed later by Schiff, Bernstein, and others, show that the muscles and tendons are insensitive to many mechanical, thermal, chemical, and electrical stimuli which are effective in other tissues. Sherrington, however, showed that the tendons may be the starting-point of reflexes. Compression of the tendon of the tibialis anticus of the cat invariably produces a reflex in the adductor femoris. Reflexes can be obtained from other muscles. On pinching the muscles of a curarised rabbit, Kleen obtained a fall of arterial pressure. Before that, Sachs obtained reflex convulsions in a strychninised frog, by exciting the central stump of a nerve to the sartorius. Sherrington showed that the knee-jerk phenomenon may be reflexly inhibited by compressing or otherwise stimulating one of the leg- muscles. He also saw that the sudden relaxation of a muscle passively pulled on often discharges a reflex in some other muscle : that the direct stimulation of the muscular nerves gives rise to vascular reflexes, often also to alterations of respiratory rhythm ; produces antagonistic effects (by lowering muscle tone) in other groups of muscles; causes decerebrate rigidity to disappear; and tinally may induce reflex contraction of other muscles. From these facts we may conclude that the more or less appreciable sensations aroused by excitation of the sensory nerve organs of the tendons and muscles are not unimportant to the regulation of movements. Other facts, however, indicate that the muscular sensations due to excitation of the afferent nerves of the tendons and muscles remain, under normal conditions, almost entirely below the threshold of consciousness, and are only of negligible, certainly only of secondary importance, as factors in the complex sensations that accompany voluntary movements ; and that greater import- ance attaches, on the contrary, to the excitations arising from the afferent nerves of the articular tissues. This theory was maintained by Kauber, Duchenne, and Lewinski, more particularly on the strength of clinical observations, and was more fully developed by Goldscheider (1889) on the basis of accurate experimental research. It seems a priori probable that the sensibility of the articular tissues as a whole should be of predominating importance in the genesis of the sensations by which movements are reflexly regulated. Every change in the relations between two articular surfaces corresponds to a simple movement, while there is perhaps no muscle that takes part in a single movement only, nor any move- ment that is not the result of the associated and variouslv ii SENSIBILITY OF THE INTERNAL ORGANS 99 graduated action <>f several muscles. Owing to their simplicity tin- force, amplitude, and direction of the movements of the joints is readily appreciated; the simultaneous contraction of several muscles, on the contrary, can arouse no compound resultant sensation unless there is a separate consciousness of the difference in intensity of the single elementary components of the sensation and their associations. In the clinical case described by Striimpell (p. 90, Figs. oG, :i7; the loss of the sensations of posture and of active and passive movement in the right hand depended not on paralysis of the muscles and tendons, but on the insensibility of the joints. In fact, in Duchenne's case (p. 90), where there was loss of cutaneous and muscular sensibility in the left upper limb, while articular sensibility persisted, the patient was aware of the posture and of active and passive movement. Lewinski (1879) had under observation an ataxic patient who in standing erect and walking felt as if his right knee were turned in (as in genu varum), and was obliged to look to convince him- self that it was straight like the left knee, but when he lay down in the horizontal posture the illusory sensation ceased. As the sensibility of the skin and muscles was the same on both sides of the leg, Lewinski concluded that the anomalous sensation was due to a diminution of sensibility on the inner side of the joint. Under ordinary conditions, the sensation in standing, excited by the weight of the body, is uniformly distributed over the whole surface of the joint; when sensation is defective or absent on the inner surface the patient feels as if both articular surfaces of the knee were compressed on the outer side, and were not in contact on the inner side, as if the leg were bent outwards, making an angle that opened externally. The anomalous sensation disappears in the horizontal posture because the cutaneous tactile sensations correct the illusion and supplement the defect in articular sensibility. Lewinski further saw that if passive movements were executed at different joints on this ataxic patient he only became aware of them when the surfaces of the joints were pressed strongly one against the other. From this he concludes that there is no doubt that sensations of posture depend exclusively on the compression of the two articular surfaces, and sensations of passive movement on the constantly changing points on those surfaces that are com- pressed during the changes in the relation of the articular heads. The important experiments of Goldscheider further support this theory. He caused his limbs to be moved passively, and recorded on an apparatus the speed of the movements and their angular value. He found that in many joints quite small move- ments, frequently less than one degree (0 '72 - '22), could be appreciated, 100 PHYSIOLOGY CHAP. According to Goldscheider, the speed of the movement is important, as well as its amplitude. A movement that was imperceptible but close to the threshold of excitation became appreciable when its velocity was increased. It is possible also to establish numerical relations between the two main factors of the passive movements. The liminal speed, that is the minimal degree of angular movement per second, varies in the different articulations from about '25 to 1'4. Goldscheider holds that the perceptions of posture and of passive movement depend fundamentally on the deep sensibility of the articulations. Not only are vision and touch not indis- pensable to them, but the sensibility of the muscles and tendons are of no appreciable importance : the minimal angle of excursion necessary to give the perception of the passive movement remains the same, whatever the initial posture of the articulation. Again, the result does not alter after the skin of the limb has been anaesthetised by electricity : when the cutaneous sense of pressure on the skin is thus eliminated, perception becomes more acute. When, on the other hand, the joints are made insensitive, the perception of movement becomes blunted, and the movements must be of a wider range to be appreciable. Hence, according to Goldscheider, the articular surfaces are the exclusive starting-point of the sensations by means of which we directly perceive the passive movements of our limbs. Of course the perception of active movements also depends upon the compressions and excursions of the articular surfaces ; but other factors intervene here the tension of the tendons, and probably also the changes in form of the active muscles, besides the passive traction of antagonist tendons and muscles. In fact, according to Goldscheider, the sensibility to active movements is more delicate than to passive movements, although the difference is not very large. VII. The discussion of the sensory phenomena connected with active voluntary movements would be inadequate and incomplete if we did not take another important factor into account. Besides the more or less obscurely appreciated sensations which accompany movement, and which are aroused at the periphery by excitation of the terminal sensory organs of the articular tissues, tendons, and muscles which we have considered under the general name of muscular sense, we have to consider the central sensation that precedes the movement, which coincides with the volitional act and gives rise to the efferent current along the motor paths. Johannes Miiller, Helmholtz, Wundt, Bain, to cite only the most eminent authorities, maintain that we have not only a sensation of the movement executed, but also a sensation of the movement willed ; that the sensation of the active movement is directly asso- ciated with motor innervation ; that we perceive the intention ii SKXSIBILITY OF THE INTERNAL OKCANS 101 before the fact; that the idea of the contraction precedes ;md does not follow the nio\einent. lint this theory of a central sense of innervation, as opposed to that of the peripheral muscular sense, finds no supporters in view of the progress and development of the theory discussed above. Some physiologists still maintain that in all voluntary acts a. central feeling of innervation is associated with the multiple peripheral sensations; the majority, however, deny Bain's theory even in this restricted form, and maintain that the sense of effort results exclusively from a consensus of afferent elementary sensa- tions. We must weigh the arguments for and against the two theories before deciding in favour of one or the other : - () Certain phenomena observed on paralysed patients were formerly adduced in favour of the central sense of innervation. If a paralysed person is invited to make a movement with his paralysed limb he puts out all his force without success, and is fully conscious of the effort he makes, although this cannot depend on any excitation from the peripheral organs of the paralysed limb. But it is obvious that this and other similar arguments adduced in support of the theory of an innervating sense are of little value. It has in fact been pointed out by Vulpian and others that when the paralysed person attempts to move the paralysed limb, he throws into action a number of non-paralysed muscles in other regions, and the sensation of effort felt may be due to the movements performed by these muscles. (J) If the sensation that accompanies the movement were due solely to peripheral excitations, there would, according to Wundt, be perfect parallelism between the sensation and the muscular contraction. But, as a matter of fact, we know by experience that the sensation does not depend principally on the extent of the movement effectively carried out, but on the force of the impulse that emanates from the motor centre. In proof of this we may cite the fact described by Delboeuf. If any one repeatedly exerts the whole force of his hand on a spring dynameter, he has the illusory sensation of using the same effort each time, and is surprised to see the rapidly decreasing values in a series of ten or twelve efforts. There is evidently no parallelism here between the sensation of effort, which remains uniform, and the movement actually carried out, which rapidly decreases. So that we may conclude that the first is central in origin and does not depend on the second. (c) Weir-Mitchell argued in favour of a central sense of inner- vation from the illusory phenomena observed after amputations, of which he made an exhaustive study from over 100 patients. It has been known since the time of Johannes Miiller that nearly all persons whose limbs have been amputated (94 in 100) have the illusion that the lost limb is still in its place, and though this feeling 102 PHYSIOLOGY CHAP. may be vague or disappear it is readily called up again by any influence affecting the stump (Vol. III. p. 201). Weir-Mitchell shows that the illusion of the presence of the lost limb is per- sistent, and may be so vivid that some persons who have under- gone amputation are more certain of the existence of the missing than of the remaining limb. That sensation rarely extends, how- ever, to the whole limb. In a third of the cases of amputation through the thigh, and half the cases with amputated arms, there is a feeling that the missing foot or hand is nearer the trunk than in the corresponding intact limb. The most interesting point for the argument is that there are subjective sensations of movement in the amputated limbs. The patient is nearly always capable of voluntary change in the phantom of the missing limb, and can produce sensations of flexion and extension, if not of the whole of the joints, at least of the fingers or toes of the missing limb. Generally speaking, these voluntary efforts are injurious and produce itching at the stump; but in some cases the patient imagines complete freedom of movement in the missing hand, and says, " My hand is open, my hand is closed, now I am touching the thumb with the little finger, now my hand is in the position for writing," and so on. From these and other interesting phenomena which he describes in detail, Weir-Mitchell concludes that the will to move and the consciousness of movement are synchronous, and occur simultaneously in the centres. At each volition the consciousness of the act to be performed, with its qualities, surges up in the mind. These phenomena are erroneously attributed to impressions coming from the periphery. (d) Z. Treves assumes the existence of a sense of innervation, by which we have a direct appreciation of centrifugal impulses sent out by the motor centres, because we habitually regulate the volitional impulse in such a way that the external change effected by the muscles brings about the desired effect, both in amplitude and speed, with the least expenditure of energy on the part of the muscles, independently of sensations conveyed from the peri- phery by the muscular sense. A proof that this regulation of the volitional impulse really exists and is central in origin is given by the experiment of the bottle (quoted by Johannes Miiller in his Text-book), in which if an empty bottle which the subject believes to be filled with a more or less heavy fluid is raised, it acquires unexpected velocity, and almost precedes the movement of the arm it flies, in Fechner's picturesque expression. This excess of energy expended when the subject does not know if it is full or empty would not appear if the intensity of the centrifugal impulse depended solely on the peri- pheral sensations due to muscular activity. In analogy with this are the phenomena described by G. E. Miiller and Schumann relative to certain errors in the estimation ii SENSIBILITY OF THE INTERNAL ORGANS io:>, of weights. If a comparatively long series of tests is made with ;i rather heavy weight, and the expenmonfor suddenly lias to lift ;i lighter weight, it will seem excessively light. If, on the contrary, the weight is made heavier, it appears much heavier than it really is. This also shows that we usually predetermine the volitional impulse, and measure it according to previous experience for the weight we are about to lift, and that these errors of judgment depend OH the disproportion between the energy employed and the mass actually raised. Treves adduces another familiar experience in support of tin- same point. If in coming down a staircase one step is higher or lower than the rest, we are apt to fall or stumble, because the foot is moving at a rate corresponding to the rhythm of the previous descent, and is not adapted to the unequal step. (e) To prove the existence of a sense of innervation, Treves adds some ingenious remarks on the education of the volitional impulse. He points out that the less the motor impulse (which results directly from the volitional impulse) is sufficient, i.e. adequate to the mechanical task imposed, the greater will be the sensation of effort. The physical basis of this sensation may be numerically expressed by the reciprocal value of the product of the resistance into the square of the velocity imparted. But if the sense of effort is mainly based on the degree of tension given to the muscle, and on the time this tension lasts till the desired aim is reached, it follows that the education of the volitional impulse which serves to reduce the sense of effort to its minimum must lie the result of previous experience : this cannot be explained unless we admit the sense of innervation, by which we are able to graduate the volitional impulse, and with it the motor impulse, and adapt it to the desired end. This idea of Treves agrees with Mach's proposition : that what we term will is no more than the sum of those states associated with the previsions of the effect that precede a movement, of which we are partially conscious. This sum must be something more than the mere mnemonic ideation or representation of movement, and something other than the sense of effort that accompanies the actual move- ment; both these in fact are not seldom opposed to the mechanical effects foreseen and actually obtained, as for instance in Delboeuf's experiment quoted above, in the so-called "cramps" of different professions, the ataxic movements of diabetes, and the like. The sum of the central conditions antecedent to the movement must, as Mach points out, form the content of the sense of innervation, directly perceived as such, and thus constitute the initial factor in every voluntary movement, even when by long practice it has passed into the region of the unconscious. Mach, whose definition of a voluntary act has just been cited, 104 PHYSIOLOGY CHAP. seems to admit these logical conclusions. For, in the 5th edition of his Analysis of Sensations, he inclines if not directly joining with those who admit the sense of innervation at least to leave the question open. We must now turn to the fundamental arguments of the opponents of a sense of innervation, including the psychologists William James and Miinsterberg, and the majority of living physiologists. They may be reduced to three main propositions, which we will consider in turn : () The first objection to the theory of a sense of inner vation is derived from the clinical case recorded by Striimpell, quoted above (p. 90). Iii this case the motor paths are intact, because with the aid of vision the pat ient is perfectly able to carry out any movement at command. The paths from the hand and fore- arm, of both superficial and deep sensation, are, on the contrary, completely interrupted. The conditions for the so-called innerva- tion sense are therefore intact, while those for the muscular sense are interrupted. Seeing that with his eyes shut the patient is unaware of the Hexed or extended position of his fingers, is unable to maintain the position of the hand assumed when his eyes were open, or to carry out correctly the movements he is told to perform, there is here sufficient reason for denying the existence of a special central sense of inuervation. The force of this objection is undeniable. It does not, how- ever, seem to us to cancel the weight of the above arguments in favour of a sense of innervation. We have seen that there are exceptions to the fact that amputated persons are " aware " of the lost limb, and that still more frequently they are unable to move the joints imagined in it. Strumpell's case, which deals not with an arm amputated in toto, but merely with interruption of the sensory paths, may count as one of these exceptions. In any case it shows that integrity of the motor paths is not enough to secure perfect execution of voluntary movements, and that we do not yet know all the internal conditions necessary to the normal functioning of the sense of innervation. (&) The second objection is founded on certain experiments of Bernhardt (1872), subsequently confirmed by Goldscheider. In order to decide whether in judging of the weight of a body we employ the peripheral muscular sensations only, or a central sense of innervation as well, Bernhardt made comparative experiments and used alternately, in lifting weights, a voluntary contraction and a contraction produced by a direct electrical stimulus. In a first series of researches made on the muscles of the leg he saw that the difference between two weights is less well distinguished when the muscular contraction is produced electrically. But in a second series of experiments on the flexor muscles of the fingers he no longer found the same difference, and the judgments of the ii SENSIBILITY OF THE INTERNAL ORGANS 105 weights raised were not perceptibly altered when iln- movement was excited by the clcclric;il stimulus. From tln-se experi- ments it \vas concluded that the supposed sense of imiervat inn does not exist, because the. muscular sense is adequate to suhsei \c l he estimation of the dil't'erenees in weight. On closer investigation, however, these experiments, which otherwise gave no constant results, only show that in judging weights the sense of effort, due to the resistance which the muscles encounter in lifting the weight, is more important than that of the central innervation sense, which we are compelled to admit on other irrel'utahle grounds. (c) The third objection to the sense of innervation is drawn from the fact that independently of sensations of peripheral origin we are not able to prove any direct and unmistakable central sensations of innervation. Ferrier more particularly uses this argument in opposing the theories of Bain and Wundt: " If the reader will extend his right arm and hold his fore- linger in the position required for pulling the trigger of a pistol, he may without actually moving his finger, but by simply making helieve, experience a consciousness of energy put forth. Here, then, is a clear case of consciousness of energy without actual contraction of the muscles either of the one hand or the other, and without any perceptible bodily strain. If the reader will again perform the experiment, and pay careful attention to the condition of his respiration, he will observe that his consciousness of effort coincides with a fixation of the muscles of his chest, and that in proportion to the amount of energy he feels he is putting forth, he is keeping his glottis closed and actively contracting his respiratory muscles. ... In the contraction of the respiratory muscles there are the necessary conditions of centripetal impres- sions, and these are capable of originating the general sense of effort." ! This objection is easy to meet. If the feeling of inner vation is to coincide with the motor impulse, that is with the centrifugal wave of excitation sent out along the motor paths, it must obviously be absent when we imagine that we send it out, but do not really do so. The whole of Ferrier's reasoning merely shows that the sense of innervation cannot function unless there is a simultaneous muscular contraction, so that it is impossible to separate the sensations of central from those of peripheral origin. But this does not refute the theory of a sense of innervation if other powerful arguments speak in its favour. On the other hand, it may legitimately be maintained that the central sensations of innervation, particularly in habitual move- ments, normally lie beneath the threshold of consciousness. The same may be said of the sensations of peripheral origin that 1 Ferrier, The. Functions f >/' Jim In, 1876, p. 2'J3. 