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HUMAN PHYSIOLOGY
Prof. LUIGI LUCIANI translated from thb italian
Prof. J. N. LANGLEY, F.R.S.
In 5 vols. llluHtruted. 8vo.
Vol. 1. Circulation and Respiration. ISs. net.
Vol. II. Internal Secretion — Digestion — Excretion — The
Skin. ISs. net. Vol. HI. Musouliir and Nervous Sys to ins. IBs. net Vol. IV. The Sfln=e Organs.
Vol. V. Hetabolistn — Temperature — Re|)Toduct{on/ ete. llnl/u Prcis.
LONDON: MACMILLAN AND CO.. Ltd.
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HUMAN PHYSIOLOGY
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HACMILLAN AND CO., Liuiteu
LONDON ■ BOH BAY ■ CALCUTTA ■ 1IAIIII4B
THE MACMILLAN COMPANY
THE MACMILLAN CO. OF CANADA, Lrn.
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HUMAN PHYSIOLOGY
BY
PROFESSOR LUIGI (jUCIANI
TRANSLATED BY
FRANCES A. WELBY
WITH A PREFACE BY .1. N. LANGLEY, F.R.S.
IN FIVE VOLUMES VOL IV
BDITBD BY
GORDON M. HOLMES, M.D.
THE SENSE ORGANS
MACMILLAN AND CO., LIMITED
ST. MARTIN'S STREET, LONDO.V
1917
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COPYRIGHT
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PUBLISHERS' NOTE
Volume V., completing the work, and dealio^ with Metabolism, Temperature, Reprixhiotioii, Sti^'a of Life, Death, and Bacee of Mankind, ia now in the press, and will appear in <hie coiirao.
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CONTENTS
CHAPTER I
Cdtakeous Sensibility
I. Vitfereaee betveeo modalilji tnd quality of aeoBAliotia. Johannes Miiller'a law of Bpecilic energies. 2, Different intonaities of seneatiajiB. Weber's law and Fechner's law. 3. Ttanararuiation of seusatiunB into peroeptiona : philosopbical theories. 4. Four njodalities of cutaneous sensation, according tu Bill, Ooldscheider, and v. Frej. G. Cutaneous nerre-endings for aecsationa of preisnre, pain, cold, and heat. 6. Phy- siological analysis.of thermal sensations (heat and cold). 7. Touch and pressure sensations. S. Capacity Tor localising cutaneous sensatiotis. S. Pain seosatioua. Bibliagra[ihj.
CHAPTER n Sessibilitt of the Ihteiinal Orqans
1. Gasaification of internal sensations. 2. Common aensation of the body or menaetlhaia, 3. Pain in the internal organs and tissues. 1. Alimentary needs (hnnger and thirst]. 5. Sexual desire. fl. The mukcular sense ; sensibility of muscles, tendons, and joints. 7. Inner- ration sense in the centres of voluntary movement. 8. Active tactile peroeptions and their components. 9. The aubconsciona sense of muscular tone and its voriatious lu referenco to the functions of the labyrinth. BibUograpby.
CHAPTER III
1. Taste-buds the peripheral organs of the aenae of taste. 2. Toiite area mapped out by the physiological method of adequate etinmli. 3. Qualities of taste. 4- Mechanics of taste. 5. Correlation between tbe chemical and phyaical constitution of sapid substances and the intensity
ii PHYSIOLOGY
and quality of the excited tastes. 8. Inadequate stinmli ; the eo-oalled electrical lante. 7. Pathological atterationa of tast« -sense by disease or paiaona. S. Specido energies of the nerves of taste. Bibliography.
CHAPTER IV
The Seitbb of Shell ......
I. Peripheral organs and nerves of smell. 2. External meohanism
of olfactory function. 3. Eicitation of smell by odorous substances in the form of gas or of ai^ueous solutions. lillectrical excitation of smell. 4. Chemical and physical properties of odorous substances.. 6. Classi- fication of odours. 6. Determination of olfactory acuity (olfactometry and odorimotry). 7. Specific energy of the olfactory apparatus deduced from the phenomeoa of ]iartial anoaiitia and partial olbctory fatigue. 6. Corrections and compensations of odours. 9. Physiological and psychical value of olfactory sensations. Bibliography.
CHAPTER V
The Sense of Hb&riho ......
1. The organ of hearing. 2. Fonctiona of the external ear. 3. Functions of the tympanic apparatus (tympanic membrane and chain of ossicles), i. Functions of internal ear muscica (tensor tympanj and stapedius). 6. Functiooa of tynijwnic cavity, Eustachian tube and fenestra rotunda (cochleae). 8. Structure of organ of Corti, and dis- similar vibratory properties of the rods, basLlar membrane, and tectorial membrane. 7. Compound tones, noises, simple tones, and diHerences of pitch and strength. 8. Limita of the perceptive capacity for tones, and faculty of discriminating between ditferent tones. 8. Timbrt or quality of simple and compound tones. 10. Acoustic phenomena perceived on the simultaneous production of several tones. 11. Theory of perceptiati of simple and compound tones. 12. ConBonaoce and dia- sonance of tones ; musical chorda. 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 Mbch&nisu op the Eye . . . :
1. Oenetsl anatoniy of eyeball. 2. Formation of retinal images ; underlying optical principles. 3. Optic conatanls of the eye. 4. Static refraction of the eyo {aninetrojiia and ametropia). 5. Eefraction of the eye ; mechanism and innervation of accommodation. 6. Far point and near point of clear vision ; range and s)ieed of accommoda- tion. 7. Normal imperfections in the dioptric apparatus of the eye.
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CONTENTS
8, Dioptric importance of the iris. 9. Mochsnism and innerration of pupil in accamiiiodatioii ; theory of pnpil -reflexes. 10. AbsorptJOD and reflection of light in the eye ; opbthalmoscop; and ekiaicopy. Biblio- graphy.
CHAPTER VII BsnuAL EictTATtoN AND Visual Stiudlation
1. Histolc^cal structure of retina. 2. Direct and indirect vision (blind spot, retinal elementa the receptors of light-atininli). 3. Ob- jective phenomena of retina! stimulation (Tisual purple, migration of pigment, contraction of epithelial cells and inner segment of cones, electromotive phenomena), i. Colour vision ; efl'ecta produced by dilTerent kinds of laminoaa radiation ; limits of their visibility. 5. Visual acuity ; phages of visual senutions ; pogitive and negative after- images. 6. Betinal 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 stimnlos and retinal adajitatioa to light and darkncas. 8. Duplicity theory of functions of rods and cones. 9. Colonr mixtures ; complemontary colours. 10, Colour contrast ; successive and simultaneous. 11- Theories of achromatic and chromatic viaioiL 12. Colour blindness ; partial and total. Bibliography.
CHAPTER Vm
Oculak Moveuests and Visual Perceftioii8
1. Articulation of eyeball in its socket ; its external muscles ; its movements and possible positions. 2, Isolated and associated move- ments of iU muscles. 3. The innervation and co-ordination of the eye- movements, 4. Simple binocular vision and the horopter. (>. Di- plopia. 6. Conflict between the visual images of both eyes and tlie phenomena of binocular contrast. 7. Spatial perception in monocular and binocular vision- 8. Stereoscopic binocular vision ; the stereo- scope- 0. Psycii CI- 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. Otiginof the aqueous humour. Bibliography.
CHAPTER IX
PSTC HO- PHYSICAL PHENOUENA of CONBOIOt'flHESS AKD SLEEP
1. The range of mental life includes u processes. 2. States of complete and i Subconscious activity ; its great inijiort;
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X PHYSIOLOGY
geoius. t. r>evelo|imctit and integration of ths mind a ru[iGti<in of the siiliconsvious, based an (layclio-pliyiiicsl proves^s as diBtiiict from con- xciuLis ]irocL'8Se9. 5. Diriiutegiatianii of (lerBonalit; (double coiiacions- iisBs, si-coiidary pcrsaiiality, altpiiiating personality]. S. Pliysiology of slevp. 7. Tlieories of sleep. 8. Paycliology of iilco|i. 9. Dreams. 10. Telejrathic phenomena. Vitalism and niatorialism. Bibliograpliy.
INDEX OF SUBJECTS ......
INDEX OF AUTHORS
ERRATA
Page IS, table, side-bracket, for "nieoodermal"read " mesodermal. "
„ 39, line 8, for "stJIoid" read "styloid."