106 PHYSIOLOGY CHAP. accompany the movement. In our habitual movements we are not aware of overcoming resistance, so long as it is confined to the weight of our limbs. But this does not prevent us from regulating the impulses in voluntary acts, so that they perfectly fulfil their purpose. In regard to the iunervation of the eye- muscles Mach remarks : " Thanks to the organic arrangement and long practice we straightway employ the in nervation necessary to fixate any object, of which the image falls upon our retina. Innervation is only disturbed when the external motor forces are not associated with the voluntarily measured innervation." It is a matter of common knowledge that the sensations originally present in our acts become less and less vivid with practice, till at last, as they pass into the region of the unconscious, they become mechanical - - or automatic, as they are usually termed (an ambiguous and unfortunate expression). So that the absence of any clearly perceived sensation of the act of innervation is not sufficient to justify the statement that it was not originally more or less conscious. Such are the delicate mechanical movements by which the artist performs a musical piece 011 different instruments, as contrasted with the long and tedious practice required before the piano or violin can be mastered. Again, while it is fully proved that the motor disturbances in ataxy produced by disease of the dorsal roots are due exclusively to the diminution or loss of the muscular sense, it would be a bold assertion to declare that all cases of disturbance of voluntary motility can be explained without the assumption of the inner- vation sense. This would lead, as Trevcs pointed out, to the conclusion that we can never foresee the external consequences of our voluntary acts, and never avail ourselves of the most favour- able conditions, in order to reduce the sense of effort to its lowest degree. G. E. Miiller and Schumann who deny the sense of innervation speak of a voluntary adaptation to resistance which they attribute to the tendency of motor and sensory activities of certain intensities and rhythm to become automatic by habit. This differs little, as Treves rightly points out, from the idea of an education of the impulse and accompanying conscious and primitive gradation of iunervatiou, which these authors expressly denied. The logical conclusion from the whole of this discussion is that voluntary acts are normally regulated by sensations of peripheral origin, which we have considered under the head of muscular sensations, and by those of central origin, which are known as innervation sensations. VIII. In the last chapter when discussing tactile or pressure sensibility we were unable to bring out its full importance from the psychological point of view, because the perceptions and ideas ii SKNSM'.ILITY OF T1IK INTKKXAL OEGANS 107 with which it is connected are nearly always intimately connected with the central and peripheral sensaiions that coincide \\itli voluntary acts. In the same \\ay the preceding remarks on the muscular and innervation senses do not sufficiently em])hasise their psychological importance, hecause in analysing our per- ceptions of movement we cannot separate them from the tactile sensations with which they are nearly always accompanied in life. Bernstein rightly distinguishes hetween -itassive and active tactile sensibility: the former comes into play when a body is brought into contact with, or exerts pressure on, the immobile cutaneous surface, e.g. on applying the two points of Weber's compasses to the skin; the latter when we pass the hand or lingers to and fro over the surface of a body, and move or lift it, so as to discern its form, size, resistance, weight, and other accessory physical characters. This last is the usual application of the tactile sense; but it is plain that in using active touch, and in touching objects, the tactile sensations must be combined with muscular sensations or the sense of movement. Now that we have analysed these sensations separately it will be well to put them together and compare them, the better to understand their nature and relative physiological and psychological im- portance. We have seen that we possess the capacity of localising tactile sensations more or less precisely at different points of the skin, according to the relative number of the touch spots and the higher or lower threshold of excitability in the different regions. Muscular sensations, on the contrary, are very vaguely localised. Generally speaking, we do not feel the contraction of the muscles, which are the active organs of movement, but only the movement or displacement of the limb. It is only on focussing our attention sharply that we succeed in vaguely localising sensation in the joint, or the muscle or group of muscles, that is contracting. Normally we localise the muscular sensation according to the signals received at the same time through the senses of touch and vision; and when these are excluded we localise the motor sensations according to the mnemonic signs which we possess of the visual and tactile sensations by which they are usually accompanied. If, for instance, with the eyes closed we trace figures in the air with the extended forefinger, we are as plainly aware of the tracings described by the end of the finger as if we saw them with our eyes, while in reality the surface of the h'nger- tip receives no impression. The physiological basis of this percept is certainly formed by the central impulses and simul- taneous muscular contractions of the limb which displace the joints in various ways, and must vary with the variation of the angles, straight lines, or curves that we trace in the air with the 108 PHYSIOLOGY CHAP. finger-tip. Why then do we not perceive the figures traced by the finger in our joints and muscles, but in a different and far-off spot which is not the seat of any excitation ? Evidently because from long habit we explore the objects which surround us with our eyes and fingers, so that the memory of the things felt on the skin and the extent of the space seen are necessarily revived each time the contraction of the muscles moves the articulations of a limb, even when we do not see them move, and when the sense of touch is hardly, if at all, excited. This observation is of great importance, because it proves incoutestably that the so-called " muscular sensations," which as we have seen are principally due to the sensibility of the joints, are in themselves only forms of the common sensibility with which all the internal organs supplied with afferent nerves are provided. Accordingly, we are unable to objectify the muscular sensations or transform them into perceptions, without the collaboration, direct or indirect, actual or mnemonic, of tactile and visual sensations. And those authors are wrong who hold the muscular sense to be a special sense, like the tactile sense, the visual sense, and so on. Just as the tactile sense is complementary to the muscle sense, so we may say that muscular sensations reinforce tactile sensations and contribute in developing their capacity of localisation. As Weber pointed out, we may reasonably hold that all our sensations, including the cutaneous, are at first devoid of the power of localisation. They represent simple states of con- sciousness, differing in quality and intensity, but giving no notion of place, and having no local sign. On the strength of certain researches of Preyer on the chick embryo, it may be asserted that in the ontogenetic development of the senses common sensibility in the obscure form of internal bodily sensation appears first; cutaneous sensibility only begins to appear at the tenth day of incubation. Before that it is possible to apply every kind of mechanical, chemical, and electrical stimulus to the skin without evoking the slightest reflex movement ; but at the end of the fifth day the embryo exhibits automatic movements due to internal ex- citations. The same facts were noticed in the mammalia embryo also. So that of foetuses in general, including the human, it may be admitted that obscure muscular sensations precede those of the special senses. This is highly important from the psychological point of view. How does the capacity of tactile and cutaneous localisation develop in the new-born animal? We may legitimately assume that its development is promoted by the activity of the muscles and the resulting muscular sensations. If Weber's tables for the delicacy of tactile sensibility are consulted, it will be found that he gives first place as surfaces of extreme sensibility to the tip of Ji SENSIBILITY OF TIIK INTERNAL OliCANS Id!) the tongue, the red edges of I he lips, and the ends of the lingers (see p. 42). Tliis dominant development <>f tactility in the tongue anil lips is satisfactorily explained it' we admit that the muscular sensations reinforce the po\\er of cutaneous localisation by the fact that amongst the earliest, most important, and most eager movements of the new-horn is that of sucking, which is evoked hy an instinctive, central impulse, and is accompanied arid regulated hy the sensations derived from the activity of the lingual and lahial muscles. The great delicacy of tactile sensihility in the linker-tips and palms of the hand, again, is due to the fact that it is hy exercising the active touch of the hand, accompanied hy various movements of the limhs, that the infant seeks and finds the hreast of its nurse, that the growing child gathers its first experiences from its own hody or from external ohjccts, that the adult, lastly, accomplishes the many actions that enahle him to carry on different manual trades. The development of muscle sensihility and the corresponding improvement in cutaneous localisation take place very slowly in children, judging from the difficulty with which they learn to touch ohjects, direct their hands to a given spot, make their first steps, and so on. In adults, according to Goldscheider's data, the muscular sense reaches a high degree of delicacy owing to the sensibility of the joints. Ami yet when a person with closed eyes is made to imitate with one arm movements that have previously been carried out with the other, the range of the movement being the same but its direction altered, or when the conditions of experi- ment are otherwise changed, there are marked discrepancies between the movement the subject believes himself to be making and that really carried out. This does not agree with the delicacy of discrimination between active and passive movements described by Goldscheider. Other experiments of Beaunis and Stanley Hall demonstrate the normal imperfection of the muscular sense when it acts alone in controlling the direction, range, and rate of a movement. Two symmetrical movements carried out with the two upper limbs, with every intention of making them equal, invariably show a preponderance to right or left according to the idiosyncrasy of the subject, quite apart from right- or left-handedness. When a thread carrying a weight is supported by the finger, there is a sensation of something external to the finger which offers resistance, but this is obviously not an elementary sensation but a resultant of various factors. The discrimination of different weights was proved by Hering to rest on the comparison of differ- ent elementary sensations of tension, position, excursion, and rate of movement in addition to tactile sensibility. This is why weights are better appreciated when they are raised than when 110 rHYSIOLOGY CHAP. they rest upon the motionless hand, as Weber iirst pointed out. Our judgments are based less on tactile sensations than on the complex kinaesthetic sensations by which muscular acts are accompanied. According to Merkel's experiments, when weights of between 200 and 2000 grms. are estimated by counter-pressure on the scale- pan of a balance, the liminal sensibility is about T a -g-th of the whole weight if the linger remains at rest, while if the scale-pan is compressed by voluntary movements it is about -^y-th. The data collected from various authors (Weber, Fechner, Jacoby, Gold- scheider and Blecher, Langlois and Kichet), however, differ too much for any positive value to attach to this experiment. According to G. E. Miiller and Schumann, in raising two weights for purposes of comparison we generally employ the same motor impulse for both weights, and our judgment is based essentially upon the different rate at which they move, since from previous experience we estimate the one that moves faster as the lighter. According to Jacoby, the latent time of a movement is an important factor in judging of weight. For a given weight a given lost time corresponds with a certain intensity of iunervation effort, and if the effort remains constant the latent period is pro- portional to the value of the weight. Another factor in the discrimination of weights, according to Jacoby, is the facility with which the movement can be stopped, which varies according to the weight raised. The analysis of the factors in the judgment of weights made by Z. Treves in his ergographic studies led him, on the other hand, to hold that the object of the judgment is not so much the weight in itself as the intensity of the effort, which is essentially due to two factors, viz. the average muscular tension and its duration. This element of judgment, however, is strictly dependent on the central impulse of innervation, and varies indirectly to the latter. So that the enormous variations and errors that generally occur in this class of observations must be interpreted as the indirect expression of the fluctuations of the motor impulse, which is the expression of a neural act akin in its nature to attention, and is highly unstable and insusceptible to direct control. With the same weight the physical factors on which our judgment is based may vary considerably with the variation of the impulse. And the impulse, Like all voluntary acts, fluctuates widely, even when it is directed to a given end, with known conditions of resistance. Treves proved this directly by showing that these oscillations of impulse occur also in rhythmical movements with a maximal voluntary impulse, and may, particularly in long-pro- tracted work, have the effect of reducing the effort so much as to mask the progressive muscular deterioration. SENSIBILITY OF THE IXTKKXAI, OEGANS 111 The above discussion is necessary to give the student s idea of the dill'crcnt sensory fax -tors of central nr peripheral origin, which necessarily enter into the formation of the so-called active tactile perceptions (in which the cut a neons sensations are associate! I with a preponderance of various kinaesthetic elements) and of the different values ascribed by the physiologists who have studied this dithcnlt subject to the various factors concerned in the dis- crimination of weights. IX. In Chapter VII. of the last volume we discussed the Hind-brain at length as the seat of the organs of subconscious sensations, on which the normal tone of the muscles largely depends. We saw that these subconscious sensations are main- tained by a number of afferent paths which are in direct or indirect relation with the cerebellum and spinal bulb. Of these. afferent paths we emphasised the importance of those represented by the vestibular roots of the eighth cerebral nerves, by which C. FIG. 44. Model of the left labyrinth of human ear; A, from outer side; /:, from inner side; C, from aticive. 'j. (Henle.) s., superior; p., posterior; <., external (lateral), semicircular canal ; a., ampullae ; ,:./., aqueduct of \estibule ; f.o., fenestra ovalis (vestilmli); /.;., fenestia rotunda (cochleae); /., coiled tube of cochlea. the so-called non-acoustic labyrinth is innervated (Vol. III. p. 461). At this point we may discuss the many experimental facts that have been collected with reference to this most delicate peripheral sense-organ, and the various theories put forward for their interpretation. A full account would, however, exceed the limits of this text-book, and we must confine ourselves to discussing the most important to our own point of view. We must begin with a brief description of the anatomy of the Internal Ear or Labyrinth, referring the reader for greater detail to anatomical text-books. The internal ear is morphologically divided into two parts the Cochlea, innervated by the ramus cochlearis, and the Vestibu- lum, consisting of the three semicircular canals, the utricle and the saccnK innervated by the vestibular branch of the eighth nerve. Physiologically, too, this division seems to be justified. (See Vol. III. p. 405.) The cochlea is a later formation than the vestibular organs. In fishes it is quite rudimentary, and is represented merely by the lagtna, which is a small appendage of the saccule. In 112 PHYSIOLOGY CHAP. amphibia it is much more developed, and in reptiles it develops 3V. FlO. 45. Membranous labyrinth of l.'t't -i.l.- >i-cn from \vittiout. (McrkH.) <'., cochlea; !>.<., cluctus coehlrai is ; Sue., sacculi- ; ('//., ntricli'; g., superior; <<., external (m lateial): p., posterior semicircular canal ; ".'., aqueduct nt' vi-~til.nl. ; C.r., ranalis Fio. 46. Diagram of entire human amlituiy nr^an. (Deliicrrc.) 1, auricular lobe; 2, external auditory meatus ; a, tympanif membrane; 4, stapes attached to base of fenestra vestibuli ; 5, bony part of Eustachian tube; 6, its cartilaginous parts; 6', mouth of tube; 7, vestibular cavity filled with perilymph; S, semicircular canals and utricle; ft, promontory; 10, feiipstia cochleae (the arrow indicates tin- tympanic opening of the cochlea) ; 11, tympanic cavity tilled with air ; 12, cochlear duct filled with endolymph, united to saccule of vestibule by a narrow junction canal ; 13, scala vestibuli ; 14, scala tympani terminating in fenestra cochleae ; 15, apex of cochlear canal, where the two walls unite at 15' ; 115, cochlear aqueduct ; 17, vesti- bular aqueduct ; IS, endolymphatic sac ; 19, parotid region. progressively from the chelonians and ophidians to the saurians and crocodiles. It is only in these last and in birds that the II SENSIBILITY OF THE INTERNAL ORGANS 113 cochlea gradually acquires a spiral arrangement. Finally in mammals it reaches its greatest, development in the form of a long twisted tube, with one and a half to four or more spiral turns. The cochlea, with the nervus cochlearis which forms ;i delicate end-organ within it, undoubtedly represents the t" utiicular branch nt" tin; niihtli ii'-rve; h., hair-cells; ;<-?.>., i>crilyiiij)liatic s|iace. cipal divisions of the membranous labyrinth is that of Scarpa (1772), who attributed to the ampulla and utricle the capacity of conducting sound-waves from the bones of the skull, whereas they are conducted by the tympanic cavity to the cochlea. This theory rests exclusively on the anatomical fact that the vestibular labyrinth is in closer relation with the bones of the skull, while the cochlear labyrinth is in more direct connection with a special apparatus for conducting sounds by the air. Duge"s (1838) propounded the far more satisfactory hypothesis that the saccules of the vestibule are excited by noises, that is by irregular sound-waves, and measure the intensity of these, and thus estimate their distance, while the neural apparatus of the cochlea is capable of excitation by musical tones. Helrn- lioltz supported this theory without adducing any conclusive proof of it. It was founded on a phylogenetic concept which led to the assumption that the cochlea, the best developed organ, was intended to convey the finer and more complicated auditory sensations in the higher vertebrates, while the saccules, which i ami tm i >iis. (Sapijry.) ], Circumvallate papilla^ ; -j, foramen caecum ; H, rangiform papillae ; -i, Nlit'onn Mini conical papillae; .^trans- verse ami oblique snlci ; (i, mUCOUS xlanils ami lymphatic t'cillicli-s at l>asr ,,{ touiiin- aihl on - ; 7. tonsils; B, ti]. ufcpi-luttis ; '.i, naninin epiglottidis. The anterior two-thirds of the tongue (Fig. 51) are covered on the dorsal surface, tip, and edges with a mucous membrane richly supplied with papillae visible to the unaided eye. The circumvallate papillae, 7-12 in number, can be distinguished at the border between the two anterior thirds and the posterior third of the tongue, and fonn the lingual V : the fungiform 128 PHYSIOLOGY CHAP. papillae, much more numerous and smaller, are disseminated all 3 h n< . 52. Semi- diagrammatic dia\\iim of section of a fimgilorin papillae , and four conical papillae '', of mouse'-, tongue (Knsaii ami I'anasci); c, taste-tmds ; , four gustatory cells ; h, two gustatory cells and one sustentaculai cell ; c, three siistrntarnlar cells. or peribulbar ramifications). Some authors (Fusari and Panaschi) stated that the gustatory cells are in direct communication with nerve-fibres and thus form the cells of origin of the peripheral nerve- fibres (as occurs for the olfactory mucous membrane, infra). But the latest researches of Retzius and Lenhosse'k confirm the con- clusions of Sertoli (1673) and Krohn (1875), that the nerve-fibres are merely in simple contiguity with the epithelial cells of the taste-bud, and penetrate between and branch round them (Fig. 56). The nerve bundle that penetrates the bud, and the fibres that run to the flat epithelium FIG. 56. TWO isolated gus- of the papilla, come from a very fine sub- tatory cells and one sus- -.i T i i i_ i j_ tentacular c.-ii from a epithelial nerve-plexus which contains numer- to^ffiuSalii 6 "^ ous sma11 cells of a neural character, although them in ciub-shaped bulbs! this has been contested by some authors (Figs. (Arnstein ) / \ o 52, 53). This plexus receives nerve-fibres from a second plexus at the base of the papillae, which is formed from the fibres of the glossopharyngeal and chorda tympani nerves. This second plexus contains ganglion cells of different forms, some of which are in relation with the cervical nerves, others with the sympathetic system. The work of Kiesow and Nadoleczny and of Schlichting has recently confirmed the fact that the chorda tympani contains gustatory fibres. That the taste-buds really represent the. peripheral organs of taste is proved by the fact that they lie in the oral cavity, and in those papillae alone from which taste sensations can be excited, Ill THE SENSE OF TASTE 131 while iu other parts of the buccal and lingual mucous membrane where there is no sensibility for taste, nerve-endings similar to those we have described for cutaneous sensibility are very abundant. These observations are confirmed by the fact brought out by von Vintschsrau and Honigschmied that after unilateral division O ^J - of the glossopharyngeal nerve, which as we saw in the last viilume (III. p. 401) is probably the only specific nerve of taste, niie half of the taste area of the tongue becomes anaesthetic to caste, and after about four months the corresponding buds dis- appear, their elements being replaced by ordinary epithelial cells. All doubt as to the functions of these organs was removed by the p IOt 57, TransM'isr ''ticn i>i i-]ii^lnttis ot'a human foetus at seven months (male). (Kit-sow.) The taste-buds are shown on the surface of the tongue. experiments made by Kiesow, with Hahn, on the taste sensibility of the human larynx. The taste-buds are found not only in the anterior two-thirds of the tongue but also in the mucous membrane of the posterior third as far as the epiglottis, in the portion of the soft palate that lies above the uvula, in the anterior pillar of the fauces, in a portion of the posterior wall of the pharynx, and lastly in the anterior or lingual and the posterior or laryngeal surface of the epiglottis, and on the inner surface of the arytaenoid processes of the larynx (Fig. 57). All other regions of the oral mucous membrane the median part of the dorsurn of the tongue, the lips, hard palate, uvula, tonsils, cheeks, and lower surface of the tongue are normally destitute of taste-buds. 132 PHYSIOLOGY CHAP. Finally, it is worth noting that during development the number of taste-buds continually decreases. In fact it has been demonstrated that in children the median portion of the dorsum of the tongue, as well as other regions of the oral cavity, are provided with taste -buds, and therefore with gustatory sensi- bility. This was first discovered by Urbantschitsch and sub- sequently confirmed by Kiesow, Stahr, and others. Stahr found forms of gustatory papillae in the centre of the infant's tongue that entirely disappear later on. Ponzo made further histological researches under Kiesow, and found taste-buds in the human foetus on the anterior and posterior pillars, as well as in the epithelium that covers the palatine tonsil and the mucous membrane that covers the nasal surface of the lateral parts ution of the end-organs of taste. In investigating gustatory sensibility it is usual to adopt either sapid substances or the galvanic method. Aqueous solutions of sapid substances are brought into contact with the oral cavity by means of small sponges fixed to the end of a wooden or metal rod (Verniere, 1827). Soluble solid substances may be applied directly by a brush (v. Vintschgau). When single l>:i].ill;ie or inter-papillary spaces are to be tested all physiologists n<'\v prefer Oehrwall's method, i.e. the use of tiny brushes with blunt points .-.Unrated with the solution. In testing extensive surfaces diffusion oi 11 u id 134 PHYSIOLOGY CHAP. can ! avoided by employing discs of gelatin or elder pith with a surface of the desired form, impregnated with the solution (Zwaardemaker, Quix). Kiesow stained his fluids slightly, to avoid the errors that may arise from diffusion. In making a methodical research it is well to have a series of substances corresponding with the primitive tastes. Sugar can be used for the sweet tastes, quinine for the bitter, common salt for the saline, hydrochloric or sulphuric acid for the sour taste. Concentrated stock solutions can be kept of all these substances, from which solutions of different strengths can be prepared at any moment. Kiesow (1894) and Hanig (1901) used the follow- ing solutions for their important researches in Wundt's laboratory sugar 10 per cent, hydrochloric acid O2 per cent, quinine sulphate O'l per cent. The mode of application differs according to whether the taste-property of a certain substance is to be tested, or if it is desired to map out the sensi- bility to taste with the oral mucous membrane. In the first case it suffices to pour a given amount (about c.c.) of the chosen solution on the tongue, at a temperature of about 37 C. The subject then withdraw.