„ 63, bottom line, for " u re to -genital " read "uro-genital."
„ 81, lines 15 and 17, for " Aronson " read " Aronsohn."
„ 82. Figs. 27 and 28, for "clitorifi of female " niad "human clitoris." „ Si, Fig. 31, for "female clitoris" read "human clitoris." „ 89, line 23, for " Hansen " read "Hensen."
90, line J2, for "Oley" read "Gley."
,, 130, line 7, for " Panaschi " read "Panasci,"
„ 138, line 1, for " Kastal" read "Kaatle."
„ 148, line 10 from below, for "NaBr," read "NaBr."
,, H7, lino 15 from below, for "Cii" read "Mn."
„ 162, line 11 from below, for "hairs" read "cilia." .
,, 170, line 4 from below, for "Rumsberg" read "Rumberg."
„ 175, line 18 from below, for "sulphate" read "sulphide."
„ 175, lino 17 from below, fur " plioaphato " read " phosphide."
,, 1S5, line 9 from below, for " hypochloride " read " hyjioclilorite."
„ 197, line 13 from below, for " Kuaa " read " Kiiss,"
,, 203, Hue 19 from below, for " place " read "plane."
,, 223, lino 8, for "Cladni" read "Chladni."
„ 267, line 12 from heloiv, for "Malte" read "Matte."
„ 267, Fig. 108, for "A. E. Schafer"rcad " E. A. Schafer."
„ 269, lines 16 and 23, for " Brudi " read " Briicke."
„ 298, line 12 from Lolow, for " notable " read " not able."
„ 314, line 23, for " psrocentliesis " road " ])aracenteais."
„ 336, Fig. 181, for "G. N. (iolding-Bird " read " C. H. Golding-Bird."
„ 351, line 20 from bclovi-, for " '0001 " read " O'OOl,"
„ 396, bottom line, for "non-i>aratlel" read " a- xym metric."
„ 398, line 7, for " Panigrossi ' read " Panegrossi."
,, 410, Fig. 195, for "stairca-sc " read "ladder."
„ 420, line 12 from below, for " Beaiessi " read " Benussi."
„ 430, line 19 from below, for "covered" read "coloured,"
„ 460, bottom line, for "oniric" road "oneiric."
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CHAPTER I
CUTANEOUS 8KNSIBILITY
Co:<niNTs. — 1. Difference between modalUy and quality of sensations. Johannes UiUlcr's law of specific energies. 2. Different intensities of Bensationa. Weber's 1>« and Fecbner's law. 3. Transformation of sensations into peraeiitiona : philosophical theories. 1. Four modalities of cntaueous sensation, aooording to Blii, Goldscheider, and v. Frey. 6. Cutaneous nerre-endings for sensations of pressnn, pain, cold, and heat. 6. Fhjsiological ana!;sis of thennal sensations (beat and cold). 7. Touch and pressure sensatians, S. Capacity for looalising cDtsDeous sensations. 9. Pain sensatioos. Bihliography.
Movement and SeoBatioD are the two extremes of the processus of animal life \)j which the orgamsm 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 of of other men.
All organs of the body that are supplied with afferent nerves continually send information of their functional state to tlie central 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 emei^e vaguely and indefinitely in consciousness as a more or less decided sense of well-being or the reverse. Or again, the messages from the different oi^ans to the centres may definitely cross the threshold of coDScioosness 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 physiolt^oal
VOL. lY 1 B
2 PHYSIOLOGY chap.
fact intimately associated with special states of coDsciouBneas. Bat very few of the impresBiona that reach us from the outer world, or from our own hody, enter completely into consciousneae, because the attention can only be focussed upon a small part of the impressions received.
Sensations are diBtingiuBhed, according to the most funda- mental difference in the paycho - physical phenomena which constitute them, as internal and external. Internal senaations 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 we are fully aware of them ; often, however, they operate unconecioualy and modify our . mentaUty, without being distinctly perceived. Such are the senaations of pain, hunger, thirst, nausea, fatigue, sexual desire, etc. The name coenaesthesia (from koivwi, common, ourdi^o-is, sensation) is often applied to the collective internal sensationa aroused in the centres by the excitations that reach them from the viscera, muscles, and surface of the akin.
External sensations are more exact and definite in character, hence they are also known as "specific senaations." They are frequently converted into perceptions, which are objectified sensations, i.e. sensations referred to an external cause by a psychical act which includes a judgment, even if an unconscious one. Consequently external senaations are the substrate of all our liigher mental functions and all our knowledge.
The phyaiological neural processea that accompany the sensory phenomena arising out of the activity of the senses are, for the most part, not available to external objective observation; so that in dealing with the senses the physiologist is compelled to borrow freely from the paychologieaf terminology derived from subjective observation, and the missing physiological analyaia of phenomena is to some extent replaced by introspective analysis. The justification of this method depends on the validity of the law of paycho-physioal parallelism, which assumes not only that functional relations exist between somatic and psychical processes — which is indisputable — but also that for each state of con- sciousness and each psychical change there ia a corresponding state and change in the concomitant neural process — which is not and in the present state of our knowledge cannot be demonstrated. The law of paycho-physical parallelism is thus no axiom, as many have supposed, but ia merely an empirical and provisory hypothesis, wtiich enables the physiologist in dealing with the highest functions of the nervous system to remain on the positive ground of controllable laws and phenomena, instead of straying into the region of metaphysics and speculating
I CUTANEOUS SENSIBILITY 3
on the natare of peyohical phenomena apart from any material sobetratc;
L The Sfjue-organs are the peripheral instruiuents of uur aenaatioDS. Most of the senaory nerves are so arranged at the periphery as to present a surbce on which the various changes in die enTironmenC can operate. With this object the nerve- endings of the sensory fibres are provided with specific moohaniams which are partially non-neural (aense-oi^aDS in a strict sense), and are so formed and adapted as only to allow certain definite external alterations to act on and excite the corresponding nerve-fibres, while abeolntely or relatively excluding the action of any other form of external stimulation. It is excluaively by means of the sense -oi^n formed by the eyeball that we perceive those rhythmical vibrations in the ether which we call li$ht; it is exclusively by the cochlea of the internal ear that we are aware of the rhythmical vibrations in the pressure of the air that we call sound; it is exclusirely by the chemical excitation of the olfactory epitheliom of the nasal mucosa, or the gustatory epithelium of the lii^:ual mucous membrane, that we are aware of smelt or taste.
The adequate atiinvlns for any given sense is that to wliich its organ is specially adapted, so that it can receive it and be effectu- ally excited by it ; aU other kinds of stimuU are inade^wite for that sense-organ. light, for instance, is the adequate stimulus for the retina, sound for the cochlea, odoriferous and sapid sub- stances for the organs of smell or taste. Electrical currents, and physical and other mechanical means which can also excite these eeoBe-organs, are inadequate stimuli.
Adequate stimuli are, as a rule, effective only when they act on the peripheral sense-organ ; they are not always capable of exciting the sensory nerve directly. The most vivid hght fails to excite visual sensation when it falls on the stump of the optic nerve ; load sounds are not perceived by the stump of the auditory nerve, though to this there are some exceptions. Chemical, thermal, and mechaoical stimuli can take effect aloug the course of the olfactory, gustatory, and tactile nerves; but they must be of greater intensity than is required to evoke sensations of smell, taste, temperature, and touch when they are applied to the peripheral end-organ. Adequate stimuli therefore become effective only when may act on the terminal sense-organs, which have presum- ably beeh adapted to them by a long evolutionary development. Inadequate stimuli, on the contrary (so lar at least as we know), can act on any part of the sensory nerve aloug its course, and are less effective, or even ineffective, when applied to the peripheral sense-oi^n.
Little is known at present about the specific arrangement of the sense-organs, whereby they are specially excitable or bus-
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4 ■ PHYSIOLOGY chap.
ceptible to one particular atimulus while absolutely or relatively inexcitable to stimuli of other kinds. It is not & sufficient explanation of this fact to say that in certain cases the influence of the inadequate stimuli ia hindered or impeded by the topo- graphical position of the sense-organs. The auditory apparatus, for example, is shielded from the action of light, of mechanical impacts, of various vapours ; the visual apparatus is well protected from mechanical and chemical stimuli. On the other hand, the auditory cells are perfectly accesaible to sound-waves, the visual cells to light, the olfactory cells to tbe air inspired, the gustatory cells to tbe food-stufis ingested. These statements, which neglect the internal constitution of the aenBe-organs, fail to explain the specific susceptibility of the latter to given stimuli. What is the intrinsic organic condition that prevents the peripheral organs of taste and smell from reacting to light, warmth, or mechanical pressure (which are adequate stimuli for the visual and cutaneous nerves), while they are excessively sensitive to certain chemical stimuli ? From the teleological point of view, we know this must be so. If it were otherwise, if the organs of taste and smell were not specifically predisposed to react to chemical stimuli, but reacted with the same facility as the skin to heat, contact, and pressure, they would be incapable of conveying to our consciousness any precise intimation of the nature of the chemical stimuli to wliich they are adapted. The same may be said of the specific adaptation of tbe retiua to light, the cochlea to sound, etc. But even if the teleological connection between the specific nature of the sense-organs and their specific function is plain, we are still ignorant as to the internal nature of their respective structures, on which depends their specific excitability to different kinds of stimuli. In all probability, as Fick assures us, there are in the peripheral sense-organs compounds of a highly unstable molecular constitution, which are decomposed by sUght impacts, and thus develop energy which acts on the nerve as au effective stimulus.
The senses can be distinguished and classified either by their anatomical situation, or by the nature or quahty of the stimulus adequate to excite them, or. lastly, by the kind of sensation which they arouse in consciousness. These different categories mostly coincide. Thus vision is the sense of the eye, for which the adequate stimulus is light, which produces visual sensations ; hearing is the sense of the ear, and is excited by tones and noises which arouse auditory sensations in consciousness ; taste is the sense of the tongue, and is excited by sapid substances that arouse
fuBtatory sensations ; smell is the sense of the nose, and is excited y odorous substances which evoke oll"actory sensations. But when we apply the anatomical test we must further distinguish a cutaneous sense, a muscular sense (inclusive of tendons and joints), and a visceral sense ; according to the nature of the stimulus, we
I CUTANEOUS SENSIBILITY 5
mnet add to the cutaneoas eenee a pressure sense, a temperature sense, and a pain sense; lastly, according to the quality of sensation, the thermal sense must be subdivided into a heat sense and a cold sense. The peychologioal claasigcation, 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 quahty. 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 candle is stronger or weaker than the sensation evoked by a certain solution of sugar, a given mosical note, a sensation of pressure or temperature in the skin. If, on the other hand, we compare the sensations appreciable within each modahty, we can indeed recognise quahtative differences; but. these are not so profound as to make impossible a reciprocal transition Irom one to the other, or a comparison and judgment of their greater or less ^milahty, 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 setuations that arise from the less well-developed
Fick (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 tUfferences in quality, between which a gradual transition is possible, as between the colours of the solar spectrum.
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6 PHYSIOLOGY chap.
This in no way iuvalidates Ihe distmction between modslity and quality of Beueation ; it merely emphasises the fact that in the higher senses the differentiation in the modality of the sensations is far more pronounced and striking than in the less developed sensee.
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 Miiller (1840) published a masterly development of tliis theory, and brought out its full importance alike in physiology and paycholt^y. It is usually known as the "Law of ^eeific energy of the seriaes" (toL iii. p. 262), and was summed up by Miiller in the following general propositions : —
(a) " No kind of sensation can be produced by external causes which cannot be equally excited in the absence of 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 commou 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 witti 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 phy8iol<^cal conditions this happens in dreams, under pathological conditions in halludnalions. The outer world can therefore make no impression on us which purely internal causes are unable to arouse.
(6) "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 Uglit 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, acconiing to the nerve-organs encountered at the different pai'ts to wiiich it is directed.
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1 CUTANEOUS SENSIEILITT 7
(c) " The specific seoBatioDS of each sensory nerve can be evoked by different internal and externa] stimuli." ..." Sensa- tion is not the transmission to consciousness of a quality or state of an external body but of the qnahty, or the state of a sensory nerve as produced by an extrinsic cause, and these qualities differ in 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. Bat 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 enei^es, which are, its vital qualities, just as contraetihty 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 aaditory 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 same vibrations of a tuning-fork that produce a note in the ear excite a sensation of vibration in the skin ; the same ether waves streamii^ &om a lamp produce light through the eye and a sensation of warmth on the skin. In order to obtain sensations of 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 hving 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" (MiiUer).
How does the excitation of the sensory nerves arouse the different conscious sensations in the brain ? Of what cliaracter is the active state of the sense-organs which generates in us the different modalities of sensation? In every age philosophers
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8 PHYSIOLOGY chap.
have sought to answer this question, but no reply is poBsible &om the standpoint of experimental science. This ia one of the transcendental problems to which Du Bois-Beymond replies i^noramuB et igjiorabimits. But the same answer had already been given by his master Johannes Miiller. "The nature of thu 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 [soperty of the optic nerve, of taste, smell and touch, remain eternally unknown like the &nsl causes in natural philosophy." The modern philoeophioal 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.
(d) " 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 oi^n. Ae we have already seen (iii p. 262), this question is still unsolved, though the theory Johannea 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 speciJic 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, %.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 modahty only, but also the qualitative differences that occur within one and the same modality of sensation, depend on specific enemes of the neurones that build up the sensory organ, or whemer they can be explained on the assumption tl^t the individual fibres of a sensory nerve are capable of serving diSerent forms of excitation or activity. This question will be discussed in relation to each of the several senses.
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1 CUTANEOUS SENSIBILITY 9
IL It is only within certain limite of intensity that external agents are effective stimulL The minimal strength which is necessary to produce a sensation is known as the liminal intensity, or tkreiJwld siimviut. The least perceptible increase of stimula- tion beyond this value is termed the limtTial differe-nce, or thTeshold of difference. Every incremeDt 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 ttie peripheral sense-oi^an, which is the more rapid and complete in proportion as the stimulus is excessiva
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 ie stronger or weaker than, or eqoal to, another.
Speaking generally, it may be said that sensation increases — within certain limits — with the strength of stimalns, but not proportionately to it ; doubUng or trebling the stimulus does not double or treble the intensity of the sensation. CommoD 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 ticldog 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 oi;gan 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. bow 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-cutAueous sensi- bility he met with a aurpriaingly 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
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10 PHYSIOLOGY chap.
neoeasaiy to add the same fractiou of the weight (average ■^, aocoiding to Weber), whatever its absolute value — whether in ouDceB, pounds, grammes, or kilogrammes.
Later observations by a number of investigatore have proved that, withio 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 ahsolute measurement of sensation, or of exact deter- mination of the ratio between sensation and atimuluB.
The same cannot be said for the so-called "psycho-physical law" which Fechuer (1860) 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, E the stimulus, C the constant represented by the liminal difference, and Feohner's "formula of psycho -physical measurement" is obtained: 5= Clog .fi, i.e. sensation is proportional to the logarithm of the stimulus.
Fecliner's theoretical interpretation of Weber's law is open to serious objections, Fechner assumes that the value of the hminal difference remains the same at all points of the scale (f?4=confltant), while experiment shows that Weber's law only holds good within certain limits, and that the value of iS'a alters at the extremes of the strength of stimulus. Fechner further assumes that the amallest appreciable increase of a sensation represents its unit of magnitude, and that all sensations result from different sums of sueli 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 Fechner that every appreciable increment of stimulus invariably excites a sensation of the same value, and that these sensations together Bummate 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.
Ill, Up to this point we have discussed sensations, and their different modahties, qualities, and intensities. But psychologists
I CUTANEOUS SKNSIBILITY 11
mean by the term " seoeation " the Himplest and iDclivisible state of consciouBDeas, by which we appreciate any alteration, e.g. light, colour, a sound, a taste, etc., without associating with it any relation to internal or external csosea. Pure and simple sensationB, as such, exist only in the new-born, in whom the sensory centres are incompletely developed. In adulte, sensations are converted by a peychical 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 ooraing 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 pliysiology of the senses it is often no easy task to distinguish in apparently simple sense-perceptions the dementary sensations of which they are compoeed.