- his tongue and applies it to the palate, and has to say immediately whether the solution has any taste, and if pn-.-ible, what the taste is. He must not know beforehand what substance is being used. The mouth must be rinsed with distilled water before parsing from one substance to another. The sensation aroused by one substance must have entirely dis- appeared before proceeding to the next. This requires about five minutes. If the titration of the respective solutions is known, it is easy to determine the liminal stimulus, that is the minimal quantity of the substance that can arouse a definite sensation. The above rules and precautions must be observed whenever the taste sensibility in the oral cavity is to be mapped out. In every experiment it is essential to find the liminal value for each individual and for each substance, i.e. the weakest solution that can be appreciated as taste in general or as a specific taste. It should be noted that with increased concentration of the solutions of some substances the quality of the taste alters as well as its intensity. In order to control the veracity of the subject's judgments, and to test his attention, pure distilled water can be used on the brush at the outset or occasionally during the experiment. Physiological investigation confirms our daily experience that the tongue is the principal organ of taste sensibility, but other areas, even beyond the oral cavity, are capable of perceiving taste. In children, as we have seen, the central part of the dorsum of the tongue is gustatory, but ceases to be so after a certain age : we must now discuss the localisation of the taste sense in adults, |' sucking and ..I' liquid aliinenlafion in general. Moreover, the penetration of liquids into the pores of the taste- Imds is t'acilit;ited in adults by tin; mechanical compression of the tongue against the teeth and jaws. But how are we to e\|ilain the different distribution of specific sensihility for the four primitive tastes': 1 No definite answer to this is possible. Kiesow, however, assuming a hereditary tendency, holds that the taste organs found in the tongue are specially adapted to external stimuli. He notes the tendency to keep substances that have a pleasant taste longer in the mouth than those which give a disagreeable sensation, the latter being either spat out at once or swallowed quickly. Kiesow attributes the small difference in the sensibility to salt of different parts of the tongue in most individuals to the fact that the saliva of the oral cavity, at least of its anterior part, is approximately equally distributed and everywhere has the same content of salt. It should also be noted that Wundt sees a close relation between these facts and certain mimic facial movements. III. An exc^ct distinction and classification of the qualitative differences of tastes, i.e. the qualities of the specific gustatory sensations excited by sapid substances in the peripheral organs of taste, is only possible by the sensorial method that is, by sub- jective appreciation of the fundamental or specific differences in the different sensations of taste. As we are almost completely ignorant of the chemical or physical properties which enable certain substances to excite the peripheral organs of taste, we cannot base any classification upon them. In daily life we make a distinction between the sapid sub- stances, based on the affective impression they make upon us rather than on the quality of their tastes : thus we discriminate between agreeable, indifferent, insipid, and disagreeable tastes. Agreeable and disagreeable substances excite different expressional move- ments of the facial muscles ; indifferent and insipid substances produce no facial movements, or at most arouse an expression of indifference or slight disgust. These reactions may be considered as instinctive reflexes, because they are involuntary; they were even noted by Sternberg in an anencephalic foetus. By means of these expressional reactions it is possible even in babies of a few months old and in many animals to distinguish clearly between the sensations aroused by different tastes in the mouth. A sweet^ taste always gives them a pleasurable sensation. even when it is in excess. Other substances, on the contrary, give a disagreeable sensation in concentrated solutions, or are indifferent if very dilute. In the first case the reaction is a movement of sucking or licking ; in the second there are efforts at repulsion and evidences of displeasure or disgust. In adults there is less predilection for sweet things, and a 140 PHYSIOLOGY CHAP. tendency develops to find pleasure in many other simple or compound tastes, acid, bitter, or salt, so long as they are in weak or moderately concentrated solutions. But the more or less pleasant or unpleasant character of different flavours varies according to the individual, and is in no necessary relation to the nocuous or innocuous effects of the various foods. The proverb that "what is good to eat can do no harm" only holds for those food-stuffs that have already been physiologically selected. The affective tone concomitant with the taste sensations must be carefully distinguished from those qualities of taste which we perceive as a property of extraneous sul (stances by which the gustatory surface is specifically stimulated. In addition to the four primitive tastes we have been dis- cussing sweet, bitter, acid, salt we commonly speak of a great many other tastes by specific names, e.g. alkaline, metallic, astringent, acrid, sharp, aromatic, alcoholic, fatty, slimy, dry, etc. Most of these tastes, however, are found on physiological analysis to be compound, i.e. they consist of several elementary constituents, in which the sensations of taste are mingled with olfactory sensations, and with the tactile, thermal, and pain sensations which are well developed in the mucous membrane of the mouth. Chevreul (1824) first noticed the ease with which gustatory sensations become associated with tactile and olfactory sensations, owing to the great delicacy of tactile sensibility in the tongue and the acute olfactory sensibility of the adjacent nasal mucous membrane. Few substances are indifferent to the tactile and thermal, sensory nerve-endings of the tongue, and so entirely destitute of olfactory properties as to be unappreciable by smell. Physiological analysis enables us clearly to distinguish the purely elementary qualities of taste in the innumerable gastro- nomic flavours. It is easy to separate smell from taste ; if the nostrils are closed, the aromatic, alcoholic, or nauseous flavours disappear. The same occurs when olfactory sensibility is abolished by a violent cold. In making a methodical experiment it is well to blindfold the subject as well as to close his nose, to avoid visual suggestion. It is then impossible by taste alone to distinguish the aromas of different wines, of coffee, tea, chocolate, oil and butter, various kinds of meat (Wing, Longet, Beclard), the alkaloids, such as aconitine and nicotine (Richet and Gley), etc. But even after absolute exclusion of smell the perception of the four tastes known as primitive, i.e. sweet, bitter, acid, salt, remains un- changed. The devices for separating thermal, tactile, and pain sensations from those which are purely gustatory are more elaborate. To exclude complication by thermal sensations, which introduce a character of hot or cold, it suffices to warm the test solution to in THE SENSE OF TASTE 141 body temperature. Tactile sensibility can be excluded by apply- ing the substance in solution not as a solid or powder, so as to avoid any mechanical stimulation of the touch spots. To distinguish the sensations excited through the pain organs from taste, it is necessary to compare the effect of the solutions on those parts of the oral cavity that have and have not taste-buds the latter being the interior surface of the tongue, mucous membrane, of the gums, cheeks, hard palate, and centre of the tongue. By this means it is found that the attributes sharp, astringent, oily, dry, etc., as applied to tastes, are due exclusively to the general sensibility of the oral mucous membrane, particu- larly that of the tongue. All these sensory attributes are in i'act produced when the test solutions are applied to the mucous sur- faces that are destitute of taste-buds, but no pure gustatory sensa- tions (sweet, sour, bitter, salt) are thus excited. Zenneck (1839) only admitted two qualities of purely gustatory sensation, sweet and bitter. Valentin (1848) maintained the same. More recently (1883), M. Duval went even further, and asserted that taste sensibility was intermediate between common sensibility and specific sensibility, and that many sensations aroused on the tongue might equally be evoked from other mucous membranes and on certain parts of the skin. On the other hand, the careful observations of Stich (1857) showed that the acid taste is not diffused over the whole of the buccal mucosa, but only when there are taste papillae. Schiff (1867), again, found that where the epidermis had been removed by a blister, the application of solution of sugar, quinine sulphate, or citric acid gave rise to different sensations which bore no resemblance to taste. He also found that any highly diluted acid which produced no sensation on the non- gustatory mucous membrane of the mouth was clearly perceived in the gustatory parts of the tongue. Tick (1864), on the contrary, showed that acids only excite the nerves of pain in concentrated solutions. This agrees with v. Vintschgau's observations of 1880, that salt solutions, sodium and ammonium chloride, and potassium iodide could only act on the non -gustatory mucosa in concentrated solutions, while in dilute solutions they are perceived solely in the taste area. Kiesow (1894) showed that not only salt and acid, but sweet and bitter as well, are accompanied by tactile sensations, which in the highest degree of sweetness may become very intense. From this it may be concluded that acid and salt, as well as sweet and bitter, are true tastes. Nevertheless, Rouget (1875) and Lannegrace (1878) admitted sweet and bitter alone to be true tastes, and held that acid and salt were pseudo-tastes or chemical taste sensations. This refinement, however, has no real foundation. In fact, these authors themselves recognised that " dilute solutions 142 PHYSIOLOGY CHAP. of acid or salt applied to the tongue produce specific sensations which are not aroused from any other sensory surface, and which give particular information about these substances under the familiar name of acid or salt taste." This amounts to saying that acid and salt are true tastes, although, unlike sweet and bitter, they may be associated with general sensations when not sufficiently diluted. Bain, Wundt, and others .admit the alkaline and metallic also among the true tastes. It was long debated whether these two tastes should be admitted among the four generally recognised as primitive, or whether they should be considered as compound sensations due to the admixture of several tastes. Von Vintschgau recognised that the metallic taste excited by electrical stimulation of the tongue is very difficult to analyse, and that in alkaline and astringent tastes common sensibility plays such an important part that it is hard to know if the taste organs also are excited. Oehrwall holds that the alkaline taste results from the combina- tion of many tastes, associated with sensations of contact. The researches of Kiesow and Hober led these authors also to the con- clusion that the alkaline and metallic tastes depend on the associa- tion of several primitive gustatory sensations; that acid and sweet are components in a metallic taste ; and that in an alkaline the general sensibility components are associated sometimes w r ith bitter, sometimes with sweet. Von Frey assumed that alkaline and metallic tastes are mixed sensations with an olfactory component. Herlit/ka showed that the so-called metallic taste is neither a gustatory nor a mixed sensation, but purely olfactory. More recently v. Frey came to the same conclusion in regard to the alkaline taste. Henle distinguished as insipid tastes the impressions produced by solutions that are poorer in salt than the saliva. The taste organs are so accustomed to the action of the buccal secretions that the latter are quite indifferent, as regards taste. But directly the concentration of these fluids is lowered, we become aware of a faint or indefinite sensation commonly known as insipid. A typical illustration of this is the sensation produced by distilled water, which is deprived of carbonic acid. According to Kiesow, a very dilute alkali also has an insipid taste, and a much diluted mixture of salt and sugar is insipid. In conclusion, the usually recognised qualities of taste are reduced to four : sweet, bitter, acid, salt. These are primitive or elementary tastes, because they cannot be further analysed. If we make of substances belonging to each of these four groups solutions that will arouse equal sensations, we cannot distinguish between the sensations aroused, e.g. by hydrochloric, nitric, sulphuric, acetic, and oxalic acid. The same applies to the bitter substances, e.g. strychnine, quinine, digitaline, morphine, in THE SENSE OF TASTE 143 picric acid, as also to sweet solutions, as sugar, glucose, and lactose. IV. The relations between the nature of adequate thermal, tactile, auditory, and visual stimuli and the quality of the sensations aroused in consciousness are well known, but the same cannot be said of the chemical senses, taste and smell, since we do not yet know in what chemical property the capacity of certain substances to act as adequate stimuli for taste or smell consists. We only know a few of the conditions that determine the taste and odour of a substance. It is generally admitted that sapid substances are adequate taste stimuli only when in a state of solution. Formerly, however, it was believed that gases were also capable of directly stimulating the organs of taste (Joh. Miiller, Stich). Carbonic acid has a distinctly sour taste; chloroform, sulphuretted hydrogen, and nitrous oxide are sweetish ; aldehyde and ether vapour are slightly bitter ; acetic acid vapour is strongly acid, and on excluding smell cannot be distinguished from some of the mineral acids. These tastes are distinctly perceptible, even when the substances are brought in the form of gas into direct contact with the tongue after drying it carefully, and closing the nostrils. The direct action of gases was disproved (Valentin, von Vintschgau) by the fact that gases, before coming into contact with the specific taste-endings, are necessarily dissolved in the fluids which fill the pores of the taste-buds. It is consequently indisputable that substances, whether solid, liquid, or gaseous, must, in order to affect the peripheral organs of taste, be in the form of solutions, or be dissolved in the oral secretions. Solubility in the oral secretions, at least to a minimal extent, is therefore an indispensable condition of gustatory stimuli. Compounds of the different metals, at the surface of which there is a difference in electrical potential, and which on contact with the tongue may give rise to electrical excitation of the taste- organs, form an apparent exception to this law. On the other hand, solubility alone is not sufficient to enable a substance to excite sensations of taste. Some gases, e.g. oxygen, hydrogen, nitrogen, never arouse sensations of taste, though soluble in the oral secretions and absorbable. The same holds for distilled water, after carefully cleansing the mouth from the food residues and sapid substances mingled with the saliva. It is more surprising that chloride or sublimate of mercury, to which individual tissues are certainly not indifferent, has no taste in certain concentrations (Sternberg). According to Graham sapid substances belong to the category of crystalloids, while colloids are tasteless. Although it is not easy to control the absolute value of this statement, owing to the difficulty of obtaining colloids in the pure state, and to the fact 144 PHYSIOLOGY CHAP. that they become altered and decomposed on contact with the buccal secretions, the difference in the two classes of substances nevertheless indicates that to be capable of stimulating the organs of taste, all substances must be in a state of true and perfect solution ; colloids only form pseudo-solutions, and exist in fluids in the form of groups of several molecules. As all food-stuffs except the sugars are insoluble in water, and therefore quite devoid of taste, it follows that the gustatory characters by which we are able to recognise food-stuffs in general (proteins, colloidal starches, fats) depend on the minute traces of soluble crystalloid and volatile substances mixed with them. In order that the substances dissolved in water may come into contact with the nerve-endings of the taste-cells, it is necessary that they should first diffuse in the fluid contained in the pores of the taste-buds ; this causes a certain delay in their chemical action, and possibly a certain temporary separation of the active components of the solution, if the rate of diffusion differs. The taste papillae are so arranged that all substances intro- duced into the oral cavity in sufficient amount must come into contact with them. The roughness of the lingual surface owing to the projection of the papillae, and the numerous interpapillary depressions, are conditions that promote the retention of the solid and liquid particles in the interpapillary spaces and the diffusion of the dissolved substances to the gustatory pores. The movements of the tongue increase the surface of contact between the organ of taste and the contents of the mouth. Pick held that these movements increased the excitability of the gustatory nerve-endings by mechanically stimulating the organs of taste. He went so far as to declare that the substances applied to the dorsum of the tongue when it is stationary are scarcely perceived or not at all. But the subsequent researches of Oehrwall and others disproved this statement and showed that the lingual movements merely increase the surface of contact between the active substance and the sense organs. At most it may be said that the pressure of the tongue against the hard palate facilitates the penetration of the fluids into the interpapillary spaces. The salivation of the bolus, for reasons that can readily be appreciated, facilitates the solution, diffusion, and perception of the flavours of solid and semi-fluid substances. The secretion of the salivary glands and of serous glands that open into the fossae of the circumvallate papillae also exert a protective function upon the gustatory organs, because in the case of unduly strong tastes there is a reflex stream of buccal secretion which diminishes their action on the specific nerve-endings. Haller assumed an erection of the lingual papillae in gustatory activity, but this is disproved by the observations of Bidder and Oehrwall. Ill THE SENSE OF TASTE 1-1". V. What is t-lii! physical ur chemical properly that confers ii|u>ii certain bodies the capacity of stimulating the taste organs ( Why do some of them amuse a sensation ol' sweet, others of bitter, others of acid, others lastly of sa.lt ^ Why do other substances that are equally soluhle and diffusible remain inert as regards taste: 1 These are the fundamental questions that arise wlien \ve attempt to determine the relations between chemical structure and the sense of taste. At first sight it does not seem difficult to discover a series of correlations between the chemical composition of bodies and their taste. Almost all acids have a sour taste, many salts a salt taste, a large number of alkaloids are 1 utter, many carbohydrates are sweet. It might reasonably be supposed that some of the properties which enable us to classify these substances into definite and distinct chemical groups represent the cause or constitute the origin of their respective flavours. But this correlation is more apparent than real, because not all compounds chemically known as acids give an acid taste, not all salts a salt taste, not all alkaloids are 1 titter, nor are all sugars sweet. There are bodies of very different constitution from the sugars which arouse a sensation of sweetness, such as the glucosides, saccharine, chloroform, and certain mineral salts, as lead acetate and salts of beryllium. On the other hand, it is interesting to note that one sugar, rf-mannose, has a bitter taste, while many other mineral and organic substances of varying chemical composition which do not belong to the alkaloids have the same bitter taste. Lastly, there are compounds of closely allied chemical composition, with different tastes. Such are the two asj laragines (Piutti), of which the dextro-rotatory is sweet, and the laevo-rotatory tasteless, although they do not differ chemically, but only in their optical properties. These enigmatical facts have so far received no interpretation. The first advance in the correlation of chemical structure and taste was made by the researches of Gley and liichet (1885), who compared the liminal values, as taste stimuli, of the chlorides, bromides, and iodides of different alkaline metals lithium, sodium, potassium, rubidium the atomic weights of which differ in tin- relations of 7, 23, 39, 87, although their chemical properties are very similar. They found that equimolecular solutions were required for liminal excitation of the taste organs, i.e. such as contained approximately the same number of molecules of the twelve ; different alkaline salts, whatever the absolute weight of these molecules. Hence, according to Gley and Richet, it is legitimate to conclude that their physiological action on the organs of taste is a chemical effect, because it takes place according to the laws of ordinary chemical action. Another interesting series of researches into the gustatory property of acids was made by (Jorin (1888). He employed VOL. IV L 146 PHYSIOLOGY CHAP. e(|uimolecular solutions of a. great number of acids with tlie object of determining whether the intensity of the respective tastes is in any relation with their acidity in a chemical sense, that is with the amount of soda they are capable of neutralising. By patient research he arrived at the conclusion that the acidity of different equimolecular solutions of acids is so much the stronger in pro- portion as their molecular weight is lower, and that, generally speaking, the intensity of the acid taste of a molecule of any acid depends on the relation between the weight of the acid hydrogen (i.e. that which can be replaced by a metal) contained in tin- molecule and the weight of that molecule. Another important question was attacked by Hb'ber and Kiesow (1898) : do substances soluble in water, or of which the molecules are capable of ionisation (electrolytes), owe their taste to the free ions or to the molecules that are not dissociated, i.e. are electrically inactive? Can free ions of different kinds give rise to different sensations of taste when they act on the gustatory organs? All these possibilities seem probable in view of the fact discovered by Hober and Kiesow, that a single salt solution may give rise to a whole scrie> of gustatory sensations which differ not only in strength but also in quality. We know by the laws that govern the dissolution or dissocia- tion of salts that in solutions of minimal to medium concentra- tion the cleavage of the molecules into kations and anions, respectively, is complete, and that it is only in more concentrated solutions that non- dissociated elect rie.