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 Jxom 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 are 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 patholc^cal stimulation of the sensory nerve-trunks, are pro- jected externally : we refer them not to the place at which they are really excited, hut to that to which we are accustomed to
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12 PHYSIOLOGY chap.
refer the correspoDding normal peripheral excitations, as was shown in the cl^pter on the general physiology of the nervous system (Vol HI. p. 201>
This leads us to grave philosophical queBtioud which can only be briefly touched on. How does the objectiiicatioQ of seueory perceptions come about? How are we able to diatinguish the outer world from ourselves? Sinoe we do not actually feel external objects, but only the changes which 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 ue ? These problems are as important as they are hard to solve, and the interpretations given to them by peycholi^ts 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 phenomenaiism 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 outer 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 nativistio ideaUsm 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 irom 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 fects, 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 Wundt, the first of whom specially brought out the importance of association of the various sensations and perceptions, while the second laid stress on the unconscious reasoning processes.
The new-bora 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 chOd moves its limbs and begins to exercise its cutaneous and muscular senses. Tactile sensations ore 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 uncooaciouB 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 seuses, touch alone is sulhcieut 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 conmion 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 ad^^nce was made.
Pochlin (1691) was the first who insisted on the anatomo- pbysiological distinction between tactile and thermal sensibility (ealoris et frigoris scTisus). Erasmus Darwin (1794), in his famous Zoimomia, 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 nonnaL 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
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14
PHYSIOLOGY
valuable resultH which still coDBtitute 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, cold, and preawre spots by Blix (1882), confirmed by Gold- Hcheider (1883) and Donald- son (1885). Another marked development of the physio- logy of the cutaneous senses was the work of v, Frey (1894-97), which showed tliat in addition to the above there also existed in the sktn a fourth sense-oi^an con- stituted by pain spots.
The work of Herzeii (1886) and Goldscheider (1898) on the paralysis pro- duced by compression of the nerves of a bmb 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 more strongly depressed and disappear more rapidly thaa sensibility to heat and pain. Ponzo (1909) showed that stovaiiie by its peripheral action produces local anaes- thesia to stimuli of touch,
wcmn ii»insuuiDciii,c<iun>uui.iitiiiuwineuii P^°' *"^ ''"''^' ""^^^ SCUSi- pylLnder <cin. IndUineter, dliWedintWTnai/byii bility tO heat StiuiuU is 16-
':hkit''iS^"S;.'tSb2r?'L';fnKj««{S.°^"r' tained. In this respect the STLnS^r°Si^:^™i^!''A""u,?SLii^''iiJj work of Strauflkys (1899) on ij i» KKwed on, uj ™n7 the oioiting Bortkte f, the reappearance of seusi-
TlieendorattiFnnonieUT'Ji, tomeuunthetem. blllty ID portlOUB Of SklU
tEr^%hi^°'^^''tL"ufp'o'f"CtiJi^°u»'! grafted for surgical purposes
Thfl Mcitipg niirfaue may be v>ri«d by using III* ig gf great importance. It
proves that tactile or pres- sure sensibility ap^wars first, wtiile 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, ticklmg, sexual
CUTANEOUS SENSIBILITY
15
^
n
pleasure, etc, or whether theae ehould be eonaidered as ^)ecial modifications of the senses of touch and pain.
A clear idea of the form, arrangement, and number of the different aensorj points on the akin may be obtained by briefly reviewing the experiments on which the discoveries of Blix, Goldscheider, and v. Frey were founded.
The simplest apparatus will e cold spate. Snutll metal rods '
dipped into cold oi hot water, would be auitabte, except that they have to be changed so frequently, and that it is inipoBsible to be certain that they always act on the skin at uniform temperature. The contrivance of Blix, which (oiiJ<ists in a small hollow metal cylinder, through which flows a constant stream of water at unifonn temperaturp, in more reliable. Alrutz and Kieaow made various alterations in thin apparutUB, so that it can be used for different purposes. The most perfect is the thermo-aesthesiometer of Veress (Fig. 1), which la used for mapping out the thermal sensibility of small cutaneous areas of 2 or 6 mm. The end of the apparatus can be unacnwed and readily replaced by surfaces of different sii'.eB, or by a blunt point, when required for the investigation of heat spols.
For pre:«ure poiut« the simplest and easiest method is that of v. Frey — with the so-called exploring hairs or briatlea Huir« of varying thicknesa (horse-hair, woman's hair) are fixed to the end of a rod, the length of which varies from 1 to 4 cm. Fig. i gives the lat*8t form of v. Frey'» hair-aeathesiometer. The anterior graduated half of the metal cannula runs backwai^ 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 Bcale-pan of the balance, the umoiint of pressure necessary to bend it lightly con be determined ; this, of course, increases or dimiuiehea according as the length of the hair is less or greater. The millimetre scale of the instrument serves for the empirical graduation of the dwree of pressure required to bend the hair according to the length of the eipoeed portion.
The same aesthesiometer may be used to determine puiii
eints if the exposed portion of the hair is so short that it will nd only at a pressure sufficient to evoke a sensation of pricking.
If a moderately cool metal point iB brought into contact with the skin, without pressure, the sensation of cold is evoked only at circumscribed apots, distant 1-2 mm, from each other. These are the cold spots of ^'i'i«the»r^' Blix. If the metal point used for exploring the akin ia motflr oi v. much cooled, a eensation of cold can also be obtained p^^uoif^ii from other surrounding areas ol' the skin ; but it is '"'' always less intense, proving that it dependa on transmission of the stimulus to the true cold spots. If the skin is tested with a hot metal point, sensitive apots are found which react in the same way by sensatious of warmth. Theae are the heai spots of Blix. Exploration of the skin with gentle tactile atimuli, as by hairs, gives Blix' pressure spots. Finally, the same method will detect V, Frey's pain spots.
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16
PHYSIOLOGY
These numerous sensitive points for cold, heat, pressure, and pain are not superposed, but are distributed over dlBerent 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 arenotequidistantnorregularly
distributed, and consequently
preMote ipott bUck.
Fm. 4.-DI)li:lbntlaiiortliermiilapat on pilnuT inrbc* or left ftmBrm (Kieaoir.) Cold ipoU muke gneo, nriD apota nd.
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 jo'essure.
The number of points is much greater according to Goldscheider than to Blix. Eiesow'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
right-hand cold spuU .
gives the distribution of the thermal points according to Eiesow 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 Itairs 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 " gooae-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
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I CUTANEOUS SENSIBILITY 17
avenge, tberef<»e, 12-13 cold spots and 1-2 heat spots per sq. eta
Blix foond that in the hair-cUd regions of the akin, which he estimates at about 95 per cent of the whole, the pressure points coincide with the hair papillae ; other pressure points that can be detected here sjid there where there are no hairs probably correspond to mdimentaiy hair papillae. But 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 lipe, lip 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 luts not yet l>een estimated. Od the back of the hand v. Frey found 100-200 in every sq. cm.
Once the poeitioD 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 Miilter's law of the specific enei^es — by showing that they react by the same form of sensation (warmth, cold, pressure or pain) when excited riot only by adequate but aUo by inadequate stimuU. 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 atimuluB, or with a point heated to 45° — v. Frey's paradoxical aenmtion of told.
The legitimate conclusion from these results is that the skin is provided with at least four distinct sets of sensory nerves, for the seneations 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 Burbce correspond to these sense-organs in the layers below them.
V. Before attempting to solve the question whether four different organs or termiual 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-fibres that innervate the skin form a deep nerve-plexus in the subcutaneous panniculus adiposus. Most of Vb.e fibres of this plexus run towards the surface of the skin, and after crossing the reticular layer of the cutis reach the subpapillary layer, where they form a second plexus lens rich in fibres, the so-called superficial 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 adiposua From the superficial nerve- ptexus still more numerous fibres branch off to end in special corpuscles in the different layere of the cutis — the reticular,
YOU IV c
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18 PHYSIOLOGY chap.
Bubpapillarf and papillary layers. Some nerve-endingB even reach the rete mucosum of the epidermis, more exactly the stratum germimitivum or layer of cylindrical celb and the prickle or polyhedral cells, where they end not in complex corpuscles but in simple sweUiogs or bulbs.
The following table from Ruffini, adopted also by Crevatin and Dogiel, indicates the topography of the different nerve-endings present in the various Layers of human skin : —
!& I Stratum c
Stratum lucidum JLayere without nerves.