-illy inactive molecules are present. Hober and Kiesow studied the taste of certain saline solutions in relation to the different degrees of their ionisation, and recognised that the taste sensation aroused as a whole by the solution of an electrolyte (or ionisable salt) is the resultant of a certain number of different elementary taste sensations, which are severally excited by the ions. Highly dilute solutions of alkalis which are completely or almost completely dissociated have a sweet taste ; in stronger concentration they have a characteristic soapy taste, which is probably due to the unsplit molecules. The salt taste of a series of salts (NaCl, KC1, MgCl , (CH 3 )NH ? C1, (C 2 H,)NH 3 C1, NaBr,, Nal, K 2 S0 4 , Na 2 S0 4 ) is due to the anions set i'ree by the dissociation of the molecules. In other salts, on the contrary, the effect of the kations predomi- nates, and there is no salt taste. This is the case with the salts of magnesium, which are bitter, and the salts of beryllium, which are sweet. The work of Hober and Kiesow thus shows that three of the primitive qualities of taste, salt, sweet, and bitter, can be produced by the free ions. As Richard also showed that acidity is excited by hydrogen ions, we may conclude that all four in THE SENSE OF TASTE 147 elementary qualities of taste ran be elicited by UK- action of ions on the gustatory nerve-endings, Hober and Kiesow believed that by comparing the tastes of the solutions of a great number of electrolytes the taste of many kinds of ions and molecules could be determined. I Hit this would not explain the origin of the different tonalities of taste, because nothing is known about the relation of the latter to the peripheral taste organs on which sapid substances act. Still it is shown by the work of these authors that the compound taste of many substances results from the sum of the tastes of their individual components, broken up by the dissociative force of the water. Herlitzka arrived at somewhat different results. On examining the taste of over seventy salts he came to the conclusion that the taste of the salts was due to the free ions and not to non- dissociated molecules, and in confirming the conclusions of Richard and Kahlenberg that the acid taste is due to hydrogen ions, and of Hober and Kiesow that the salt taste is dependent on the anions, he affirms that the elementary kations (excepting, of course, the hydrogen ions) have a sweet or bitter taste, or both together. The taste of a salt would thus result from the conflict between the taste of the anions and of the kations, some salts having only the taste of one, some of the other, some of both. Attempts have also been made to determine the relations between different tastes and the relative position of the atoms and the various groups of atoms in the molecule. We are mainly indebted to Haycraft (1887), Steinberg (1898-1903), and Herlitzka (1908-9) for these interesting researches. Haycraft found that the molecular weight of a substance, even if it affects the intensity of a taste, has no important influence upon quality of the taste sensations. On the contrary, the different groups of Mendeleeffs system show a marked agree- ment in their gustatory qualities. The chlorides of the first group (Li, Na, K, Cu, Eb, Ag, Cs, Au) all have a salt taste, while the sulphates have a bitter-sweet taste. The chlorides of the second group (Mg, Ca, Zn, Sr, Cd, Ba) are bitter-salt, except the salts of beryllium. The chlorides of the seventh group (I, 01, On, Br), again, have a bitter taste. Another fact discovered by Haycraft is that the organic compounds containing the group OOOH have an acid taste; the alcohols, with the exception of the lowest members of the series, are sweet. Sternberg, too, attempted to ascertain the relations between given groups of molecules and certain tastes. He called these groups sapiforous, and included in them the groups OH, NH. 2 , N0 3 , which in various combinations of the organic molecules have different tastes. Herlitzka took up the relations between the taste of the ions and their position in Mendeleeffs periodic system, and pointed 148 PHYSIOLOGY CHAP. out that tliis relation, as expressed by Haycraft, must have been different if he had considered the taste of the salt as a whole, and not that of the kations. Herlit/ka employed Ramsay's scheme to bring out the relation between the taste of the kations and their position in tin- periodic system. From his researches as a whole he was led to formulate the hypothesis that the excitation of the peripheral taste organs by means of the salts is produced (like the excitation of other elements of the body) by a change in the state of the colloids, with alteration of their electrical potential. According to Herlitxka the metallic sensation (metallic taste, more correctly metallic smell) is characteristic only of a small number of salts, all belonging to the heavy metals; and that only when the metal is present in the form of elementary ions, never in the form of complex ions. AVith some metals, moreover, the metallic smell is perceived when they are present in the form of a certain ion, but is absent with forms of ions of a different valency. According to this author the threshold of stimulation for the metallic sensation is extremely low, and varies for the different salts between and ^ - : their solutions not only have no taste, but many of them do not give the characteristic reaction of the respective ions. In these the molecule is com- pletely dissociated. For the salts of the weak acids in which the process of hydrolysis is more active, the threshold is slightly higher. The metallic smell consequently appears to be a property of elementary, dissociated ions. According to von Frey the alkaline taste (smell) depends on the liberation by the alkalis of the volatile bases (methylamine) that result from the products of the decomposition of the epi- thelium from their salts. VI. Special attention should be paid to the fact that the end-organs of taste differ specifically from the other sense organs possessed by the tongue in common with the skin, in being incapable (so far as is known) of excitation by mechanical and thermal stimuli. Among inadequate stimuli of the taste sense we need therefore only consider the action of the electrical current. Sulzer (1752) first noted that on applying two different metals to the tongue a special gustatory sensation resulted which he compared to the taste of rust. Volta (1*792) repeated Sulzer's experiment without knowing of it, and found that the special taste was due to the passage of an electrical current ; in fact he obtained the same effect on stimulating the tongue with the current from his pile. Volta inclined to the opinion that the electricity acted directly upon the taste organ. A few years later Humboldt (1797) suggested that the electrical taste was 111 THE SENSE OF TASTE 149 due to the- products of the chemical decomposition produced in the tongue by the passage of the current. These divergent opinions gave rise to two theories which for a century disputed the effects of the electrical current on the organ of taste. Humboldt's explanation assumed a more definite form when it became known that the passage of an electrical current through a solution of alkaline salts (sucli as the buccal secretion) caused its electrical decomposition, so that acid was liberated at the anode and alkali at the kathode. This fact is sufficient of itself to explain why, on passing a galvanic current through the part of the tongue that is supplied with gustatory sensibility, there is an acid taste on applying the anode (the kathode being placed on the neck or some other part) ; when the kathode is applied to the tongue there is, on the contrary, an alkaline taste. But this explanation is opposed by an ingenious experiment ma,de by Volta and confirmed at a later date (1860) by Eosenthal. If the tip of the tongue is dipped into a small tin vessel filled with an alkaline solution, and held in the moist hands so as to pro- duce a weak current, the acid taste is equally perceptible. If this taste depended on electrolytic action it ought not to appear under these conditions, because the acid would at once be neutralised by the alkali in the vessel. Rosenthal adduced other experiments in support of Volta's opinion. If two persons are in circuit by means of placing the moist hand of one on the positive pole and of the other on the negative pole of an electric battery, and the tips of their tongues are brought into contact, the first will be aware of an acid taste, the second of an alkaline. Both tongues are under identical conditions, being separated only by a thin layer of buccal secretion, and the sole difference is the direction of the current passing through them. How then can there be an alkaline fluid on the one side and an acid on the other at the point of contact of the two tongues? Eosenthal also experimented on the stimulation of the tongue by an electrode formed of red litmus paper, which became blue on contact with the buccal fluid, before the passage of the current. During the passage of the current when the paper acted as anode, the subject perceived the characteristic acid taste, but the paper did not turn red, showing that no perceptible amount of acid was liberated at the anode. Hermann objected to the first two experiments that electrolytic decomposition might take place in the depths of the lingual tissue and not at its surface; for the third it was remarked by Valentin that the taste-endings might lie more sensitive than litmus paper. But there are other points in connection with the electrical taste that claim attention. Bitter (1798) stated that on passing an electrical current for a long time through the tongue, 150 PHYSIOLOGY CHAP. the electrodes being at a certain distance from each other, and on then breaking the current, the original acid taste at the anode lirfame first slightly bitter and then alkaline, while the alkaline taste at the kathode simultaneously became acid. These observations were repeated and confirmed with variations by Hermann and Laserstein (1891), Shore (1892), and Hofmann and Bun/el (1897). Hermann and Laserstein showed that it is not the oscillations in the intensity of a constant current but the current itself which produces the electrical taste. The com- paratively weak effect of induced currents is due to their brief duration. On applying the anode to the tongue there is a decidedly acid taste during the passage of the current; on Apply- ing the kathode the taste becomes alkaline. But the acid taste is more pronounced than the alkaline, and is not neutralised by alkaline fluids. On breaking a weak galvanic current there is an acid after-taste even when the taste has been only slightly alkaline during its passage. Shore obtain. -d similar results. Hofmann and Bunzel stated that when the kathode is applied to the tongue there is a burning sensation during the passage of the current, accompanied by a bitter taste; on opening the circuit there is a faint acido-metallic taste, which is stronger in proportion as the current had lasted longer. According to these authors the primary taste is due to electrolysis, and the acid after-taste at the kathode is a contrast effect, comparable to the phenomena of the same order observed in the visual organs. Certain of von Zeyneck's results (1898) also tend to show that sensations of taste produced by electrical currents are the effects of electrolytic dissociation either of the superficial fluids of the mouth or of the fluids that irrigate the cells of the taste buds. He believed, on the strength of exact electrical measurements, that on first passing a minimal ineffective current through the tongue, and then gradually increasing its potential, the anodic and kathodic sensations of taste appear with the intensity of current that produces electrolytic dissociation. VII. It seems not improbable that there may be cases of complete congenital absence of the sense of taste, such as are known for the other special senses. A priest known to us in our native town declared that he had been unable from birth to distinguish the different flavours in his food, so that the choicest dish or the coarsest food, coffee with or without sugar, quinine or salt, wine or vinegar, were to him alike indifferent. Unfortunately, he died before we had the opportunity of testing by exact scientific methods his ability to taste (and smell), which might then have been controlled by post-niortem examination. Up to the present no clinical case has been recorded to confirm physio- logical inductions as to the specific nerves of taste and their in THE SENSE OF TASTE 151 precise localisation in the pel ipheral taste organs. Such a case would IK- of the highest interest. Partial congenital detects, on the other hand, have hem frequently reported, particularly the total or partial absence of taste at the tip of the tongue. Still more common are individual variations in the localisation of sensibility to the lour primitive tastes, already discussed (p. 134). Not infrequently the distinc- tion hot \veen s;ilt and sour is little marked, but this may possihly depend on the different use of the two terms in the language of different individuals, rather than on their dissimilar capacity of discriminating between the two tastes. Loss of the sense of taste under morbid conditions (ageusia) is not uncommon owing either to peripheral lesions or to central lesions on one or both sides of part or of the whole of the gustatory area. Such paralysis may affect the power of perceiving different tastes in different degrees. Alteration of the specific qualities of taste (parageusia) is fairly frequent in central diseases as the precursor of a more or less complete paralysis of taste. In a clinical case observed by Nagel unilateral paralysis of taste was preceded by a state in which the patient perceived all tastes on the affected side as salt. In hysterical persons partial or total suspension of gustatory sensations is frequent. Kiesow has recently demonstrated that true hallucinations of taste may be present even in persons who are comparatively normal. Keal gustatory or hallucinatory dreams have also been noted (i)e-Sanctis, Kiesow, and others). The temporary, partial paralysis of taste which can be pro- duced experimentally by the direct application of certain poisons to the gustatory organs is also of great interest. Von Anrep (1880), Knapp (1885), observed that cocaine kept for a certain time in contact with the lingual mucosa abolishes all power of taste. Aclucco and U. Mosso (1866), on the contrary, found that solutions of cocaine hydrochlorate, if not too concentrated, inhibit sensibility only to bitter tastes, while for other tastes it is altered little, if at all. Oehrwall, Shore, Kiesow (1894) on repeating the experiment showed that the action of cocaine affects all qualities of taste. Oehrwall, however, recognised individual differences, and Kiesow discovered by accurate determination of liminal excitation that cocaine acts more strongly on bitter, less on sweet, still less on salt and sour taste. Shore arrived at approximately the same conclusion. Von Anrep further showed that puncture by a needle at the edge of the tongue pencilled with cocaine caused no pain. On the other hand, Kiesow brought out the interesting fact that even strong applications of cocaine do not cause the tip of the tongue to lose its sensibility to pain and thermal stimuli. And as cocaine affects gustatory sensibility soon after its application, 152 PHYSIOLOGY CHAP. and the cutaneous sensations later, he employed it to discriminate the tactile qualities of sapid substances, and thus proved that acid and salt are true gustatory sensations. Edgeworth discovered that the masticated leaves of Qymnena silvestre have the property of completely abolishing the taste for sweetness. Hooper (1887) observed more accurately that sensa- tions of sweet and bitter were entirely removed by gymnenic acid. Shore found the action of this acid to be most intense for sweet, less for bitter, still less for salt, and nil for acid. Kiesow con- firmed these results, but found weak action even on the acid taste. He concluded that, the action of cocaine is more extensive than that of gymnenic acid, and that the latter acts on sweet almost as cocaine upon bitter. According to Kiesow, gymnenic acid has no et't'ee.t upon the tactile, thermal, and pain sensibility of the tongue. Fontana (1902), under Kiesow's direction, found that eucaine-.Z? has a similar action on taste to cocaine, i.e.. predominantly upon bitter. Ponzo found that stovaine at a certain concentration abolishes the sensations of sweetness and bitterness, while it is still possible t" perceive salt and acid, though faintly. He also found that the period of anaesthesia is succeeded by a period of hyperaesthesia which is limited to certain gustatory sensations, and which he holds to be of central origin. Stovaine produces this hypergeusia for salt tastes ; cocaine for sweet and bitter. Herlit/ka observed that .^th normal solution of chromium nitrate produces hypoaesthesia for sweet and salt, and in a less decree for bitter: also that cobalt chloride causes gustatory par- aesthesia for a period of 24 hours, during which all fluids taste salt. Artificial changes of the normal mean temperature of the tongue, again, may depress or temporarily inhibit the excitability of the peripheral organs of taste. Weber first noted that on plunging the tongue for 30 to 60 minutes in water at 40 to 42 K, or into iced water for the same time, the sweetness of sugar was no longer perceptible. This observation was confirmed by Guyot, Aducco and U. Mosso, and Kiesow. Kiesow found that 10 minutes' action of iced water or warming to 40-51 C. sufficed to make the tongue insensitive not only to sweet but also to other strong tastes, with the exception of acid, which could still be perceived under these conditions. The temperature of the solutions employed as taste stimuli, again, affects the threshold of gustatory sensibility. According to Camerer (1880) the optimum of the stimulus is between 10 and 20 C. Taste sensibility, on the contrary, according to Kiesow, does not alter within the limits of temperature at which the solutions employed do not arouse decided sensations of heat or cold. Above and below these limits gustatory sensibility diminishes. in THE SENSE OF TASTE 153 VIII. Among the most important facts in the physiology of taste is the observation that none of the four qualities, sweet, acid, salt, bitter, can be subdivided into ut her components. Each taste is an elementary quality that exhibits only quantitative differences; it is not possible to pass gradually from one taste to another, as in the colours of the solar spectrum and the tones of the musical scale; as yet we have no rational criterion for arranging the four tastes in a given order in series. We have seen, in speaking of the topography of taste, that certain substances arouse different sensations according as they are applied to the tip or the base of the tongue, and that in general the sensibility to the four different primitive tastes is differently distributed over the different segments of the taste area. And we have seen the possibility of paralysing one or other of the gustatory qualities by means of specific poisons. These facts collectively suggest, on analogy with what takes place for the different sensitive points of the skin, that the respective taste papillae differ specifically in regard to their excit- ability by the four primary tastes. There is no proof of this in the histology of the peripheral organs of taste, which fails to show any difference of structure between the taste-buds of the papillae, such as it is possible up to a certain point to see in the nerve-endings for cutaneous sensibility. It is not possible to test the individual taste-buds like the specific sensitive points of the skin. In many cutaneous regions these are far apart, while the taste -buds are grouped in large numbers in each papilla. About 400 buds lie in each of the circumvallate papillae ; a lesser number in the fungiform. Each bud, moreover, represents not a single nerve-ending, but a bundle of sensory elements, so that it is not possible, as in the skin, to excite each element individually. Still, it is interesting to excite the fungiform papillae separately, to see if each of them reacts equally to the four different tastes, or particularly or exclusively to one or two tastes alone, according as the specifically dissimilar nerve-endings conjectured are contained in each in an equal or unequal degree. This research, which is the application to taste of the method of Blix for the skin, was undertaken successfully by Oehrwall, a pupil of Blix (1891), on the fungiform papillae of the tip of the tongue ; on these it is easy to carry out a series of methodical experiments continued over several days upon the same group of papillae, previously marked and numbered, so that they can be easily recognised without any confusion. Highly concentrated solutions were employed (40 per cent sugar, 5 per cent tartaric acid, 20 per cent sodium chloride, 2 per cent hydrochloric, acid), and brought into contact with the ends of the single papillae by 154 PHYSIOLOGY CHAP. means of a tine brush, the point of which was smaller than the papillae. The sodium chloride solutions were soon abandoned, because the resulting sensations were not sufficiently strong and distinct. To accelerate the signalling of the sensations, the subject was forewarned of the quality of taste to be excited. Oehrwall investigated the gustatory sensibility for sweet, bitter, and acid on 125 papillae. He began by showing that the mucous membrane between the papillae was insensitive to tastes, and further found that 27 out of the 125 papillae examined did not react to the test solutions. The 98 papillae accepted as gustatory reacted as follows : To acid, 91. Exclusively to acid, 12. To sweet, 79. Exclusively to sweet, 3. To biti-r, 71. Exclusively to bitter, 0. To sweet and acid, 72. Exclusively to sweet and acid, 12. To bitter and acid, (IT. Inclusively to bitter and acid, 7. To swcd, bitter, and acid, 00. sum up we ni;ty s;iy that of the OS gustatory papillae the reaction was : To acid but noi t<> sweet, 19. TII Mveet luit not In acid, 7. To acid luit not to liittcr, 24. To bitter but not to acid, 1. T" >\Veet but 111)1 tn bitter, 15. To bitter but not to > \\ei-t, 7. r'nuii these results we may conclude (a) that not all the fungi form pa.pillae of the apex of the tongue are gustatory ; (&) that not all the gustatory papillae are sensitive to all the three tastes investigated; (c) that many (38 to 98) do not react to one or two of the tastes. We must also bear in mind that there is another difference between the various papillae though it is more difficult to estimate it exactly; not all the papillae that are sensitive to a taste appreciate it with the same intensity; the reactions to each taste may vary in strength in the different papillae. Consequently the differences between one papilla and another are not merely qualitative but are quantitative also. Oehrwall also experimented by the electrical stimulation of isolated fungiform papillae with a brush electrode. He found that an acid taste only appeared in the papillae which reacted to acid. He failed to arouse a sweet or bitter taste, because in order to avoid electrolytic effects he employed the induced current, with which there is a sensation of warmth and of vibration that disturbs the experiment. With the constant current Oehrwall always obtained an acid taste at the anode and a sensation of warmth in the papillae that were sensitive to acid. At the kathode he obtained a bitter or sweet taste as well as a sensation of in THE SENSE OF TASTE 155 heat. Ho also found thai all i In- papillae investigated, including the non-gustatory, were sensitive to tactile and thermal stimuli. Goldseheider and Schmidt also tested the papillae \\itli solu- tions of quinine and sugar mixed; they sometimes obtained a sweet taste only from one papilla and hitter only on another. Kiesow's researches on the same subject were both a control and a continuation of those of Oehrwall. An important differ- ence between the methods of the two observers is that Oehrwall's subjects were aware of the nature of the substance employed, while Kiesow kept them in ignorance. The substances used were solutions of sodium chloride (which Oehrwall gave up), sugar, hydrochloric acid, and sulphate of quinine. The hydrochloric acid was in 0'2 per cent solution, the others in almost saturated solutions. The results practically agree with those of Oehrwall, and show that the greater part of the papillae investigated do present marked functional differences. Of 39 papillae examined, 4 gave no reaction to any of the four substances. The other 35 (excluding doubtful reactions) gave the following results : 18 reacted to salt. 3 to salt exclusively. 26 reacted to sweet. 7 to sweet exclusively. 18 reacted to acid. 3 to acid exclusively. 13 reacted to bitter. to bitter exclusively. This table shows that of the 35 taste papillae 9 did not react to sweet. 