/Stratum granuloaum . I
1
Rete mucosum ■ "l Layer of prickle cells . 1 Layers of longer nerT«s. IStntum germiuatiTum . /HederiformezpaiiBioiu-
jupportiug membrane.
(Ueissner's corpiiBclea. Domel'a corpUBdes. 1 Ruffini'a papillary endings. lOoIgi-Uauoni corpuscles.
f Meiwner's corpuscles. SubpapUlary layer . . -I Dtmel's orbonform lermiuations.
lOo^i-Mazzoui corpuscles.
Reticular layer .... Dogiel's iirboriform terminations.
fPocini's corpuscles. Dogiel's arboriform terminations.
As shown ill this table, the most superficial nerve-endings of the skin he in the two deepest layers of the rete mucosum or Malpighian layer. laugerhans (1868) first saw that certain nerve-fibres, after losing their iiiychu sheath, penetrate the epidermis to form a network with loose meshus, and then spread in independent aud varicose branches through the epithelium, to the outer limit of the layer of prickle cells, where they terminate in bulbs (Fig. 6). Phylogenetically, these represent the oldest form of nerve-endingB in the vertebrate epidermis.
The so-called bederiform nerve-endings he in the Malpighian layer close to the sweat-glands. The nerve-fibres of which they are formed come from the superficial plexus of the skin. Near the epitheUum they lose the myeUn 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 Banvier and Dt^iel
I CUTANEOUS SENSIBILITY 19
believe to be sensoiy in character, like tlioee found in the oUactory
Ml Uunugh FpidarmiiothiiDunliiDd. <IUn'
■npflrUci&l bomf lalH ; up., iwollen hor "" " *
U^lpJ^Llvi Ufer : coDaiatlD^ of p-, priakli
from Uda pleins vulcou nervB-flbrllH cai
id c, slonmMd Italia t
iiliiBinliiBli>"toiluni iiro7lJiecu1Li von; Ji« H>lpl(|hlii> liyr.
and guetatory organs. Phylf^euetically, the hederiform endings
with terminal baskets represent the latest form of nerve-e in the vertebrate epidermis ; they occur only in mammals.
iding
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20 PHYSIOLOGY chap.
Meiesuer'H corpuscleB, which lie in the papillae and eub- papillary layer of the eorimn, were diBCovered in 1852 by Meissner and R. Wagner in the cutaneous papillae of the hande 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 lesa 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 liomogeneous, 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- scribed by Dc^iel represent a transitional form between the typical nerve-endings of Meissner, which are collected in a n corpuscle enclosed in a capsule, and the
Fin. K.-Mfli9snflri toT]>aiu:[^ [n t. Don - typical Hcrvc - endiugs, which do iuIniliMibM'<i'ihi'"rtdi''H*igh?; '"-''' ^^^"^ ^*'*' corpuscles, but remain nuHiniiLai. (itanvier.) », n, two free within the papilla, without a sur-
nerTB.nbrei, piuuInK to Uia cor- ,■ i li i- ^ j
puHi<: 1, a, urminii ruiome roundmg capsule ; these were first de- SSlKr^Kie. ""*'"'"''" scribed by Ruffini (1892) in the l>apil]ae that contain no Meisaner's cor- puscles, under the name of papillary bulbs. Dogiel's corpuscles consist of two parts i one closed, lying at the base of the papilla, the other open towards its apex. The former dilfers in no respect from the typical Meissner's corpuscle, the latter resembles one of the many forms of free nerve - endings described by Eufiini, 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 tliat they are also present in anbcutaneous con- nective tissue, as well as in the subpapillary and papillary layers. Their external form' and dimonsiDUB vary ; tliey consist of a lamel- lated capsule and an internal core of fibrillary connective tissue
I CUTANEOUS SENSIBILITY 21
with iiiauy nucIeL Two or more braucliea of a iiiTvt' - fibre penetrate (he core, and there loae their sheath and become attenuated. The j»le fibres divide and subdivide into a large number of branches, which do not form twisted convohitions, but run a tortuous course to the end of the core. The branched fibres for the most part present numerous varicosities of different shapes
Fli,. C.-'DoalEri cdrptuclB. n. VulciHe nbr«, pmigtiiK to tli" corpuscle; b, li, cluKil portion of eorpnide, coiRipondlDg lo ths tus oT the iuplll> ; c, trm part, cormpoodlng to ttis ipui ot tbeptpllli, formal of non-mysLinated vuicou nbna.
and sizes (Fig. 10). In others the varicosities are scanty, and the appearance of the terminations is totally different (Fig. 11), In others ag^in, according to Crevatin and Dogiel, one or more dehcate 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 myeUuated fibres (Fig. 12).
In the subcutaneous Ifttty tissue there are two other charac- teristic forms of corpuscles besides the Golgi - Mazzoui bodies :
^ioogle
22 PHYSIOLOGY chap.
Paoiui's and Ruffini's corpuscles. The former, already discovered by Vater, were described in delail by Pacini (1840), who saw them
Pin. 1ft— Two OolRl.Uiiianl C0iTmMl« mnnecMd with m Ungle blfurenWd nBrvB-flbni. (HufflBL)
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
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I CUTANEOUS SENSIBILITY 23
lated tissue and a ceDtral core penetrated by the inedullated fibre, which runs through it direct to the end, where it branchee and ends in an eolai^iuent.
Between the largest Pacinian eorpuBcles, that are plainly visible to the naked eye, and those of Golgi-Mazzoni, which can only be detected with the microacope, there is an uninterrupted series of intermediate or tranBitiooal forms. One very rare variety of Pacinian 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 bulbs (Fig. 15).
'nt li.-Oolg1-M»i«nico™iKl(. (Cr«- nUa.) BHltlnlJieinfffLnilednem- ■bn, ■ Bne nan . n jell nitsd Dbn peneUmtw Into the corpuKle And rbrmn a aetwotk In the npiuls, u deacrtbed
The corpuscles which Buffini discovered in 1891 have in common with Pacini's that they are found in approximately equal uombers in subcutaneous cellular tissue, and like the Pacinian bodies are of very variable dimensions. They are cyhndrical and spindle-shaped. A capsule consisting of a few thin lamellae closely applied leather can be distinguished from a supporting bundle of fibrillary connective tissue and elastic fibres, between which the nerve-fibres penetrate and expand in the form of a non-myelinated ramification. Sometimes the nerve-fibres enter laterally (Fig. 16) j at other times they enter at one end of the spindle (l';ig. 17).
Buffi ni's corpoBcles also present many variations. The
^ioogle
24 PHYSIOLOGY chap.
cutaueous iitiive-platea'of Crevatiii, and tlie arboriferous terinina- tionfl of DogieX, have been deecribed under this name, but differ in certain very important morpliologicat characters.
I^n, 14.— PuJnlBn bodr, from nt'H niBnnMi?. Migniltsd. <IUniiQr.) n, Fednncia, <rithnwv«-
ot Hhatb ; a, bnnched nervA^iiding At the dietal eiul or tliA COT^ ; iJ, lip« iif»puatinff Uit tanlcii or the corpuscle; /, chanod tbrongh the tunica bnverifld by the nArn-llbn; c»
All theBe eiid-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 wliole of the rest of the skin
1 CUTANEOUS SENSIBILITY 25
where there are hairs, reprenentiiig, according to v. I'rtiy, about 95 per cent of the total cutaneous areii, the various corpuscles we have referred to are not absent, bub become less frequent, 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 regious^this is the nerve-plexus, which can be seen round the hair follicles beneath the mouth of the sebaceous glands. Arnatein (1876) with the gold chloride method first succeBsfuUy 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 auQular and longitudinal fibrils. The latter
Pin. IS.— Rninui
corpoicjfl, ihoi
are highly varicose and more external ; they rise along the hyaline layer towards the surface of the skin, and terminate in wide disc-
^iooyle
26 PHYSIOLOGY chav.
like enlargements (Fig. 18). The nervea of human hair have not
Fio. IT.— Rnfflnl'i oorpiucle, Id wbich the Bbno ptnetnte into ons tai ot the spindls. (RutDnL C.. Cipsnlo ; H,, ahe«lb of Hsnle ; >.(., >o»t™t«ciiLar Umub ; n-t, nerve.eniling.
yet been described and studied; but everything leads us to con- clude that they are similar to those of the hairs of other mammals. As regards the specific sensory function of the aeveral I'orms of cutaneous nerve -endings, it must be confessed tliat our knowledge has p/ made little progress. The peripheral organs lor appreciation of pressure are undoubtedly represented in all parts of tlie skin provided with hairs M l>y the ahove-described nerve-plexus in the outer ahuath of the haii'-root. ^ Blix, and more recently v. Frey, /■( have demonstrated tliat a pressure point corresponding with each hair lies near the point at which it 4_^ emerges, on that side from which the hair follicle slopes.