17 salt, 17 acid. 22 bitter. This confirms the fact already brought out with another method by Kiesow and Ha'nig, that at the tip of the tongue sensi- bility is maximal to sweet and minimal to bitter, the reverse of what is observed at the base of the tongue. Kiesow further noted the interesting fact that within the small space of a single fungilbrm papilla four senses may be represented taste, touch, pain, and temperature; the sense of taste and the thermal sense can, moreover, be present in different qualities of sensation, as sweet, acid, warm, cold. Kiesow also observed effects of peripheral fatigue wliich Oehrwall neglected. And lastly, he found that in dealing with these minute gustatory surfaces it was often difficult to distinguish between the salt and the acid tastes, as he had previously noted in his experiments upon children. These results of the effects of isolated excitation of separate gustatory papillae seem to afford direct evidence that the different qualities of tastes are based on a specific differentiation of proto- 156 PHYSIOLOGY CHAP. l>l;isni iii the cells of the taste -buds, or of the nerve-endings distributed to them. To explain the fact that some papillae react to a single taste, others to two, others to three, others again to all the tastes, we must admit that the qualitative differentiation of the gustatory sensibility of the epithelial cells or the nerve-endings of the taste -buds is variously developed in different papillae. The specifically dissimilar nature of the peripheral taste organs also causes their dissimilar reaction to toxic substances. Oehrwall, on the strength of these results and of the fact that the four primary tastes are discontinuous qualities which cannot be arranged in series like musical tones of different pitch, came to the conclusion that the four tastes cannot be considered as different qualities of one sense, but are different modalities, i.e. four distinct senses. In the same way the sensations of heat, cold, and pressure, which were formerly considered to be different qualities of one form of sensibility (the tactile sense), are now recognised to lie different modalities of distinct senses. But the phenomena of contrast and compensation observed between different gustatory sensations, as between different colours, seem to contradict this theory of the plurality of gusta- tory senses, since they show that the tastes are intimately related to each other as different qualities of one modality of sensation. Johannes Miiller noted that after masticating the root of the aromatic calamus milk and coffee taste sour ; that sweet things take away the flavour of wine, while cheese increases it. Oehrwall, however, failed to confirm the first statement; generally speaking, he found that bitter did not increase the sensibility to sweet. On the other hand, he showed that sweet did not increase the sensibility to acid, but considerably depressed it. The observation of Aducco and U. Mosso, that when a dilute solution of sulphuric acid acts on the tongue for 5 to 10 minutes it alters the organs of taste to such an extent that distilled water is perceived as a very sweet fluid, is a more obvious contrast phenomenon. If a dilute solution of quinine sulphate is applied instead of distilled water, there is a sweet sensation at the tip of the tongue, and the bitterness is only perceived at its base and lateral edges. Solutions of formic, citric, and acetic acid do not act like sulphuric acid ; hence the action of the latter is not exclusively due to its acidity. Kiesow made a special study of contrast and showed that after excitation of the tongue with weak solutions of hydrochloric acid and salt distilled water is perceived as sweet. Laserstein saw that after the action of a 1*5 per cent solution of soda distilled water seems to be sweet. Nagel also found that on washing out the mouth with a solution of potassium chloride (which produces a faint taste of indefinite character) the gustatory in THE SENSE OF TASTE s are altered in much the same \v;iy as by suljilmric acid, so that pure water tastes sweet. Another interesting fact was noted hy /unt/ and Heymen^ I" the effect that a solution of sodium chloride and quinine, so weak as not to arouse any distinct taste, is still sufficient to exaggerate the taste of a solution of sugar. There is at present no really satisfactory solution of these facts, which correspond more or less to contrast phenomena, but they do not appear to favour Oehrwall's theory, but rather to support that of a single gustatory sense. Kiesow, moreover, holds that Oehrwall's theory is contrary to direct experience, and brings out the psychological fact that the different gustatory qualities, however they may differ among themselves, have none the less something in common which distinguishes them collectively from every other category of sensation. In defence of his theory that the four elementary tastes correspond to four distinct sense-organs, Oehrwall also questions the phenomena of compensation. He holds that on mixing two or more tastes (except when new chemical compounds arise from the mixture) it is not possible to form a new taste, and that the taste of the ingredients can always be recognised in the mixture. Brticke expressly stated that some gustatory sensations are able to compensate each other respectively, without thereby reciprocally neutralising the stimulating substances. But the instances he adduced are not convincing, nor are they comparable. with the results obtained by mixing two complementary colours which neutralise each other and yield the sensation of white. Sugar compensates or corrects the bitter taste of coffee and the acid taste of lemonade, but not in the sense of producing a new taste. Both tastes persist, and there is a mixed and more agree- able sensation. Sensations of contact, again, may obscure or modify the affective tone of a sensation, e.g. mustard and pepper frequently do so. The clearest example of a compensatory effect in gustatory sensation is mentioned by Kiesow, who, on mixing weak solutions of sugar and salt in a certain ratio, obtained an insipid alkaline taste which recalled neither sugar nor salt. If more concentrated solutions are mixed the phenomenon of compensation is no longer apparent. If a mixture of two substances is taken into the mouth, one strongly sweet, the other bitter, various sensations are per- ceived at different times and on different spots of the tongue, sun ic 1 litter and others sweet. Kiesow saw that on combining the majority of primitive tastes in certain proportions, but always very dilute (sweet and salt, sweet and acid, sweet and bitter, salt and acid, salt and bitter, acid and bitter), they are respectively diminished in different 158 PHYSIOLOGY CHAP. degrees, but it is difficult completely to abolish the two component tastes. It is by these effects of compensation of different tastes, as well as by the varied association of gustatory with tactile, thermal, and olfactory sensations, that the flavour of nauseous medicines is corrected and the different ingredients of food materials are so combined as to convert the four primary tastes into innumerable complex flavours. To conclude, the effects of compensation of tastes support the doctrine by which all tastes are considered as different qualities of one and the same modality of sensation. BllJLIOGRAPHY General Monographs on Taste, comprising earlier and recent literature : V. VINTSCHGAU. Hermann's Handbuch d. Physiol., 1879. E. GLEY. Art. "Gustation," Dictionnaire em-yclopedique d. sciences medicales. Paris (no date). .MAIHHANH. Le Gout. Bibl. intern, de psycol. expur. Paris, 190:'.. N. VASCIDK. Art. "Cimt," Dictionnairu dr physiologic, Ch. Richel;, 1906. (Most complete monograph on Taste, comprising all literature between 1754 and 1905.) H. ZWAAKHKMAKKI:. Ergcbuisse der Physiologic, ii. Wiesbaden, 1903. W. NAGEI.. Nagel's Handbuch der Pliys. d. Mi nx lien, 1905. Important Experimental Contributions are to be found in the following works : R. SCHIRMEK. Inaug. Diss. Gryphiue, 1856. R. SCHIKMKK. Deutsche Klinik, xi., 1859. KICK. Lehrbuch der Anat. und Pliysiol. d. Sinnesorg., 1864. RICHET and GLEY. Compt. rend. Soc. de Biol., 1SS5. ADUCCO and U. Mosso. Giorn. R. Accad. med., 1886. GOLIISCHEIDEU and SCIIMIHT. Zentralbl. f. Physiol. iv., 1890. HERMANN and LASKKSTKIN. Zentralbl. f. Physiol. iv., 1890. NAGKL. Zeitschr. f. Psychol. u. Pliysiol. d. Sinnesorg. x., 1892. KIESOW. Wundt's Philos. Studien, ix. x. xii., 1894, 1896. HOBEU and KIESOW. Zeitschr. f. physikalische Chemie, xxvii., 1898. HOBER and KIESOW. Arch, per le scienze mediche, xxiii., 1898. DE-SANCTIS. I sogni, 1899. STERNBERG. Zeitschr. f. Psychol. u. Physiol. d. Sinnesorg. xxii., 1899. ROLLETT. Archiv f. d. ges. Physiol. Ixxiv., 1899. WUNDT. Vblkerpsychologie, i. 1, 1900. KIESOW and NADOLECZNY. Ibid. i. 1, 1900. KIESOW and NADOLECZXY. Arch. ital. de biologie,.xxx., 1898 ; xxxvi., 1901. LUCIAXI. Fisiologia dell' uomo, 4th ed. iv. KIESOW and HAHN. Ibid, xxvii., 1901. OEHRWALL. Skand. Arch. f. Physioi. ii., 1891 ; xi., 1901. STAHR. Zeitschr. f. Morphologic u. Anthropol. iv., 1901. HAENIG. Inaug. Diss. Leipzig, 1901. HAENIG. Wundt's Philos. Studien, xvii., 1901. FONTANA. Giornale della R. Ace. di Med. di Torino, vol. viii., year Ixv., 1902. FONTANA. Zeitschr. f. Psychol. u. Physiol. d. Sinnesorg. xxviii., 1902. WUNDT. Grundziige d. physiol. Psychologie, lii., 1902. HOEBER. Zeitschr. f. Psychol. u. Physiol. d. Sinnesorg. xxxiii., 1903 ; xxxvi., 1904. SCHLICHTING. Zeitschr. f. Ohrenheilkunde. xxxii. in THE SENSE OF TASTE Pow,.i. Giornale d. K. Ace. ill Med. s. I'sychol. xiv., 1909. l\n-n\v. Atti del IV Congresso intern.
  • ', falx cerebri ; 10, cerebral hemispheres; 11, right orbit surrounded by orbital fat. with i-ye-muscles ; 12, great ala of s]ihcnoid ; 13, sphi'iio-maxillary cleft ; 14, adipose tissue of x\.noiiiatic fossa; !;">, biiccinalor muscle; Iti, last molar; 17, vault of palate ; 18, zygoma ; ]'., left orbit. physicians diagnose many eruptive diseases at once by their odour; by it wine and oil merchants know the good and bad qualities of their stock-in-trade. Wardrop tells of a man born blind and deaf who distinguished his acquaintances by their smell. I. The specific olfactory sensory region consists of a limited portion of the mucous membrane of the nasal fossae. Seen in transverse section (Fig. 62) the nasal fossae appear as an irregular VOL. IV M 162 PHYSIOLOGY CHAP. triangle, the apex being represented by the roof, and the base by the floor of the nasal cavities. The median septum and the floor are smooth, while the lateral walls are subdivided into three irregular cavities or meatuses by the three turbinate bones. The posterior ethmoid cells open into the superior meatus, the frontal sinus, the median ethmoid cells and the maxillary antrum into the middle meatus. The functional use of these bony cavities and their communications with the rnuc'ous membrane of the nasal fossae is quite unknown. The nasal cavities are divided into two regions: an upper, known as the olfactory region, and a lower or respiratory region. These can be distinguished by the eye, owing to their colour. In the first the mucous membrane is yellowish (locus luteus), in the 11 XII FIG. 63. \ei\es nt" nns.il septum, seen from rijjht side. \. (Sappey, from Hirschfeld and Leveille.) I, olfactory bulb; 1, olfactory nerves passing through foramina of cribriform plate, ami desc-eiidm;.' t<> ! distributed on the septum ; 2, internal or septa! twig of nasal branch of ophthalmic nerve ; M, naso-palatine nerves. second reddish (Schneider's membrane). Between the median septum on the one hand and the upper and middle turbinals on the other there is only a small fissure, the sulcus olfactorius or olfactory groove. The respiratory region has a ciliated epithelium and numerous acinous glands, while the olfactory region is covered by an epithelium that has no hairs and is provided with tubular glands. Fibres of the trigeminal nerve are not only distributed all over the respiratory region of the nasal mucous membrane, but also send branches to the olfactory region. The olfactory region is the part to which are distributed the fibres of the olfactory nerve which take origin in the bulb of the same name, traverse the pores of the cribriform plate of the ethmoid bone, form a thick plexus with narrow elongated meshes, and end in the mucous membrane of the upper third of the septum, and the pars olfactoria of the upper turbinal (Figs. 63, 64). It was formerly believed that the olfactory region also IV THE SENSE OF SMELL 163 extended over a part of the middle turbinal (Schwalbe), because the yellow portion above described is more extensive than the olfactory epithelium proper, and covers, particularly in the foetus .mil in 'w-horn animal, a certain portion of the middle turbinal as well. But the work of Max Schultze and the measurements of von Bruun showed that the region innervated by the olfactory nerve is confined in adults to a portion of the upper turbinal and of the septum. Von Brunu carried out his investigations on two adult subjects, aged from thirty to forty. He made sections of the nasal mucous membrane, and was thus able to determine the true \tension of the olfactory epithelium. In the first subject the 12 Flo. 64. Nerves of outer wall . (Left.) Oils ..I tin- i.lt'artmy u-^ion. Highly magnified. (M. Sehultze.) 1, from the frog; 2, from man: n. epithelial cell, extending into a long ramified process; b, olfactory cells; c, their peripheral processns ; r. their extremities, seen in 1 to be prolonged into fine hairs ; d, their central filaments. FIG. 66. (Right.) An olfactory cell, human, (v. Brunn.) n, central process prolonged as an olfactory nerve-fibril ; b, body of cell with nucleus ; p, peripheral process passing towards the surface ; c, knob-like termination of peripheral process ; 7i, bunch of olfactory hairs. view that the olfactory sense has its seat in the cerebral ven- tricles, and that odorous particles reach it through the foramina of the cribriform plate, was first questioned at the end of the eighth century, when the Greek monk Theophilus Protospa- tarius recognised the olfactory nerve as the organ of smell, by means of which the odorous vapours are carried to the brain during inspiration, and the superfluous moisture is given off in expiration. As evidence that the olfactory nerves are the specific nerves of smell Schneider adduced an observation by the Bolognese anatomist, Eustachio Budio, who in 1600 claimed to have known a youth IV THE SENSE OF SMELL 165 \\lin was destitute from birth ol' any sense of smell, ;unl in whom the post-mortem examination revealed absence of the olfactory nerves. Diemerbrok and Mery attributed the capacity of per- ceiving smell to the nasal branches of the lifth nerve as well, but without convincing the majority of physiologists, by whom the function is attributed wholly to the olfactory surface. Bellingeri (1818) and Cloquet (1828) supported this view. Magendie, on the contrary, sought by numerous publications 1^1824-41) to revive the earlier view of Diemerbrok and Mery, and stated that no positive proof was forthcoming to show that FIG. 67. Diagram of the connection of cells and fibres in the olfactory bulb. (E. A. Schafer.) oJf.c., cells of the olfactory mucous membrane ; ulf.n., deepest layer of the bulb composed of the olfactory nerve- fibres, which are prolonged from the olfactory cells; gl., olfactory jloineruli, containing arborisations of the olfactory ner\ e-libres and of tin- dendrous of thf mitral cells ; //i.e., mitral cells ; a, their axis-cylinder processes paisin.u towards the nerve-fibre layi'i-. n.tr., of the bulb to become continuous with fibres of the olfactory tract; these axis- cylinder processes an- seen to give off collaterals, some of which pass again into the deeper layers of the bulb ; n', a nerve-fibre from the olfactory tract ramify in.", in the grey matter of the bulb. the other nerves to the nasal inucosa (sensory branches of the trigeminus) did not participate in the function of smell. But he evidently interpreted as effects of olfactory sensation the reflex acts that can be excited in dogs deprived of the olfactory nerves, by means of irritating vapours capable of acting on the tactile and sensory nerves of the nasal mucous membrane. This was demon- strated by Eschricht, Bell, Bishop, Joh. Miiller, Duges, and Ficht. The two last observers, who had no true olfactory sensibility, were susceptible to the excitation due to the vapours of acetic acid, ammonia, and the like, which provoked sneezing. Bidder, Wagner, Longet, Vulpian, pronounced against Magendie's opinion ; 166 PHYSIOLOGY CHAP. Malherbe, Giannuzzi, and Claude Bernard in favour <>r it (Bernard on the strength of a dubious clinical case). The experimental results and clinical observations of Valentin, Schit'i', and Provost in our opinion leave no room for doubt that the olfactory nerve is the exclusive nerve of smell. II. The specific olfactory surface is well protected by its remote position against pathological processes as well as inadequate stimuli; on the other hand, it is easily accessible to adequate stimuli, i.e. to such as can arouse olfactory sensations, save \\hen a nasal catarrh or any other circumstance closes the olfactory groove. Odorous substances may reach the nasal cavities and the olfactory surface in two ways: by the nostrils, penetrating with the air introduced during inspiration, and by the clioauae, with the air expelled during expiration. In both cases it is necessary in order to produce a |iriveption "I' smell that the air-current shall reach the olfactory surface. When an odorous substance is brought under the nose, there is no sensation of smell, even when the nostrils are open, so long as the breath is held or breathing per- formed through the mouth. Even in ordinary quiet respiration the olfactory sensations are not always very plain, particularly with weak-smelling substances. To obtain clear sensations, it is necessary to breathe deeply, or better to make rapid, short, and repeated inspirations by sniffing. It is doubtful whether this act is accompanied by active dilatation of the nostrils (Bidder, Fick, Valentin) or whether they are not more or less tightly closed (Bell, Diday, Funke, Braune, Clasen, von Vintschgau, and others). Diday, to bring out the importance of constriction of the nostrils in sniffing, observes that after forced dilatation by the introduction of a glass tube into the nose, almost every olfactory sensation ceases on breathing in an odoriferous substance. Apart from this purely secondary question these facts show that in ordinary respiration air that penetrates the nostrils does not enter by the olfactory groove, while in sniffing some at least of the air does reach that region. A. Fick showed by a very simple experiment that the anterior portion of the nostril is more important in the function of smell than the posterior. When a rubber tube connected at the other end with a vessel containing an odorous substance is introduced into the nose, no smell, or at most a very slight odour, is per- ceived, if the mouth of the tube is directed towards the middle or lower turbinal. If, on the contrary, it is turned towards the roof of the nasal fossa, in the direction of the olfactory groove, a per- ceptible sensation of smell is obtained. If the posterior portion of the nasal aperture is stopped olfactory acuity remains intact ; if, on the contrary, the anterior part is blocked the sense is consider- ably weakened. iv THE SENSE OF SMELL 167 This explains the interesting observation of Beclard, that persons who have lost the projecting portion of the nose by disease or injury have no sense of smell. In such cases the air passes directly through the choanae, without ascending to the olfactory region. In order to determine the path by which the air -current normally passes through the nasal cavities, Paulsen (1882) in Ivxner's laboratory performed an interesting experiment on the head of a human corpse. He sawed through the cranium in the middle line to expose the nasal fossae, and then applied small pieces of red litmus paper to different regions of the nasal mucous membrane at short distances from each other; after this In- reunited the two halves of the skull. He then set up artificial respiration through a bellows of approximately the same capacity as the lungs, which he attached to the trachea, and passed air containing ammonia vapour through the nostrils, when the litmus paper turned blue in the parts of the mucous membrane over which the ammonia passed. The results of these experiments were quite clear. The reaction of the litmus paper showed that the inspiratory air- current describes a curve in the nasal cavity, first passing upward, and then turning towards the choauae. Moreover, the air which penetrates through the anterior portion of the nostril rises higher than that which enters by the posterior portion. On reversing the direction of the current, i.e. when the air charged with vapour is driven from the choanae to the nostril, the result was quite different ; the curve described by the current of air follows a somewhat lower level than in the previous experi- ment. Zwaardemaker, Franke, and more recently Danziger and Eethi, essentially confirmed the results of Paulsen, although they varied his method in different ways. Zwaardemaker used the 1 faster cast from one-half of the nasal cavity of a horse in which the septum had been replaced by a glass plate. A glass tube was inserted into the posterior part, and the soot of a petrol lamp placed in front of it blown through by means of an aspirator. This could lie followed by the eye, and it was seen that the region innervated by the olfactory nerve remained free from soot. Franke sawed through a human skull in the middle line, stained the whole of the mucous membrane black, replaced the nasal septum with glass, and blew white tobacco smoke through the nostrils by a bellows, which showed up well through the glass partition on the black ground. This experiment, like the preced- ing, showed that in ordinary quiet breathing the air inspired through the nostrils did not reach the olfactory region, but described a curve over the middlf nieatus, and middle and upper part of the septum. The observations made by Kayser on the living subject also 168 PHYSIOLOGY CHAI>. agree with the above. He caused a Hue magnesia powder to be aspirated, and then, with the rhinoscope, investigated the parts of the mucous membrane to which it specially adhered. Since then it is proved that the iiispiratory current as such does not reach the olfactory region, it is clear that the odorous molecules cannot be carried thither by the stream of air, but must reach it by some other means. We know from Bloch's work that the temperature inside the nose is above 30 , so that Zwaardemaker's hypothesis that odours penetrate to the sensory end-organs of smell by a process of gaseous diffusion remains the most probable. The act of snitting which draws the air-current higher into the nasal cavities must undoubtedly facilitate the diffusion 'and penetration of odours into the olfactory region. Bidder assumed that smells can only be perceived during inspiration, and denied that they can be carried to the olfactory region during expiration as well. But Paulsen's experiments showed that the expiratory current takes the same curved path as the iiispiratory, only running somewhat lower, which may impede, but cannot hinder, the diffusion of odours in the olfactory region. On the other hand, it is easy to show that odorous substances breathed in through the mouth and breathed out through the nose may give rise to distinct olfactory sensations. That these are weaker than those excited by inspiration through the nose is sufficiently explained by the fact that the odorous substances inhaled through the mouth must pass through all the air-passages, where they may be partially absorbed before being brought into contact by the expired air with the olfactory region. Again, during mastication of solid food and particularly during the deglutition of alimentary boluses and of fluid, the vapours and odoriferous particles exhaled by the foods and beverages may on passing through the choanae above the soft palate reach and excite the olfactory surface during expiration. This fact is important, because it establishes the intimate relations and associations between the senses of taste and smell which we dis- cussed in the last chapter. The mechanism 1 y which the olfactory sense is excited during a meal depends principally on the fact that at each act of swallowing the soft palate is suddenly raised, on which the air saturated with odorous exhalations is driven from the choanae towards the olfactory region at a pressure, according to Tick, of about 30 cm. water. When deglutition is completed, there is a deeper expiration than usual, and