In regions tliat have no hairs it can be affirmed with great prob- ability that Meisaner'a corpuscles correspond to the pressure points.
Pia. 18.— SpcUon throusta ■ litilr uhI ball nii i. i> tii. i n
■iiHth or at mwniHed ISO timcnu The results of B1l\ and V. Frey m '^K; Lil.'tSLta^intara.^r !^tl\ f^ct agrec with the old view on which hiir; i.t, tunic* oiteiM; \.(., hysiine Meissner's Gorpuscles Were always lield 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 -stimuh, and
CUTANEOUS SENSIBILITY
27
to the iact that &ppreciatioQ nf pressure is hmt iu cutaneous scaia.
Yon Frey also suggested with much probability that the pain spote, which are most abundant in tlie skin, are served by the free terminatiouB of the superficial nerve-plexus, which supply the epithehum 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. Frey in certain regions would have to be much more numerous and closer together. The fact tliat the cornea, which v. Frey foaad to be destitute of any specific sensibility except pain, is
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 tlie 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. By eUmination it may be said that Dc^el'e corpuscles, HufKni's papillary endings, and the Golgi-Mazzoni corpuscles are the organs for tlie sensation of cold, while Pacini's and EufBni's corpuscles functiun, at least in the skin, aa organs for the sensation of heat. The fact that the
^ioogle
28 PHYSIOLOGY chap.
latter lie in the deepest layer of the Bkin ^rees well with V. Frey's stfttement that the heat spots are the most difBcult to determice and have a longer reaction time. On the other hand, it appears probable from an interesting observation by t. 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 bother, 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 K H. Weber showed in 1851.
We saw that it is easy by means of punctlform stimulatiou 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.
Goldseheider, in order approximately to map out the distribu- tion of thermal sensibility, experimented on different cutaneous r^ons 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 aa it contains many or few thermal points for heat orcold. It must be noted, however, that the degree of sensibility is not proportional to the number of excited sensory points, bet^use 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 tfiat 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 the^e 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 Goldseheider there is no
I CUTANEOUS SENSIBILITY 29
i^ion in which sensibility to wanuth is more developed than that to cold. This holds good both for the covered and for the uncovered regions. Where the sensibility to heat is highly developed, tliat to cold still preponderates, both in intensity and in extent There are — as we have said — regions in which sensibility to cold is more or lees acute while sensibility to warmth is very low or entirely absent.
The varying thermal sensibility in different cntaueous areas depends not only on t)ie 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
Weber and Nothuagel attempted to map out thermal eensibility 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 cyUnders, at a temperature of 15° for cold and 45°-49'' for heat. More recently Veress (1902) has ^ain investigated sensibility to heat on himself by means of his 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
(b) 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.
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30 PHYSIOLOGY chap.
(d) Sensibility to heat is not unifcarm in the extremities; Borne distant parts are more 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 tliat in those cutaneous r^ons which are peculiarly adapted to tactile sensibility (as the hand in generaJ, 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 are 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 be 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 experimeuts 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 lai^er surface. One finger alone can be plunged into water at a temperature at which the immersion of the whole limb would be paini'ul.
The reaction time for sensations of cold and that for tactile sensations are equally sliort ; 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 penetcate 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 Xiesow's latest work, the reaction time to pain sensations is much shortened if Hharp-poiiitcd stimuU 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 31
precedes that of heat Further, the excitation of an7 spot by cold produoes a mort; lively sensation, that reaches its maximum more rapidly than excitation of the same spot by heat. Accord- ing to V, Frey this difference is not apparent on exciting the two thermal spots hy electrical stimuli. From this he concluded that the nerve-organs of the warm spots lie in the dee)H!r layers of the akin, and those of the cold spots in the more superficial layers.
The physical properties of the thermal agents, again, have an influence on the eCTects of excitation. Stimuli may consist of solid, liquid, m- gaseous bodies, and may act by conducting heat or by irradiation; they may be good or bad thermal conductors; their thermal capacity may be 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 giadually increasing strength can be excited. This, however, apphes only to intensity of seusation 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 bed 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 mimmal 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. T)mnlK!rg 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 d^ree of beat required to evoke a thermal sensation.
The importance of the smoothness or roughness of the surface of the body 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
^iooylc
32 PHYSIOLOGY chap.
in the skin. So loug as the temperature of any part of the skia remaine constant between certain mean limits, there is no ex- citation ; but as soon as the temperature of this region changea, either from external or iuteiiial reasons, thermal sensations at once arise.
Normally a slow, continuous thermal current flows 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 the rise or fall of temperature in the end-organs. According to Weber it is these chaugcs 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, he writes, as If we could detect the process of rise and fall in the tem[>erature of our skin much better than the degree to which the temperature rises and falls. Since the discovery of si>eciliG organs for cold and heat, it has become possible tu 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 for heat liy 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 akin were growing warmer. Fechner aud Vierordt also noted that it is possible to feel a prolonged sensation of heat or cold without any cliange in the temperature of the environment. These facts led Hering to conclude that not only thermal changes, but the alraolute degree of cutaneous temperature as well, may act as a stimulus of thermal end-organs.
When the toiiiperature of thii environment remains fairly constant we are nut as a rule aware of any thermal sensation, although the different parts of the skin may have a very difTerent
I CUTANEOUS SENSIBILITY 33
temperature, according as thej are exposed or covered. The temperature that produceB no thermal seoBation is not at &aj definite pcnnt 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 &om 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 considerahle limits without producing any persistent thermal sensation. It might be Buppoaed that this adaptability depends upon variations in the blood^supply to the skin, which to a certain extent protects the peripheral thermal end -organs horn the oscillations of temperature in the environment. Thuubei^, 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 phytialogieal zero-point (Hering) of thermal sensibility.
Starting &om tUs fact Hering maintains that any intrinsic temperature of the thermal o^ans above the physiological zero- point is perceived as heat, and any temperature below the zero-point as cold. Tlie 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 tempeniture of the end-organ appreciated as heat causes an upward displacement of the zero-point : any temperature appreci- ated as coM, 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 fondamentally 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. Bnt the so^iaUed paradoxical sensation of cold cannot be explained either by Weber's or by Hering'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, £iesow, Thunberg, VeresB, Bsder). All cold spots react by a sensation of cold when brought into contact with a warm point. When a thermo- aestheeionieter is applied over an extensive surface, cold spots are stimulated as well as heat spots, but the sensation of warmth
TOL.IV D
^ioogle
34 PHYSIOLOGY ' chap.
predominateB aad maskB the opposite senBation of cold. Thunberg, however, by chooBing an appropriate form of stimuluB 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 paradojiical 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 Si^ar (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 condylomata.
VII. Sensations of pressure, like sensations of heat and cold, are elementary, and cannot be split up into simpler components. Fressiure sensations enable us to appreciate the surface contact of external objects independent of their temperature.
MelBsner beheved it possible to distinguish sensations of simple contact from pressure sensatiom, 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 outaueoua 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 shghtest 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 sensibihty to thermal stimuli differ from those of sensibility to tactile stimuh.
An exact comparative determination of tactile or pressure sensibility in the different regions of the skin is very diflicult. The pressure exerted by a gas or iiuid 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 tlie hydrostatic pressure when the entire body is bathed in water. The pliysiological effect depends not only on the amount of pressure exerted ou the skin, but also on
I CUTANEOUS SENSIBILITY 35
the aumber of superficial cutaoeous elements on which the piessme acts. The effect aeeinB to be greater or more easily perceived in indirect relation to the area of akin compressed. Von Frey'a method of determioing the tactile sensibility in different regions of the skin by hairs is based on this fundamental obaervation.
With this method it is easy to preve that the parts most sensitive to mechanical agents are the tip of the tongue, the red port of the Upe, 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 senability is most developed, which is s valid argument in favour of the theory that the two forms of sensatioQ arise in different end-organs.