the air of the pharynx charged with the odours exhaled by the food is driven through the choanae. The olfactory sensations thus aroused can, as Chevreul showed, be easily eliminated if the nostrils are kept closed with the finger. Nagel rightly pointed out that the appreciation of smell through the choanae is of higher biological importance, particularly iv THE SENSE OF SMELL 169 for man, than that which takes place through the nostrils. Man, in fact, does not snuff up his food while he eats it, like the animals that have an elongated nasal aperture placed near the buccal orifice. Man uses the sense of smell (in combination with taste) much more during mastication and deglutition than during the act of putting the food into his mouth. The popular theory that smell is the sentinel of the respiratory apparatus, as taste is of the digestive apparatus, cannot be accepted unreservedly in the light of physiological experience. We are protected from breathing noxious air by the nasal branches of the trigeminal, and not by the olfactory nerve. Irritating gases, even when they are capable of arousing olfactory sensations, are specially perceived by the sensory branches of the nasal mucosa. The chief importance of smell is, in association with taste, to perceive the quality of foods, to influence their selection, to excite appetite, and reflexly to promote the digestive secretions. The suppression of smell is dangerous to man because it disturbs all these functions, and not because he becomes incapable of enjoying the perfume of flowers or the aphrodisiac exhalations of certain secretions. III. As in the sense of taste, the adequate stimuli for smell are chemical in their nature, and the odoriferous substances must come into direct contact with the olfactory surface before the olfactory end-organs can be excited. The earlier opinion that odours may act at a distance upon the olfactory organ, by special aerial or ethereal undulations, as do sound and light, is now wholly abandoned, and has no foundation. The fact, as pointed out by Longet, that odours can be carried by the wind to a distance of several miles, in itself, according to Zwaardemaker, proves the corpuscular theory of smell, and excludes the possibility that they can be due merely to vibrations. The number of substances capable of exciting smell, that is of giving out odours, is certainly very great, and many bodies that seem to us to have no smell are odorous for certain animals ; this is due to the limited development of our sensibility. Even if all the substances that are volatile, or dissociable into the finest particles, are not odoriferous, at any rate for man, it may still be said that the most penetrating and characteristic odours we know are given off by volatile substances. Certain other substances that are normally non-volatile and inodorous give off odours under certain conditions. Arsenic, e.g., which at an ordinary temperature has no smell, gives off' a strong smell of garlic on heating; resin and many metals become odorous under friction. Accordingly, it is often held that both under ordinary conditions and under special physical influences an atmosphere of minute particles emanates from the surface of many bodies, and that these may be pcrn-ivcd ly their scent, if not by 170 PHYSIOLOGY CHAI-. man, at any rate by other animals, when they reach the olfactory area of the nasal mucous membrane along with the inspired air. Tourtual (1827) stated that odours are only perceptible in the gaseous state. E. H. Weber (1847) gave support to this view by a number of experiments. He bent his head so far back that the nostrils were directed upwards, and then injected water into the' nasal fossae so that the olfactory region should be as full as possible. He found that when the water had run out, the function of smell was lost for 30 seconds, and then returned gradually, but did not become normal again for i'! minutes. A solution of sugar had the same effect as pure water. The injection of water scented with eau cle cologne produced a smell at the first moment of injection; but all olfactory sensation disappeared when the nasal cavity became full ; on emptying it smell was abolished for a time, as on injecting pure water. Valentin (1848) confirmed Weber's results, and found that on emptying the nose of the injected water the tactile nerves of the nasal mucosa recover their activity before the olfactory nerves. Fn'ihlich (1851) obtained much the same results. The loss of olfactory sensibility thus produced depends, according to Weber, on the saturation of the olfactory epithelial cells of Schneider's membrane with water, which checks their function. But it is more correct to suppose that the injection of plain water, particularly at a low temperature, alters the epithelium of the nasal mucous membrane, and causes a nasal catarrh which is sufficient of itself to produce diminution or total inhibition of olfactory activity. According to Aronsohu (1886), Weber's theory that odours are imperceptible in a watery solution is erroneous. On Kronecker's suggestion he substituted a solution of physiological saline for the pure water douche, adding an odoriferous substance and raising the temperature to 38 C. He used 0-5 c.c. oil of cloves in 250 parts of saline at 38, and was able to smell it, on filling the nasal cavities by a nasal douche apparatus, for 30-40 seconds. Temperatures above the normal (38-44 C.) are more favourable than lower temperatures, perhaps because they increase the excitability of the olfactory nerves. Aronsohu also experimented with camphor, eau de cologne, cumarine, and vanilla. The degree of dilution of these odours required to evoke a definite sensation, arid also the concentration required for solutions to reach the threshold of excitation, vary. The indifferent (isotonic) solution of sodium chloride is 0-7-0-75 per cent, preferably 0-73 per cent, which corresponds with the fact discovered by Eumsberg that the tissue fluids contain 0-62- 0.73 per cent sodium chloride. Sodium chloride may be replaced by other salts, each of which has an optimum degree of concentration which is indifferent to iv THE SENSE OF SMELL 171 tin- excitability of the olfactory cells. I!' the osmometric equivalent (i.e. that which carries odours) of sodium chloride = 1, then that of sodium carbonate =2, that of sodium sulphate = 4, that of sodium and magnesium phosphate = 6. The most important fact discovered by Aronsohn is that these salt solutions which have been regarded as odourless have each their own more or less definite smell. Vasehide came to the same conclusion. Nagel, Haycraft, and Zwaardemaker all disputed Aronsohn's statements. Zwaardemaker objected that it is impossible to expel the air completely from the upper part of the nasal cavity by Aronsohn's method. If any bubble of air is left the odorous substance will be exhaled into it, and may excite the olfactory surface in the form of gas. According to Zwaardemaker the question must remain undecided till experiments on the dead subject have proved the possibility of completely filling the nasal cavity. Veress set out to solve the problem on these lines. Before experimenting on the living body he made careful studies on anatomical subjects, using the right nostril of a human head sawn through in the middle line. In this way he was able to make direct observations on the path of the fluid introduced into the nose, and further sought to determine which position of the body was most favourable to complete filling of the nose, and what amount of fluid was necessary. He obtained good results from a posture at an angle of more than 35. In his experiments on the living subject he tilted the head to postures of 50 to 80. He also tried to reproduce possible pathologico-anatomical modifica- tions, such as displacement and thickening of the middle turbinal, variations in the olfactory groove, etc. He pointed out that errors may arise also from the mucus that covers the walls of the nasal cavity, since this may contain 1 nibbles of air that are not removed until the mucus itself has been expelled : and he imitated the mucus in his preparations with a thick solution of gum. After these preliminary studies on the dead body, Veress set to work on the living subject. He discovered an error in Aronsohn's method, owing to the fact that the dbrsum of the nose formed the lowest part of the nasal cavity. Veress, on the other hand, by bending the upper part of the body forward, obtained a position of the head in which the olfactory surface really lies lowest. Although in certain positions 1 c.c. of fluid is sufficient to cover the olfactory area, he used so much that the excess ran out of the nostrils. He also examined the effect of an indifferent solution of sodium chloride at body temperature upon the olfac.tory end-organs, by first filling the nose with it, and thru replacing this by a similar solution containing the odorous substance to be investigated. Veress attributes great importance to the inlluenceof temperature, 172 PHYSIOLOGY CHAP. which is eliminated by his method. The substances examined were : eau de cologne, ylang-ylang, essbouquet, oil of cloves, oil of origanum, oil of peppermint, camphor-water, caproic acid, and caproic acid with addition of piperidine. Veress found that even when pain was avoided by careful tilling of the nasal cavity, sodium chloride specifically excited both the olfactory end-organs and the endings of the trigeminal nerve, and further pointed out that the sensibility of the olfactory area to this iiuid is altered after a strong bath. Veress speaks of symptoms similar to those that occur in coryza, which in his opinion come under the category of olfactory and gustatory sensations. As to the effect of the odoriferous substance contained in the saline, Veress says that when the two iluids are completely mixed there is a coin]Miund sensation which cannot lie accurately defined, in the production of which both the respiratory and the olfactory areas participate. In this compound sensation, according to Veress, the tactile sensations predominate, and the gustatory sensation is weak. That the olfactory area proper is really concerned in it can be controlled by the fact that its sensibility is diminished after a bath. If, for instance, when all the fluid had been removed from the nasal cavity the subject was still able to perceive the odour, Veress considered the experiment a failure, since it was doubtful whether the olfactory groove had been entirely filled. For this reason he questions Vaschide's results, because no appreciable diminution of olfactory sensibility appeared in his experiments. After much practice Veress succeeded in distinguishing some odorous substances from others, and divided them into different groups. Thus, for instance, it was difficult to distinguish eau de cologne from ylang-ylang, camphor from oil of peppermint, oil of cloves from oil of origanum, while it was easy to say if the exciting substance were oil of cloves or ylang-ylang, camphor or oil of origanum, oil of peppermint or caproic acid. But he pointed out that it was to some degree possible to identify the group to which any substance belonged, by means of its action on the mucous membrane. He compared this ability to recognise the odorous substances with that by which a man born blind recognises through his tactile sensations certain qualities of external sensa- tions which a normal individual is incapable of knowing by touch, and further claims that associative processes may take part in this act of recognition. Veress came to the general conclusion that an odoriferous substance brought into contact with the olfactory organ in the form of fluid may be regarded merely as a heterologous stimulus for that organ. Veress observes that we cannot speak of an olfactory sensation in aquatic animals in the sense in which we use it of mammals : iv THE SENSE OF SMELL 173 he refers to the work of N'agel, and contends that the experiments made by Aronsohn on fishes are not above criticism. Aronsohn offered to fishes ants' eggs dipped in clove oil or tincture of asafoetida, and saw that they retreated from the food, even if still several millimetres away from it. From this he argued that the olfactory organs were excited, but Veress thought it equally probable that the retreat was due to excitation of the tactile organs. Notwithstanding these elaborate researches, we are hardly justified in asserting that all classes of fishes are entirely unpro- vided with the sense of smell, especially as the olfactory organs are so highly developed. If it were so we should have to assume that the olfactory cells of fishes have functions other than those in air-breathing animals. Further, it is evident, as pointed out by Johannes Mtiller, that the essential part of an olfactory sensation lies not in the gaseous nature of the odorous substance, but in the specific sensibility of the olfactory organs, and in their differentiation from all other sense-organs. On the other hand, there are direct observations, the earliest of which date back to Aristotle, that tend to show that fishes possess a sense of smell which is specifically distinct from all other sensations. Milne Edwards points out that sharks often come from afar to devour the carcases thrown into the sea, and that other fishes of the same class show distaste for food that gives off odours. Other authors, on the contrary, including Nagel, agree with Veress in denying that fishes and aquatic amphibia have any sense of smell comparable with that of terrestrial animals. The question seems to us to be decided by the experiments of v. Uexkiill on Selachians (1894). He took certain specimens of Scylliutn that had been deprived of food for some time, and extirpated the olfactory mucous membrane of the nasal fossae in some, leaving it intact in others. He found a difference in the behaviour of those which had and had not been operated on. The latter, shortly after food had been placed in their tank, either loose or in a bag, became very restless and began to swim in search of it. According to v. Uexkiill washing the hands in the tank after touching sardines was enough to throw the intact fishes into a state of excitement. Those operated on, on the contrary, seemed quite unaware of the presence of food, even when it was placed close to them. These experiments seem to establish the existence of a sense of smell at least in Selachians. Other experiments by v. Uexkiill show it to be quite distinct from the sense of taste, as he found that normal dogfish will take a sardine covered with quinine sulphate into their mouths, but immediately reject it. Con- sequently, it is not taste but smell which guides them in seeking 174 PHYSIOLOGY CHAP. their food ; and they reject the unappetising morsel, not by smell hut by taste. Accordingly, even if Aronsohn's experiments on man were carried out by an imperfect method, the conclusion he arrived at, that the olfactory sense can be excited by odorous substances dissolved in fluid, agrees well with what is known for tishes. Very few researches have been made on the olfactory organ with inadequate stimuli. Among these the electrical current alone has given some positive, even if doubtful, results. Yolin failed to observe any effect of an olfactory character; Ritter obtained a special sensation similar to that aroused on looking at the sun or sniffing up tobacco (excitation of tactile and pain sense). He subsequently noted near the kathode the sensation felt before suee/ing, and occasionally a trace of ammoniacal odour : at the anode, on the contrary, there was sometimes a sensation of acid which may have been due to spread of current to the taste-buds. Mure interesting results were obtained by Althaus from a patifiit affected with bilateral paralysis of the trigeminal nerve. On applying strong galvanic currents to the Schneiderian membrane he obtained a smell of phosphorus. Aronsolm made a number of investigations by his method, passing a current through the nasal fossae filled with an isotonic solution of sodium chloride at 3S . Different olfactory sensations were aroused according as the anode or the kathode was applied. The kathodic smell occurred during the closure of the circuit, the anodic at the opening. The kathodic smell was regularly stronger than the anodic. The quality of the two sensations, which approximate to the gustatory impressions, was indescribable. When an odoriferous substance in solution was employed, its characteristic smell was altered by the electrical current. According to Valentin it is possible by mechanical stimulation of the nostrils to produce unpleasant olfactory sensations which last for some time. But other observers failed to obtain any results. Thermal stimuli arouse no olfactory sensations, even when the nasal fossae are filled with fluid at or at 50 C. IV. At present we know little of the chemical and physical properties which a substance must have in order to be an adequate stimulus of the olfactory end-organs. We are wholly ignorant of the correlation between the physico-chemical constitution of a body and the quality and intensity of the odours it is capable of arousing. Some substances that differ greatly in chemical con- stitution have much the same odour ; on the other hand, some substances that are chemically allied have a very different smell. Haycraft (1888), Passy (1892), and Zwaardernaker (J895) brought out some interesting facts in relation to this intricate subject. In the periodic system of Mendeleeff and Lothar Meyer iv THE SENSE OF SMELL 175 tlu 1 elements that form odoriferous compounds belong almost exclusively to the fifth, sixth, and seventh groups. The Hfth group contains nitrogen, phosphorus, vanadium, arsenic, niobium, antimony, didymium, tantalum, bismuth. The sixth group consists of oxygen, sulphur, chromium, selenium, molybdium, tellurium, wulfranium, uranium. The seventh group consists of fluorine, chlorine, manganese, bromine, iodine. It is undeniable that many of these elements form odoriferous com- pounds, and that a certain periodicity in the appearance of odorous and non-odorous substances exists within each series. Another interesting fact is that in some series of homologous o o chemical compounds, e.g. in those of the fatty acids and the alcohols, there is a regular and continuous change in the odour. It is particularly remarkable that the lowest members of these homologous series have very faint smells, and that the intensity of the smell continuously increases in higher members (formic, acetic, propionic, butyric, valerianic, caproic acid, etc.). In the highest members the series of odours is interrupted ; stearic acid, e.g., has no smell. Another series of regularly changing smells consists of benzol, toluol, xylol, etc. Undue importance was given in the past to the so-called odoroscopic researches of B. Prevost (1799) on the physical quality of odours. He observed that many odoriferous substances assume a characteristic rotary or vortex movement on the surface of water, which he interpreted as the effect of the discharge and diffusion of odorous particles into the atmosphere. Liegeois brought forward other odoroscopic phenomena, but expressly noted that they only appeared in substances of vegetable and animal origin, while those of mineral origin show no movement on contact with water (e.g. ammonia, hydrogen sulphate and phosphate). On the other hand, he found that some completely inodorous substances, such as sulphuric acid, potash, and soda, exhibit the same phenomenon. Obviously these "odoroscopic phenomena " afford no explanation why odorous substances excite the organ of smell : the movements are due to the surface tensions of the different compounds, and are not a specific property of odours. Tyndall also showed that the vapours of odorous substances possess a remarkable power of absorbing thermal rays, but it is very doubtful J if it is owing to this property that they are odorous. Erdmann's researches on the solubility of certain essential oils (cedar, rose, geranium) in liquid air are extremely interesting. In comparison with other chemical compounds, these odorous substances have a very high specific solubility in liquid and possibly also in gaseous air. It is probable that this property is common to all odorous substances and that it is one of the 176 PHYSIOLOGY CHAP. conditions in virtue of which they are able to excite the olfactory end-organs. V. The qualities of odours are extraordinarily numerous. No one, as Nagel justly points out, can say that they know all the substances capable of exciting specifically distinct sensations of smell; many people are not acquainted with certain very characteristic odours familiar to chemists, e.g. formaldehyde, picric acid. We cannot as a rule recognise the components in a mixture of many odours. It is also possible to make a gradual transition from one to another of two very different odours by a series of mixtures, in which the two components are present in different proportions. In this respect smell differs very markedly from taste, in which, as we have seen, there are few specifically distinct qualities of sensation, so that the components are easily recognised in any mixture. Granting all this, it is not surprising that we have as yet no true scientific classification and scale of odours. We are not even able to distinguish the different qualities of odours by different names, and to express them we employ the names of the vegetable or animal substances from which they emanate. Lastly, we cannot differentiate odours into elementary and compound. It has, however, been attempted by different methods to classify odours in certain groups or categories. Haller proposes to arrange them in three groups, according as they are pleasant, unpleasant, or indifferent : odores suaveolentes, odores inter mediae, odores foetores. The first class includes particularly the ethers and essential oils : among the foul smells are certain gases of very simple composition (sulphuretted hydrogen, carbon bisulphide, certain hydrogen carbides, etc.), as well as certain decomposition products (indole, skatole, etc.). But it is impossible to distinguish the two classes sharply from one another and to determine the odours belonging to the intermediate class, because these dis- tinctions are based exclusively on subjective appreciations which vary considerably in different individuals. Moreover, some gases, e.g. chlorine, bromine, iodine, ammonia, which have a bad smell when concentrated, are, on the contrary, indifferent or even pleasant when suitably diluted. At first sight the classifications of odours proposed by Frohlich seems better. As the nasal mucous membrane is supplied by two pairs of nerves, the olfactory and the nasal branches of the trigerninal, the first of which alone is the specific nerve of smell, while the second serves touch, temperature, and pain, the sensations generated here must also be placed in two categories, i.e. those resulting solely from excitation of the olfactory nerve, and those which are due to excitation of other sensations as well. The former are pure olfactory sensations, e.g. those produced by ethereal oils, resins, balsams, etc., which never give rise to reflex iv THE SENSE OF SMELL 177 movements; the latter o\\v ilieir origin not merely to stimu- lation of the olfactory nerve but also to that of the nasal branches of the trigeminal, as by chlorine, iodine, bromine, nitric acid, ammonia, oil of mustard, rape, etc., which always produce reflex movements. But if the fact is more closely investigated, it is seen that very few odorous substances excite pure olfactory sensations. Nearly all, when they act with a certain intensity, affect not only the olfactory sensibility but also the general sensibility of the nasal mucous membrane. Thus, oil of juniper and of bergamot and even camphor, which Froblich considered to be purely olfactory substances, irritate not only the nasal mucosa but the conjunctiva of the eye as well. In order to give some notion of the innumerable qualitative varieties of odours the classification of (purely 'olfactory) odorous substances into nine groups proposed by Zwaardemaker may be reproduced : I. Class : Odori eterei (Lorry) (a) Essences of fruits used in perfumery (apple, pine-apple, pear, etc.). (6) Beeswax. (c) Ethers, aldehydes, ketones. II. Class : Odori aromatici (Linnaeus) (a) Camphoric odours (camphor, borneol, patchouli, rosemary, eucalyptus, turpentine). (6) Odours of drugs (clove, ginger, pepper). (c) Odours of anise and lavender (menthol, oil of fennel, arnica, thymol, chamomile). (<1) Odours of lemon and of rose (palisander, sandal-wood, cedar- wood, etc.). (e) Odour of bitter almond (hydrocyanic acid, benzole and salicylic aldehyde, nitre-benzol). III. Class : Odori balsamici (Linnaeus) () Odours of flowers (jessamine, syringa, lilies of the valley, orange- blossom, acacia, etc.). (6) Liliaceous odours (iris, nan-issus, hyacinth, violet, mignonette). (c) Vanilla odours (benzoin, balsam of Peru and Tolu storax, cumarine, heliotrope). I V. Class : Odori aiidmxtnci (Linnaeus) (a) Odours of amber. (6) Odours of musk (nitro hiilyltoluol, ox-bile, many animals, some fungi). \ . ( 'lass : Oi.h> r I agliacei (Linuaeu>) (a) Sulphuretted hydrogen, hydrogen carbide, vulcanised rubber, asafoetida, gum ammonicum, ichthyol. (6) Arsenuretted hydrogen, phosphoretted hydrogen, trimethylamine. (c) Chlorine, bromine, iodine, quinine. VI. Class : Odori e/n/i/r> u maiici (Hallen (") Odour of roast co flee, toasted bread, tobacco smoke, pyrocatech in, guiacol, creosol, acrolein, pindiue. (1>) Odour of ainylic alcohol and homologues, benzol, toluol, xylol, phenol, creolin, iiaphthalin, naphlhol. VII. Class : Odori caprilici (Linn n (a) Caproic acid and homologues, cheese, sweat, putrefying bone.-, ' (6) Cat's urine, vaginal secretion, spermatic fluid, chr-i nut (lour. VOL. IV N 178 PHYSIOLOGY CHAP. VIII. Class : Odori repugnanti (Linnaeus) (a) Narcotic odours of Suhmaceae, henbane, etc. (b) Odour of luigs, ul' ozoena. IX. Class: Odori nauseanti (Linnaeus) (a) Odour of carrion. (b) Faecal odour (scatole). VI. Delicacy of smell, or the power of perceiving slight differences in the intensity of odours, is often distinguished from olfactory acuity, or the capacity of distinguishing minimal amounts of odorous substances. But the two expressions may be used indifferently, because acuity practically coincides with delicacy of smell. Olfactory acuity differs very much for different odours ; it is measured by determining their liminal values. To find the liminal value Valentin (1855) placed small quantities of odorifer- ous substances in a large glass vessel of known capacity, and approximately determined the minimal quantity required to render the air contained in the flask capable of stimulating the olfactory end -organs; or he mixed the odoriferous fluids with large amounts of water, and then tested by smell the minimal dose of odorous substance that could be appreciated. By these methods he found, e.g., that the minimal perceptible amount of essence of roses is 1/200,000 mgrm., of tincture of musk 1/2,000,000 mgrm. Fischer and Penzoldt (1887), and Passy (1892), made further experiments on the olfactory acuity to different odours, and perfected the methods employed by Valentin. Passy dissolved the substances in alcohol, and from the stock solutions made very weak dilutions, of which he poured a small drop into an empty litre flask, and then tested by sniffing at the mouth of the flask whether the odour were perceptible. All experimental errors in this research tend to raise the threshold of excitation. The following figures, however, give some idea of the extraordinary delicacy of smell for certain odours : mgrm. per litre of air. Essence of orange . . 0.00005 0.001 Essence of wintergreen Rosemary . Ether . " . Camphor Heliotrope . Cuniine Vanilline . Natural musk 0.000005 0.0004 0.00005 0.0008 0.0005 0.004 5 0.005 0.1 0.05 0.05 0.01 0.05 0.0005 0.01 0.00005 Artificial musk (trinitrobutyltoluol) . 0.00001 0.000005 Fischer and Penzoldt made an interesting experiment on the olfactory acuity of man. They tried to determine the minimal perceptible amount of mercaptan in the air of one of the rooms IV THE SENSE OF SMELT, 170 in the laboratory. They found that 1/23,000,01)0 mgrm. of this substance diffused in a litre of air ga\r a feeble but quite distinct olfactory sensation. When we reflect on this extraordinary sensi- tiveness in the rudimentary human olfactory organ, \M- <-;m obtain some idea of the enormous olfactory acuity of certain animals in which the olfactory mucous membrane is not limited to the nasal cavity but extends as far as the frontal and sphenoid sinuses. We owe to Zwaardemaker the invention of a practical method which facilitates quantitative research into the acuity of smell. He gave the name of olfactometry to the investigation of olfactoi \ sensibility for odours in general, and of odorimetry to the measure- ment of the comparative sensitiveness to different specific odours. FIG. 68. Indiarubber olfactometer connected with a Marey's tambour. (Zwaardemaker.) Con- sists of the olfactometer tube, which runs inside a tulir of vulcanised rubber. By pulling this out more or less a different extent of the odoriferous surface is exposed. The tambour records the moment at which inspiration begins. It is connected with a branch nf the olfactometer tube by a small receiver which holds pure water, to prevent the smell of the rubber tube of the recording apparatus from affect in.n the subject. In ordinary examinations of olfactory acuity the recording apparatus is no! used. As early as 1888 he invented a very simple apparatus, the olfacto- meter, which consists of a graduated glass tube (10 cm. long, 5 mm. wide internal diameter) which runs easily inside a second tube coated on the inner side with some solid odoriferous substance, e.g. vulcanised rubber (Fig. 68). The curved end of the glass tube is introduced into one of the nostrils. If the outer tube is entirely covered by the inner glass tube, no smell is perceived on sniffing through the latter ; but if the glass tube is drawn out so that a greater or less surface covered by the odorous substance is exposed, an odour is perceived on sniffing through the olfactometer ; its intensity increases with the area of the surface exposed. Zwaardemaker proposes as the unit of qualitative measurement the sensation obtained when the rubber cylinder is exposed for 180 PHYSIOLOGY CHAP. a length of 1 cm. ; he found that this stimulus is on an average the minimum perceptible stimulus of the olfactory organ under physiological conditions, and called this unit an olfactie. But on testing the olfactory acuity of a number of individuals by the olfactometer, it is found that they vary considerably even under apparently normal conditions. In odorimetric investigations, by which it is sought to establish the olfactory qualities of different substances, Zwaarde- maker replaced the rubber cylinder by cylinders of porous clay previously steeped in solutions of the odoriferous substances (Fig. 69). The liminal value of 1 cm. length of tube of course varies for each substance, as this figure is merely the unit of minimal stimulus, or olfactie, for the smell of india-rubber. VII. The problem whether smell, like the other senses, is represented centrally by a number if specific energies is peculiarly difficult, because we are not yet able to make any systematic classification of the infinite number of odours, nor to dis- tinguish elementary from com- F.o.69. Olfactometerwithporousclavcylinder pound odours, as we can ill the which can be saturated by various odoriferous case ot taste. But even if it IS Huids. (Zwaardemaker.) The j^lass oitarto. metric tube runs inside the ] mi oils tube, which is contained within a wider ^lass tube. The test solution is introduced by a pipette through a small hole (afterwards closed by a scieu tap) into the .space between the ;_:lass tube. The cut veil end of the tube is passed into one nostril. screen prevents the odoriferous substance from penetrating to the other nostril. under existing scientific conditions to enumer- ate the specific energies corn- tube and "the outer surface ..I the porous prised in the range of olfactory sensations, this does not ex- clude us from assuming gener- ally that a certain number of specific energies must exist at the olfactory centres to enable us to perceive or recognise the quality of odours. Some interesting facts can be adduced in support of this view, and may now be briefly recapitulated. In the first place we must draw attention to the cases of partial anosmia, congenital or acquired. Some normal individuals, while possessing a well- developed sense of smell, are unable to perceive special odours. Blumenbach states that many people cannot perceive the scent of mignonette, while their sense of smell is perfect for all other odours. Joh. Mtiller recognised this partial defect of olfactory sensibility in himself. To him the scent of mignonette was merely a grassy smell. Cloquet, Mackenzie, and Eeuter noted cases of anosmia limited to the vanilla group. It is recorded that other normal individuals could not smell violets. Generally iv THE SENSE OF SMELL 181 speaking, cases of partial congenital anosmia are rare; but it may be doubted whether they are so in reality, or whether the absence of smell for certain odours has not been overlooked or left un- diagnosed, and only discovered accidentally. The Knglish chemist. 1). H. Nagel was unable to perceive the specific odour of cyanic acid, which resembles that of bitter almonds, and found the same in several of his students, though their smell was normal for all other odours. Cases of partial anosmia after illness are more frequent. Zwaardemaker draws attention to the alterations of smell after diphtheria and influenza ; in certain cases, which he investigated, the anosmia does not extend to all odours. Sensibility to certain */ smells appears to be abolished, to others it is merely weakened, to others unchanged. Winkler in his neurological clinic observed a tabetic who exhibited almost complete anosmia for the smell of benzoin, though he recognised the smell of musk. In the same clinic another patient could not smell musk, but perceived benzoin better than other odours. Parosmia and subjective or hallucinatory smells also have a certain importance in regard to the question of the specific olfactory energies. Joh. Mliller describes a patient who constantly complained of bad smells, and the post-mortem examination showed that the arachnoid had ossified in several places, and there were areas of softening in the cerebral hemispheres. A. Dubois knew a man who after a fall from his horse had for many years till his death the sensation of a foetid odour. Many physicians have observed patients who had a constant sensation of a smell of burning, similar to that produced by lighting wooden matches. Other patients complain of a persistent smell of faecal odours. It is important to note that these hallucinatory sensations are perceived most distinctly during the inspiratory act and on sniffing, as if the activity of the perceptive centre was aroused by the olfactory substances introduced with the air to the peripheral organ. We may exclude Ludwig's suggestion that the subjective sensation of faecal odours in some patients depends on reabsorption into the blood of the products of intestinal putrefaction, which directly excite the olfactory centre. Zwaardemaker proved, in fact, that olfactory hallucinations may be associated with complete objective anosmia to the odours perceived subjectively. Generally speaking, olfactory hallucinations are rare. Smell is seldom represented in dreams (Brillat-Savarin, De Sanctis, Kiesow and others), although the two last believe it is more common than is generally supposed. This agrees with the fact that it is difficult, even by a strong effort of will, to evoke memory images of the commonest smells, as we can easily recall visual, and particularly acoustic and musical, sensations. But that in certain cases 182 PHYSIOLOGY CHAP. olfactory memory images can be called up is amply proved by the researches of Kiesow on the so-called spontaneous representations. Xot all the sensations of which patients, particularly hysterics, complain can be regarded as hallucinatory, merely because they are not perceived by normal individuals. In many cases they depend on a hyperosmia or abnormal lowering of the threshold of olfactory sensibility, in consequence of which odours not normally perceptible can be detected. But there was undoubted hallucina- tion in the case of a hysterical woman who was aware of an unpleasant taste of menstrual blood some time before the com- mencement of menstruation. The partial temporary anosmia that can be artificially produced if the olfactory apparatus is fatigued by prolonged exposure to different strongly odoriferous substances is of great importance in the classification of odours. We know that smell is easily fatigued by long-continued exposure to odorous substances. Anatomists who are in the dissecting room for long periods finally cease to notice the cadaveric odours; patient^ with foetid wounds or suppurations cease to smell the foe tor that disgusts their nurses ; those who cure furs or work in drains become accustomed to repugnant smells, and fail to perceive them. Aronsohn showed that very strong odours depress the activity of the olfactory apparatus in a few minutes, and that after exhaustion a certain time, at least 1-3 min., is necessary to restore excitability. On sniffing tincture of iodine the smell was appreciated only for 4 minutes, balsam of copaiba for 3-4 min., camphor 5-7 min., ammonium sulphate 4-5 miu., turpentine 5 min. Zwaardemaker obtained more exact results with his olfacto- ineter. He constructed curves of progressive fatigue of the olfactory organ, when excited by odorous substances of constant intensity over a regularly increasing number of seconds. The measure of fatigue is indicated by the progressive rise of the threshold of excitation, i.e. the minimal stimulus perceptible after repeated stimulations of increasing duration. Fig. 70 shows four curves of olfactory fatigue, two obtained with rubber (at a strength of 10 and 14 olfacties), and two others with benzoin (intensity 3 '5 and 9 olfacties). The first glance shows that the threshold rises, owing to fatigue of the olfactory sense, with the duration of excitation, and the more rapidly according to the strength of the stimulus. On comparing the tw r o curves obtained with rubber and the two with benzoin, it is seen that the latter causes fatigue far more rapidly than the former, although the intensity of the stimulus was less. This olfactory fatigue or exhaustion observed after sniffing odorous substances for a certain time does not extend to all IV THE SENSE OF SMELL 183 odours, but generally assumes the more or less definite characters of partial anosmia. A methodical study of this interesting subject might show the best way to solve the problem of the classification of odours according to their specific energies. But the results so far obtained have not corresponded with these expectations. Among the experimental researches in this direction, those of Frohlieh and of Aronsohu promised important results. Both 01 fin iis 1 , _L 10 20 30 60 7O oo FIG. 70. Curve of olfactory fatigue. (Zwaardemaker.) The liniinal values are marked in olfactics on the ordinates ; tin- dm-aticm nf stiinulal inn in seconds nn tin- aliscissac-. The four curves (two of caoutchouc of 10 and 14 olfaeties ; two nl' ben/nin of :!..">. and '.' olfarties) show a more or less marked rise in the liminal value allei sucecssixe stimulations of inci -easing duration. Tin- length of tlie nlfactory st imulat inns is regulated by a metronome which marks seconds. The subject lakes a deep breath even two seconds. fatigued their sense of smell by a sp'ci;il odour, and then sought to determine the odours for which olfactory sensibility was still normal, or had been diminished to the same extent as for the odour experimented with. Theoretically, different specific energies must be assumed for the first, and identical energies for the second. Frb'hlich's researches led to no unequivocal practical con- clusions ; Aronsohn's, on the contrary, while carried out by a less exact method (he took no account of the intensity of the different odours) led to some results that deserve mention. He found that 184 PHYSIOLOGY CHAP. after fatigue by tincture of iodine he was able to distinguish ether and ethereal oils naturally, and oil of cedar, turpentine, bergamot, and cloves somewhat less distinctly ; his smell was, on the contrary, considerably blunted for alcohol and copaiba balsam. After fatigue by copaiba balsam he was alile to distinguish ethereal oils, ether, and camphor. On the other hand, after losing his sensibility to camphor, he could no longer smell eau de cologne, oil of cloves, or ether. The results of fatigue by ammonium sulphide were more surprising: sensibility remained perfect, or almost so, for ethereal oils and cumarine, but was absolutely lost for sulphuretted hydrogen, hydrochloric acid (7 drops in 50 of water), and bromine (1 in 1000). Aronsohn concluded from this that ammonium sulphide, sulphur- etted hydrogen, and the halogens form a single class of odours with the same specific energy. He concluded that different qualities of odours affect different parts of the olfactory nerve. Zwaardemaker and Nagel carried out similar experiments. If two odorous substances that do not affect each other chemic- ally, e.g. cumarine and vanilliue, in aqueous solution, are mixed in such proportions that tin- scent of vanilla alone is perceptible, then, after exhausting the sensibility of tin- olfactory organ to the latter, the odour of cumarine alone remains perceptible on sniffing the mixture. This result leads to the conclusion that different specific energies underlie the two odours named, although in Zwaardemaker's classification they belong to the same class, even to the same subdivision of odours. . Attempts to support the theory of a number of specific olfac- tory energies have been made from the effects of certain local or general poisons. Frbhlich found that on sniffing at 5 grms. morphia mixed with sugar, smell was perceptibly blunted. If 1 cgrm. strychnine and sugar is held in contact with the Schneideriau membrane for 20 minutes a profuse secretion of mucus is produced, which lasts eight days ; there is a simultaneous exaggeration of olfactory acuity. Internal use of strychnine also produces hyper - osmia. The internal administration of atropiue and daturine, on the contrary, inhibits the power of differentiating between odours for several hours. Experiments on the partial anaesthetising of the olfactory mucous membrane by cocaine are also interesting. The first observations of this kind date from the year 1888 (Lennox Browne, Gremt). Kiesow (1894) observed that if the nasal mucous membrane is painted high up with cocaine, olfactory sensibility decreases very much, and entirely disappears to certain smells. Goldzweig obtained similar results. Zwaardemaker, however, made the first systematic investigation on the toxic action of cocaine. He found that sensibility was unaltered to some odours and weakened to others, but the results did not conform with his classification. He further observed that the iv THE SENSE OF SMELL 185 state of anosmia was preceded hy a. brief period of h\ perosmia. Renter made a very interesting communication to tlio effect that the anosmia, produced by the action of cocaine is not only pie e.eded but- also followed by a period of hyperosmia, as the effect of the cocaine is wearing off. Eollet then observed that on return to the normal state after the action of cocaine there is a, period in which the liniinal value oscillates considerably. Kollet further experimented with gynmenie acid and produced a, long period of total anosmia, after which he found that the appreciation of single qualities of smell returned at unequal intervals. From these observations as a whole it must be assumed that the olfactory apparatus contains a certain number of component elements (which are probably more numerous than those of taste) endowed with specific sensibility to different elementary qualities of smell. But in the present state of our knowledge this difficult sul iject is far from being cleared up. VIII. In daily life, as in medicine and pharmacology, bad smells are often corrected by other more pleasant odours. In perfumery it is a common practice to mix different scents in order to obtain pleasant olfactory sensations. To form a clear picture of the effects of mixing different odours, or of their simultaneous action on the two halves of the olfactory mucous membrane, it is necessary to distinguish several possible cases. Sometimes on mixing odoriferous gases or vapours with other gases new inodorous compounds are formed. Thus ammonia and acetic acid form ammonium acetate, which has no smell. Accord- ing to Nagel an inodorous compound is also formed when the smell of formaldehyde is counteracted by ammonia. Clearly in these cases there is no physiological neutralisation of two olfactory sensations. Again, the sensation produced by an unpleasant odour may 1 it- succeeded by a stronger and more penetrating smell. In this case the stronger smell alone excites our olfactory sense, but the weaker does not disappear. It no longer excites the sense of smell, either because its liniinal value has been displaced, or because attention is concentrated on the stronger odour. The use of perfumes is generally directed to the disguising of bail smells. Preparations of creolin or hypochloride of lime are used to disguise the smell and disinfect the purlieus of public con- veniences. Tar corrects the odour of ozoena, carbolic acid of gangrene. Castor oil and cod-liver oil, which have for many people an unbearable smell and taste, are made less unpleasant by the addition of various substances. When two equally strong odours act separately on the two nasal fossae, it is possible to perceive the one or the uthcr odour alternately. 186 PHYSIOLOGY CIIAI'. Valentin experienced this on smelling ether and balsam of Peru at the same time. He concluded that there was a conflict between the two olfactory sensations analogous t<> that observed in the two visual tielils. according as attention is fixed on one or other of them. The same olfactory conflict was noted by Anmsolm between the smell of eamphor and that of cedar oil. In other cases there is no such conflict on stimulating the olfactory sense by two odours at the same time, nor does the Kn;. 71. Zwaanlemaker's double olfactometer with jmrnus tuln-s. Its i-nnsti m-tinn is tin- saun- as that of the simi>lr olfactonn-t.-i (Ki-4. r,;i). The two olfactometers are separated ]>y nx-tal diaphragms, and run on two rods marked in ri-ntimelres. Tin- ruds of the olfactorneter tubes an- introduced into the two n. >sti il>. \vli--n t \\o ditlri ent odours are simultaneously employed. Or they may be united by the "["-tube shown in tin- ti^uir, so as in stimulate only one nostril simultaneously with both odours. stronger smell predominate; but there is a more or less perfect neutralisation, and the two odours become fainter or disappear entirely. Thus Aronsohn saw that the smell of camphor dis- appeared on simultaneously smelling petrol, eau de cologne, essence of juniper, or garlic, though all these odours are weaker than that of camphor. Zwaardemaker made a scientific study of these neutralisation effects. For this purpose he employed his double olfactometer (Fig. 71). With this instrument he was able to apply a different odour of measurable intensity to each nasal fossa by introducing THE SENSE OF SMELL 187 nozzle of the olfactometer into each nostril, or both odours could be made to act on one nostril alone by uniting tin- two olfactometers by a T-junction. The liminal value, or minimal perceptible intensity of each of the odours to be experimented with, is first determined. This estimation has to be made for each nostril separately, since it is rare to find both equally sensitive. The minimal amount of each odour perceptible to each nasal fossa is the olfactie,(u\(\ corresponds to a certain length (measured in centimetres) of exposure of the olfactometer tube. When this has been determined it is easy to vary the intensity of the olfactory stimuli in measurable quantities (of 1, 2, 3 . . . olfacties), and to change the relative intensity of the two odours in the two olfactometers. By this method Zwaardemaker established that full com- pensation is obtained when the nostrils are separately stimulated with the following pairs of odorous substances in the proportions indicated as follows : In centimetres of t he olfactometer. hi Cedar wood and rubber . . . .5-5 Benzoin and rubber . ... 