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 BtimuUte an exposed nerve )e prol>ably a thousand times greater than that sufficient to excite those nerves adequately when it acts on then- 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 detfitmines the excitation, hut the deformation of the skin surface, which produces an alteration in the pressure (Druckgefdlle), and this again indirectly pioduces an active reaction of tlie 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. la the first case the pressure is greatest at the compressed sur&ice, and diminishes in the deeper parts 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
^iooyle
36 PHYSIOLOGY chap.
towards the deeper parts and in the surrouDding areas (Fig. 21). Both the poedtive and the negative alterations give rise to senea- tioDS of contact, and this with approximately equal strength 'of stitnulua showB 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 order 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 £i6sow show that the liminal value varies with the rate of
■«efghtT!S!"tl by tiBctlE
u), ud b; tiBctlon op • dlu linn In A npre»Dt the posit
pTBTiOUBly -tt
fl poaLtlva clun^ orr . ^ .
iL« ijQni.j>v vi»uH^ >jiD ■•■lAtlQiis of pTHflun Ln Ch« Hliln Art mkitmtl »t th« edges of th« freight or diic, lod become gnidiully Itma below tbe ana comprcHed or pulM 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 liminal mechanical stimulus cannot be estimated by weight, because the effect of a given weight always depends on the area of the surface of contact.
(b) 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 hmiual estflnations when the experi- ment is confined to the excitation of single nerve-endings, t.«. to single tactile spots, by means of v. Frey's hairs.
(e) If the same tactile spot is stimulated by a weight which has a constant surface of contact, so aa 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 tbe deformation ■
1 CUTANEOUS SENSIBILITY 37
takes piace. A deformatioD rapidly produced has more effect than one produced slowly. It follows that the effect of the Btitnulus IB not dependent oq the mechanical work performed, becauee different amounts of meohaiiical work may produce identical sensations, and vice versa.
(d) If on stimulating one and the same tactile spot the surface area of the etimnluB is altered, then to obtain approximately the same effect the weight and rapidity of etimulation must be correspondingly altered. Hence the results obtained with different methodB 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 lai^r tJie 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-oi^n 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 compreesed and those not compressed. If, e.g., one linger 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 increaalng pressure, since the variation is so alight that it remains below the effective threshold of stimulation.
Kiesow (1904), in a long series of patient and delicate re- searches by the most modem 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 BUx and Gold- Bcheider) in the surface unit, and then the liminal stimulus for vbe touch spot, that is the mean value of the threshold, obtained by a seriee of separate observations in different cutaneous regions. These estimations were made by means of v. Frey's hairs.
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38 PHYSIOLOGY chap.
The reBulta of these researches were published hj Xiesow in a Dumber 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 sensibUity 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 Tnean liminal values : —
Back, luedinn line, level of 3rd donal cerl«bra 1-00
AbdoTiteii, w]iit« line, midwav between uiubilicus and pubic ay mphyBi-' . 1'06
Thorax, median line, level of Sth intercoatal space 1-24
TltoTax, left axillary line, level of 5tii intertVBtal space. . . . 1'33
Thorax, median line, level of 4th intercostal space .... 1'39
Thorax, left axillary line, midway between xiphoid and iimbiliciiF= . 1-79
Left patella, middle of 1-95
Left leg, in the middle of the anterior surface l'B9
Back, median line, level of autero-Buperior iliac apine .... 2-23
Left thigh, anterior surface, about 1 cm. from edge of patella . 2-31
Back, median line, level of 7tli cervical vertebra S'T2
ITiorax, median line, level of 2nd intercostal space .... 2-77
Left leg, caif 2-96
Left arm, middle of flexor surface 3-01
Left icnat, atiloid procene of ulna ........ 3-06
Left elbow 3-09
Left forearm, upper part of fleior surface . .3.12
Left vtrint, dorsal surface, middle line 3-26
Dormm of left foot 3.38
L^t toriit, radial surface 3.49
Left forearm, middle of flexor eurfaue 3-80
IvrUt, 2.7 cm. above the joint 3-80
Left upper eyelid 7.16
Foi-«A«ne (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 minima/ liminal values (not the mean, as above), obtained the following results : —
Finger-tips of left hand 3
Edye of tower lip, middle pari. 50
Tip of Umgjie 60
Kiesow obtained the following number of touch spots in the surface unit (1 Sf]. cm.), starting from the region in which tliey are fewest ( = I ) : — ■ Leg, middle of anterior surface 1-00
Calf.
h patella, ia\A<W<i 1-60
ieyn/orwirm, middle of flexor surface 1-86
Left ana, upper [wirt of flexor surfatf ....... 2-00
Left elbow 8-43
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1 CUTANEOUS SENSIBILITY 39
Left thigh, anterior sorluce, about 1 cm. from edge of patella 2-87
Bock, middle line, level of ant. sup. iliac apise 3-13
L^foTtarm, middle of flexor sudace 3'22
Thorax, left asUlaiy line between xiphoid process and lunbilioiiH . . 3-26
Thorax, middle line, level of 2nd intercoBt^l space .... 3-86
L^ vritt, Btiloid proceea of ulna 4-10
Tloraz, middle of aiillaiT line, level of 6th intercostal space . 415
Thorax, middle line, level of 4th intercostal space 4<36
Dortum of Uftfoot, middle 4-7,'>
Baek, middle hne, level of 3rd dorsal vertebra 4-76
TTtorax, middle line, level of 6lh intercostal spaoe 4-96
Left torUl, radial a^ataxe . ......... 6.16
Left writt, dorsal surface, middle line 6-60
Left lerist, fleior surface, 2.7 cm. from the fold 6.70
Back, level of 7th cervical vertebra 8-36
CompariaoD of these tables shows that the two factoiB on which the pressure sensibilitf in the difierent regions of the skin depends (the mean liminal value and the number of ta4;tile points in the Burfaoe unit) are more or leas compensatory, but partly correspond. In other words, in certain cutaneous regions the infrequency of points is compensated up to a certain point by the lower efTectiTe liminal siimuliiB, or conversely, the higher liminal excitation ia partly compensated by a comparatively greater abundance of touch spots ; in other regions, on the contrary, both the mean liminal value and the number of tactile organs con- tribute in raising or lowering the local sensibility to contact or preesure.
If the results which Kiesow obtained for the number of touch spots in the surface unit are compared vritli those previously worked out by Goldscheider and Blix, it is found tliat they are, on an average, intermediate between those of Goldscheider, which are excessive, and of Blix, which are too low, Tliis depends partly on the difference in the methods adopted by the three workers, Airatz (1905) controlled Kiesow's results, replacing v. Frey's excitatory hair by a glaaa thread, which he preferred because it is not affected by damp; also a hair is never straight and its elasticity alters with tiae. His results are in entire agreement with Eiesow.
It is an interesting fact that Kiesow's results on the topo- graphical variations in sensibility to pressure agree surprisingly with those obtained by Weber's classical investigations with the compass, which we shall presently discuss.
Weber's observation showed that cold objects, such as coius, placed on the skin are estimated to be heavier than warm objects of the same weight and size, Kiesow showed that this depends on the fact that cold produces a positive change of pressure in the akin, in a maoner analogous to a compresaing mechanical stimulus : heat, on the contrary, hke traction, causes a negative change in the pressure (Fig. 21). So that there is in tlie first case
40 PHYSIOLOGY chap.
an increase, iu the secoud a decrease, of the action set up by the mechanical stimulus within the skin.
YIII. When any port of the skin is excited by a small sorfaca or a blunt point, we are able, even with the eyes shut, to indicate more or less exactly the place of excitation. Weber termed this capacity of localising cutsmeous excitation the " sense of locality " ; others have termed it the "spatial sense." These terms are ill chosen, as they suggest that there is a specific localising sense In the skin, other than those of contact or pressure, and of heat and cold, to which our sensations of locality or space must be referred. In reality, these sensations are only the perceptual signs of the cutaneous sensations which have already been discussed ; in otbei words, we perceive not only contacts, and positive or negative changes in temperature, but also their seat, that is the area or surface of the slan that is altered by tactile and thermal stimuli
There are two methods of determining cutaneous localisation, both of which were employed by Weber : —
(a) The skin of a blindtblded subject is touched with a blunt point, and the subject must at once indicate the spot touched. The degree ot error is measured in millimetres and indicates the degree in which the region is sensitive to localisation.