3-5 Paraffin and rubber 8-5 Rubber and wax 10 Rubber and balsam of Tolu . . .10 Wax and balsam of Tolu . . . .10 Paraffin and wax 10 10 2-5 : 14 10 3-5 : 10 10 8-5 : 14 7 14 : 28 7 14 : 70 9 40 : 90 5 10 : 20 If the relative intensity of any pair of these substances is altered, either the strongest smell alone is perceived, or there is a conflict between the two sensations, or only a very weak and indefinite sensation, or lastly, a disappearance of all sensation when there is perfect neutralisation. According to Zwaardemaker there is never, even with very strong odours, a mixed sensation, i.e. a psychical combination of the two olfactory sensations, tending to a reinforcement or sensible qualitative alteration in the percep- tion of one or the other odorous substance. One of the most interesting experiments that can be made with Zwaardemaker's double olfactometer consists in filling the one with acetic acid (2 per cent), the other with ammonia (1 per cent). On leading the two odorous substances separately to the two nostrils, a smell of ammonia or of acetic acid is obtained, according as the one or other cylinder is the more exposed. The two smells are never simultaneously perceptible. It is, however, possible to find such a relation of intensity that neither of the two odours prevails over the other, or there is at most a weak smell of one or the other. Lastly, it is pn.-sible to discover such propor- tions that on sniffing with the two nostrils no olfactory sensation results, even when the two stimuli are so strong that either, separately, would arouse an intense sensation. 188 PHYSIOLOGY CHAI It this surprising phenomenon, discovered by Zwaardemaker, took place in the open air, it could easily be explained as the < 'fleet of the chemical combination of the two odours, which would fiirni ammonium acetate. But this explanation will not hold for the double olfactometer, because the two substances are srp.-init.rd during the entire period of excitation by the nasal septum. It is therefore a physiological effect, analogous to, but more complete than, the compensations of gustatory sensations studied by Kicsow. Zwaardemaker's .-issrrtiun that the simultaneous excitation of smell by two or more different odours never elicits a comj><>tni/) The mixed odour may resemble the component odours without being identical with them ; in other words, it is always a tjualitatively new sensation. /waardemaker took up this <|iirst ion again, and admitted tin- existence of t rn>' mixed odours, but declared that they only appear when the component odours are very nearly allied, i.e. when they In-long to the same or to an allied class. When, on the other hand, two odours of different and dissimilar classes are brought together, there is not a mixed odour, but a neutralisation or con- flict between the t\\ sensations neutralisation if the stimuli arc weak, conflict if they are strong. Moreover, there arc certain variations in the strength of the stimuli, within which the effects of compensation or of struggle do not disappear. To produce a conflict the stimuli need not necessarily be equal in intensity, or of the same value in olfacties. When weak stimuli are used the intensity can only be varied within narrow limits ; but there is a wider range of variation when stronger stimuli are employed. To this Xagrl replied that compound or mixed odours may be formed by mixing odours not only with similar but also with dissimilar substances. He obtained unmistakable mixed smells with vanilline and bromine, aniyl acetate and iodine, turpentine and xylol, etc. It is true that owing to the different volatile properties of the odorous substances, and to the unequal fatiguability of the olfactory organ to different stimuli, the iv THE SENSE OF SMELL 189 sensation of a mixed odour readily breaks up into its components, and the phenomenon of conflict sets in; but theoretically it is important that a sensation of new <[ii;ility ran, even temporarily, be produced on mingling different and very dissimilar olfactory stimuli. According to Nagel, this phenomenon presents a certain analogy to what is observed for colours. IX. As regards the physiological value of olfactory sensations, it should be noted that they not in frequently excite retlex acts, in the motor system and in that of the glands, which may be useful alike to the individual and to the species. \Ve have elsewhere seen that Pawlow noted a profuse salivary and gastric secretion in the dog when the animal had merely .sniffed at its food. We also pointed out the special importance to the coming together and pairing of the sexes in many mammals of the venereal odours that emanate from the mucous glands of the sex -organs. Olfactory excitations undoubtedly play no incon- siderable part in the sexual life of man. The repugnant smells that emanate from putrefying food- stuffs, from excreta, and from certain poisonous substances induce instinctive acts directed to the rejection of these substances for food, or to removing or concealing them. At the same time it must be noted that not all foul smells come from noxious matters, nor do all noxious matters give off bad smells. There are close relations between the olfactory sensations and the sphere of emotion. All odours that reflexly excite the activities of vegetative and reproductive life constantly produce a feeling of pleasure. But numbers of other olfactory sensations are associated with a feeling of pleasure or distaste, without connoting any physiological value or significance. These have none the less a more or less definite psychical value. Smell is perhaps more capable than any other modality of sensation of profoundly altering that general affective state of the mind which we call mood. Where there is a bad smell, one becomes impatient and irritable ; in a pleasantly scented atmosphere the tone of the mind alters, and we become cheerful or gay. Another characteristic of olfactory sensations is their capacity of calling up by imagination the memory images of distant places, objects or events with great clearness. Nagel notes that the smell of tar calls up a seaport, the acrid smell of machine oil revives the memory of a sea-voyage. In conclusion it is found that certain special olfactory sensations sharpen the wits, and aid the processes of ideation and judgment. The use and abuse of tobacco to which literary persons are especially prone is partly justified by these psychically stimulating effects. 190 PHYSIOLOGY CHAP, iv BlBLlOGKAPHY The earlier scientific bibliography of the olfactory sense is all cited in the following monographs : C. Dr.MKiui,. Menioire sur 1'odorat des poissons. Societe Pliilomatique de P;ni>. Nouveaux Bulk-tins, i., 1807. CLOQUET. Osphresiologie, 2nd ed. Paris, 1821. BIDDEU. Wagner's Handworterbuch d. Physiol., art. " Riechen," ii., 1844. V. VINT.SCHGAI-. Hermann's Handbuchd. Physiol., "Geruchsinn," iii. Leipzig, 1879. \V. A. XAIU.I.. Yri-gleieh. physiid. u. anat. Untersuchungeii uli.-r dm Geruchs- und Geschrnackssinn. Bibliotheca Zoologica. Stuttgart, 1894. W. A. NAGEL. Ergebnisse vergleicli. pliysiol. Untersuchungeii tiber den Ge- schmacks- und (ii-nirlissinn und ihru Organe. Hiol. Zentralblatt, xiv., 1894. J. V. UKXK.ru.. Yerglek-hendi! sinnesphysiologische Untersuchungen. 1. Uber die Nahrungsaufnahme dor Katzenhais. Zeitachrift i'iir Biol. xxxii., 1894. XWAAKUEMAKKI:. 1 'hvsiologie dcs Geruches. Leipzig, 1895. The following art- the most important of the later publications. They also contain references to other authors cited in the text : . Arch. f. Anat. und Physiol., 1886. RETIII. Sitzuugsber. d. Wit-n. Akad. cix., 1890. K. K. Gesellsch. der Arzte in \Vi,-n, May 18, 1890. ZWAARDBMAKER. Forts.-lir. 1. Med., 18S9. Arch. f. Anat. u. Physiol., 1890. S. PAS-V. Revue scientitiquc. May s and \i>, 1897. NAGEL. Zeitschr. f. Pliysiol. xv., 1897. Handbuch d. PlivMol. d. Menschen, "Der G.-ruclissinn," 1905. ROLLET. Plliig.'i-'s Aivh. Ixxiv., 1899. KKKMASN. /fitschr. f. ang.'W. clinnif, 1 '.U)0. HAYCUAKT. Schafer's Text-book of Physiology, ii., 1900. C. RKUTEU. Onderzoekingen gedaan in lift pliysiol. labor, d. Utrechtsche hoogeschool, 1900. VASCHIDB. Corapt. rend, do la Soc. de Biol. liii., 1901. VERESS. Pfluger s Arch, xcv., 1903. H. KH-HTKXKEI.DT. Literatur /ur Fischkunde. Bonn, Hager, 1906. STEUXBEKG. Gcsehmack und Gcruch. Berlin, 1906. Recent English Literature : PAKKI.U and STABLEK. On certain Distinctions between Taste and Smell. Amer. Journ. ul' Pliysiol., 1913, xxxii. 203. PARKER. Tin- Relation of Smell, Taste and the Common Chemical Sense in Vertebrates. Journ. Acad. Natural Science, Phil., 1912, xv. 221. CHAPTER V THE SENSE OF HEARING CONTENTS. 1. The organ oi' hearing. '2. Fmirtiuns of the external car. 3. Functions of the tympanic apparatus (tympanic membrane and chain of ossicles). 1. Functions of internal ear muscles (tensor tympani and stapedius). :>. Functions of tympanic cavity, Eustachian tube and fenestra rotunda (cochleae). 6. Structure of organ of Corti, and dissimilar vibratory properties of the rods, basilar mem- brane, and tectorial membrane. 7. Compound tones, noises, simple tones, and differences of pitch and strength. 8. Limits of the perceptive capacity for tones, and faculty of discriminating between different tones. 9. Timbre or quality of simple and compound tones. 10. Acoustic phenomena perceived on the simultaneous production of several tones. 11. Theory of perception of simple and compound tones. 12. Consonance and dissonance of tones, musical chords. 13. Rising and falling phases of auditory sensation ; auditory fatigue. Entotic and subjective auditory sensations and hallucinations. 14. Binaural audition and localisation of sounds. Bibliography. WE have already seen that from both the morphological and the physiological point of view the Internal Ear has two distinct portions, one of which is innervated by the vestibular, the other by the cochlear branch of the eighth cerebral nerve, and tli.it they represent two distinct sense-organs (see Vol. III. p. 405). The peripheral organ of hearing is the Labyrinth (cochlea) ^ it h the terminations of the cochlea) nerve, to which alone the name of renexly by means of sub-conscious impulses (p. Ill) are phylo- genetically a stage in evolution from primitive cutaneous sensi- bility ; the cochlear organ, which subserves auditory sensations, represents a much later stage in evolution it is absent in lisln s. first appears in amphibia and reptiles, increases in birds, and finds its maximal perfection in mammals. The adequate stimulus of auditory sensations consists in the vibrations of elastic bodies, within certain limits of frequency and intensity. These vibrations are transmitted through the air to 191 192 PHYSIOLOGY CHAP. the organ of hearing, and penetrate to and stimulate the endings of the auditory nerve, and arouse the sensations of tones and noises in the sensory centres. I. Of the three parts into which the organ of hearing is divided (Fig. 72) the internal ear alone contains in the cochlea the terminal sense-organ that is excited by sound vibrations ; the outer and middle ear are mere complementary physical parts of the apparatus, which serve to promote and facilitate the conduction of sound-waves to the cochlea. ...13 l-'n.. 72. Hia.-iam ot the human car as a whole. (After Debienc.) 1, pinna or auricle ; i>, external auditory meatus; :<. tympanic nit-mbrane ; 4, stapes attached to fcnestra ovalis (vestibuli); 5, bony portion <>( Eustachian tube; r>. cartilaginous portion; 6', its internal orifice ; 7, cavity of vestibule tilled with pciilymph ; s, semicircular canals with utricle; it, piiinioiitory ; in, t'enestra rotunda (cochleae) with anow indicating tympanic orifice of cochlea ; 11, tym- panic cavity tilled with air; 12, cochlear canal tilled with endolymph, united to saeculus vestibuli by small canal; 13, scala vestibula; 14, scala tympani terminating in tenestra rotunda; 15, apex of cochlear canal, where the two scalae unite- at 1.7; 16, cochlear aque- duct ; 17, vestibular aqueduct ; 18, endolymphatic sac ; 10, parotid region. The external ear consists of the pinna, and the external auditory canal or meatus, closed at the end by the tympanic membrane. The middle ear, or tympanic cavity, is an irregular, hollow chamber with bony walls, filled with air. It contains the chain of auditory ossicles the malleus, incus, and stapes, with their two short muscles the internal aperture of the Eustachian tube which opens into the pharynx, the fenestra ovalis (vestibuli) closed by a membrane, to which the base of the stapes is inserted, THE SENSE OF HEARING 19:! and the free fenestra rotunda (cochleae), closed by a membrane known as the secondary membrane of the tympanum, which is directly connected with the scala tympani of the cochlea, The internal ear or labyrinth contains in two distinct parts the sensory end-organs innervated by the two branches of the C. Fii:. 73. Cast of the interior <>t' the labyi inlh. I -eft human ear. j'. (From Henle.) .1, seen outer side; ];, from iinuT si., posterior; ', external tri- lateral semicircular canals ; u., tlu-jr :iiii]iullac : a.V., ai[Ueduct n( tin 1 vestibule ; f.o., fent'.stra ovalis (vestibuli) ; /.'., fenestra ret HIM la (cciclilcai-) ; <-., spiral tube of cochlea. eighth nerve : the utricle, saccule, and the three semicircular canals with their respective ampullae innervated by the vestibular nerve, and the cochlea, innervated by the cochlear nerve. The osseous labyrinth (Fig. 73), hollowed out of the petrous bone, must 2).c i/tr- ].-,,,. 74. Left membranous labyrinth, virw.-.l e\t.-i nally. (Mi-rki-I.) Co., cochlea; /'-., cluctus cochlear is ; Sac., suerule ; E/tr., utricle ; s., ., p., superior, external, and posterim- semicircular canals ; a./'., uquaeductus vestibuli ; C.r., canalis remiiens. be distinguished from the mem lira nous labyrinth (Fig. 74), which lies within it. The space between the two is tilled with perilymph, while the interior of the membranous labyrinth contains the endolymph. The cochlea is the acoustic portion of the labyrinth ; it consists of a spiral tube, divided into two chambers by a bony septum (lamina spiralis) completed by a membranous portion (membrana spiralis). The lower chamber or scala tympani communicates by the fenestra rotunda with the tympanum ; the upper chamber, VOL. iv 194 PHYSIOLOGY CHAT. the scala vestibuli, opens into the vestibule. At the apex of the cochlea the two communicate by a small opening (helicotrema). Between the two scalae is a smaller canal, triangular in section, the scala media, or ductus cochlearis (canal of the cochlea), which is bounded by the slender membrane of Ueissner facing the scala vestibuli, and the spiral or basilar membrane tacing the ..Is ms Fie. "">. Si-ctkm thnm^h cochlea nt tin- cat. Maxnilied -"> cliaiiu't.-i >. (Snliutta.) '/.'.. duct of cochlea; \i.*., :_ r anxli"ii sjiirale ; Co., bony wall of cochlea; l.s., li<;arnentuni siiirali- ; ni.s., nuMnbraiia .sjiiialis ni liasilaris, .sii|)]MHtiii- <>i uan of Corti ; m.i:, nicinlnaiui \fstibularis or Ueissn.T's iiii-iiibraiii- ; N.c., ni-i-vus cm-hli-ai is ; -.,., scala vrstibnli ; f.t., scala scala tympani. This canal contains the endolymph of ihe^ mem- branous labyrinth of the cochlea, and within it and just above the basilar membrane is the very delicate organ of Corti, the terminal apparatus of the cochlear nerve. As shown by Fig. 75 the cochlear nerve penetrates the central canal of the modiolus, and sends its branches along the osseous spiral lamina to the basilar membrane and the organ of Corti. v THE SENSE OF HEARING 195 This brief review of its elements will elucidate the study of the complex peripheral apparatus of audition. We shall .study the morphological details more closely later on, in discussing each portion of this apparatus, with the object of bringing out its physiological importance. It is easy to show by experiment that the outer and middle ear of man are not of fundamental importance to hearing. If the external auditory passages are stopped, hearing is undoubtedly diminished more to the deeper than to the higher tones of the musical scale, when transmitted through the air. Both, however, can be distinctly perceived through the bones if the sounding- body, i.e. a vibrating tuning-fork, is applied to the skull (mastoid .ipophysis, forehead, etc.). The transmission of sounds and tones occurs normally through the bones when we listen to our own voice. The vibration of the vocal cords throws the bones of the skull into vibration and thus excites the organ of Corti independently of air -transmission through the outer and middle ear. It is an everyday observation that after stopping the auditory meati our own voice sounds much louder. Again, when the head is dipped into water sounds are transmitted through the skull, because the sound-waves of water behave like the sound-waves of solid bodies. That the external and middle ear are physical instruments for the improvement of hearing is shown by the fact that the sound- waves of the air are communicated with great difficulty to solid and fluid bodies. That is why on stopping our ears it is very difficult to understand any one speaking in ordinary tones. Apart from the difficulty in the transmission of air-vibrations to the bones of the skull, Rinne has shown that hearing is more sensitive when excited through the tympanum than it is when excited through the bones. When the vibrations of a timing-fork held between the teeth have become so feeble that the ear no longer distinguishes them, they become audible again if the tuning-fork is brought near the external ear. When the transmission of sound-waves through the ordinary air-passages becomes impossible owing to disease of the outer and middle ear, there is relatively a very pronounced deafness. If the cochlear labyrinth remains sound, this deafness can to some extent be remedied by means of Rhodes' audiphone, which consists of a broad, thin plate, which is brought into connection with the teeth, and thus gathers up the sound-waves from the air, and transmits them to the organs of Corti. II. In man the auricle or pinna is an organ of complicated form, but in other mammals it generally takes the shape of a more or less elongated trumpet, usually directed upwards, less often downwards. In the human pinna we must distinguish tin- helix (which occasionally presents Darwin's tubercle), antihelix. 196 PHYSIOLOGY CHAP. tragus, antitrasrus, lobule, and the concha which surrounds the O ' O ' * entrance to the meatus. The muscles which move the pinna as a whole are the retractor, adductor, and elevator, while those which alter its form were described by Valsalva aa the tragicus, anti- tragicus, helicis major and minor, transversus and obliquus auriculae. These muscles are well developed in animals, in which the pinna is capable of a number of expressional movements, but in man they are rudimentary and practically disused, few people (of whom Johannes Miiller was one) being able to move their ears at will. So that while, anatomically speaking, the pinna is a typical organ, containing all the parts corresponding to those in other mammals, physiologically it is an organ that is under- going functional retrogression, and has almost entirely lost the significance it possesses in other animals. Recently (1911) Ch. Fernet has proposed to cure certain forms of serni-deafness by means of auricular ////////m.sV/V.s, the aim of which is by appropriate exercises to recover the voluntary functions of the external muscles of the ear. In order fully to understand the functions of the pinna it is desirable to study it, in the first place, not in man, but in an animal in which it is highly developed, e.g. the ass, where its function is that of a trumpet, which collects and condenses the sound-waves. If we apply a long trumpet to our ear, we recognise at once that it is a great help in hearing ; the ticking of a watch can be heard at a much greater distance than with the unaided ear. The sound-waves which enter the large aperture of the trumpet with the air are reflected along its walls, so that the waves that reach the meatus are exaggerated. The ears of donkeys and horses, which are mobile, can moreover be turned towards the source of the sound, and thus fulfil the function of hearing-trumpets, whatever the position of the animal. In addition they have dilator and constrictor muscles, so that they can increase or diminish at will the intensity of the sound-waves that reach them. The human ear, on the contrary, is ill-adapted to this purpose. Its form is very unlike a trumpet, and it has become immobile from disuse. The small importance of the human pinna in audition is shown in the fact that its removal affects the delicacy of hearing very little. If the inequalities of the pinna are filled up with wax or plastidine, this has practically the same effect as amputation of the lobe. Schneider found that audition was slightly diminished ; Harless and Esser noticed hardly any differ- ence. Experiments made by Gradenigo on an individual with normal hearing who had lost a lobe showed that the perception of weak, high tones, e.g. the ticking of a watch, was facilitated, while loud, deep tones were not perceptibly reinforced. Hence it is clear that a large portion of the sound-waves that reach the v THE SENSE OF HEABING 197 pinna are reflected outwards, and do uot sensibly inc.ivusr the number of those that penetrate I IK; meatus. If the pinna were placed at an angle of 40 directed forwards ii \\ould certainly fulfil its function as an auditory trumpet, much better, although psychiatrists regard the protruding ear as a morphological sign of degeneration, and aesthetic considerations would, in any case, depreciate such an advantage. Those who are hard of hearing, however, instinctively use their hand to bend the lobe forward, and thus make it fulfil the <>Hicc of a sound-collector. It has been said that the cartilage of the ear may serve as an elastic lamina to receive the sound-waves and transmit them through the bones to the internal ear. But if the meatus is stopped with wax and a watch applied to the pinna we can scarcely hear its ticking, while if it is applied to the mastoid process the sound becomes plainly audible ; the cartilages of the lobe are consequently poor conductors of sound. Is the pinna of any importance in recognising the direction of sounds ? The appreciation of the direction of sound will be con- sidered later in discussing binaural audition. The pinna un- doubtedly has a certain importance in this connection : when we turn the ear towards the source of a sound, it is under the most favourable conditions for reflecting the wave towards the auditory meatus. When the sound comes from in front, and still more when it comes from behind, the ear is in an unfavourable position. If we use one ear only, while the other is stopped with wax and the eyes blindfolded, we are able to judge correctly of the direction of a sound, by observing how its intensity varies when we move our head in different directions. Weber maintained that we could judge the direction of a sound by means of the pinna because the sound-waves excite its tactile organs. But every-day experience teaches that the sound- waves in the air excite our sense of touch only when they are of extreme intensity, even in regions in which the body is far more sensitive than is the skin of the auricle. Buchanan and, at a later time, Kuss and Duval, Beaunis, and Gelle held that the pinna, independent <>f the movements of the head, reflects towards the meatus the sound-waves that impinge upon its anterior surface, and arrests those which reach its pos- terior surface. There would thus be an area behind the pinnae in which the vibrations would have difficulty in reaching the ear, and this could be utilised to discover whether the source of sound lay before or behind the head. To demonstrate this fact Gelle pointed out that it' a watch is held horizontally to the ear, first in front, then at the side, then behind the head, and is gradually moved farther away, so as to discover the distance at which the ticking can be appreciated, it is easy to show that this distance is least behind the ear and o i 198 PHYSIOLOGY CHAP. greatest when the watch is held laterally to the head in the axis of the meatus. Gelle demonstrated the function of the pinna in appreciating the direction of sounds by a very simple experiment. If the pinnae are eliminated by inserting the two ends of a rubber tube 50 cm. long into the auditory meati, so as to close them completely, and a watch is then placed at the centre of the loop after blindfolding the subject's eyes, the same sensations of sound are received in both ears, and they do not alter if the rubber loop is placed above or behind the head. Another older experiment of Weber confirms this. If the two lobes are bandaged closely to the skull, with closed eyes we are no longer able to distinguish the forward or backward direction of a sound, because the conditions of the conduction of sound in both directions" have become approximately equal. Weber further noticed that if the h