(6) The blunt ends of a compass or other instrument with a scale (Pig. 22 — aesthesiometer of Weber, v. Frey, Giesbach, Binet, Fonzo, and others) are applied lightly and aimultaneouely to the akin. The blindfolded subject has to say if two separate contacts are perceived, or only one. The power of localisation in the region under examination is measured by the minimal distance at which the two points of the compass are perceived separately, and not as a single contact.
According to Vierordt the delicacy of tactile spatial perception seems at many points of the body-surface to be in a certain relation with their mobility, in so far as it corresponds to the variety and rapidity, direction and range of the movement. Spatial discrimination is maximal at the tip of the tongue, which is able to move rapidly in all directions. In the skin of the limbs it increases from the proximal towards the distal regions, and is greatest at the finger-tips, the most distal segments, where the range of movements of the limb is maximal; these are also the parts usually employed as tactile organs.
Both in the skin of the limbs and that of the trunk tactile discrimination is more developed in the transverse than in the longitudinal axis, and on the fiexor surfaces than on the extensor surfaces (Weber) ; in the intercostal spaces the errors are mainly in the direction of these spaces, from which it appears that the direction of the nerves has some influence upon the direction of errors in localisation (Ponzo).
In young people tactile discrimination is better developed
I CUTANEOUS SENSIBILITY 41
than in adults, because the touch spots lie closer together (Landois). It varies considerably with different individuals even within physiological limits ; numerous observations show that it can be developed and improved hj practice. Czenuak sod Gartner found that the power of localisation is more highly developed in the blind than in normal people, end Volkmann 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 compoeitors it ia extraordinarily well developed in the finger- tips. In the highly mobile parts of the limbs, a few hours of ^isctice are enough to increase tactile discrimination to a
tvo tnm eUpi with two poinla sC tbelr odL To Tmi7 the ajitiitj of tti* taiMIt, th>rmil, or raiaftl itliniiliu Uu Uiml iTotr polati m*x b* nplicediby blont or ilurp meUI polnbL
remarkable d^ree, almost to double it. In the immobile and mix« protected r^ons, on the contrary {e.g. the skin of the trunk where it is low), even prolonged esercisen 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 ana, but also in the correspondine area of the opposite side.
tfany conditions alter the deucacy of tactile localisation. If & Hmb is raised so as to make it anaemic, or the veins are com- ynmed till there is congestion or venous stasis, spatial sensi- bility is blnnted. The same occurs when the attention is fatigued by unduly protracted tests (Alsbergj, and by the action of oold (Goltz); ^ler prolonged application of the anode of a galvanic current (Spanke) ; on pesaive distension of the skin (Czermak) ; bj certain poisons — atropine, daturine, morphine, strychnine, cannabiDe, aIi!obol,chl<nal hydrate, potassium bromide (Slchtenfels ud others).
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42 PHYSIOLOGY chap.
According to Sberriugton, cerebral cortical leBions in man disturb tactile localisation far more than any other form of cutaneous BeoBibility ; the patient, in fact, may refer a touch on the hand to the forearm.
Before discuseing the results obtained by various experimenters on the sense of localiaation in different regions, it is necessary to point out certain facta that must be remembered in using Weber's aasthesiometer. These are : —
(a) If the two ends of the compass are put down one after the other, instead of aimultaneously, the two contacts will be appreciated at a less distance.
(6) The same occurs if, in estimating the liminal distance at which the compasa-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 with indifferent fluids increasefi tactile discrimination, i.e. the discrimination is sharpened.
(e) 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 millimetrefl of the mean liminal distances for perception of the two points of the aestheaio- meter, obtained by Weber on a normal adult subject, and by Landois on an intelligent boy of 12 years old.
Adolt. Boy.
Tip of tongue !■! I-l
Palmar surtace of third phalanx 2-2 1-7
Bed part of lipB 4-e 3-9
Palmar surface of Becond phalanx 5 3-9
Palmar side of first phalanx 5
Doraal aide of third phalanx 6-8 4-5
Tip of nose 68 4-6
Bui of thumb T
Middle of palm 8.8
Middle of doraum and ed^'e of tongue 9 9-8
Uetacarpiis of thumb 9 6-8
Plantar Burfaco of third phalnni of big toe .... II-S 6-8
Dorsal surface of i^econd phalanx ...... 11-3 9
Cheek 11-3 9
Eyelids 11-3 8
Centre of hard palate 13-S 11-3
Palmar side of lower third foreanii 16-0
Anterior yiart of zygomatic region II^B n<3
Plantar side of metawirpus of liig toe 15'8 8
Uoraat surface of flrst phalanx 15-8 9
Dor^l head of metauarpiw 18 13<5
Inner part of lips 803 13-fl
Posterior part of zygoranl ic ifgioii 2i-6 20-3
■■yGoo^lc
CnTANBOrS SEUSIBIUTT
cipiul n^ if bud
. _ ^_ J wgion 271 i2-6
Domun of bud 31-6 23-6
Chin 33-8 22-6
VerUiofhead 33-8 22-fl
KttB&goiBt 361 31-6
Sacnl and gluteal M^oms 40-6 33-8
Poreum utd leg -10-6 36-1
DwBum of foot near tw^ 40-6 36-1
Stemmn 4C-1 33-8
Neck, high up Wl 36-1
Dooal spine, lower thoracic and lombar ragion M-1
Middle of neck 67-T
Middle of arm, thigh, Uck 67 7 40-6
Weber gave the name of taelUe cinU to the area within vhich the two points of the aestheeiometer are appreciated aa a single point. If in any cutaneons area the localiaation is equally developed in all directions, the circles are round, i.f. they approximate to the figure of a true geometrical circle ; but tliis 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 tacUle jUlds to taelile circUa.
These tactile circles or fields have no fixed anatomical limits, and do not correspond to the peripheral distiibution 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 appUed 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 sucli a varying peripheral distribution of the sensory cutaneous fibres in the different parts. Weber accordingly assumed ttiat 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 a /oca/ dgn of recognition of the pUce to which it is distributed in the periphery. In developing this idea Wundt concluded that on stimuUtion each cutaneous area transmits to the brain not only the impression of
v_ioo^le
44 PHYSIOLOGY chap.
coQtact but also the sign of the place at which it occurs, which he calls local colour, to be used in consciousness as a local sign. The local colour of the excitations aroused in the skin is gradually difierentiated as between one place and another by phylogenesis and by exercise. So long as the ditference is shght it is not perceived in consciousness, and the two simultaneous impressions from adjacent points of the skin may fuse into one. But when the difference in local colour increases, because the two impressions arise from more widely separated points, both are appreciated. With exercise and attention it becomes possible to perceive differ- ences in local colour that are not habitually noticed. This explains why the sensory cutaneous areas may be educated by practice.
This hypothesis is not a scientific explanation ; it merely substitutes metaphor for fact, with a view to making it more acceptabla On the other tiand, it is open to a grave objection : what has been said above shows that recognition of the place from which a sensation of contact arises is a function of the perceptive centre, and depends, as Johannes Mtiller showed and as is con- firmed by later researches, on its specif energy. The nerves merely transmit an excitation or nervous vibration which is common to all the sensations ; they do not transmit any quality, colour, or sign of recognition from the part touched.
B^nstein formulated an ingenious hypothesis to account for the phenomena observed on applying Weber's compasses to the skin. He held that when the excitation aroused in the skin by contact reaches the cortical centre it spreads more or lees widely, as occurs in the periphery with sensations of pain. On the neurone theory this central spread of excitations coming from the periphery is a natural consequence of the fact assumed by Bamon y Cajal that each sensory fibre terminates at the centre in an arborescence ; but even on Golgi's theory of the diffuse fibrillary network, which serves as a vehicle of central com- munication, it may be admitted that nervous activity spreads more or less widely through the meshes of the network according to the intensity of the stimulus. When two adjacent points of the cutaneous surface are touched, the two excitations on reaching the central surface spread and summate into a single excitation, which culminates in a point equidistant from the points of arrival from the two fibres (or (groups of fibres) stimulated. In this cose, therefore, there is only a single sensation of contact. When, on the contrary, the two points of contact are farther apart, the two excitations on reaching the centre do not summate, but two distinct apices are formed, which correspond with the points of arrival of the stimuli from the two fibres (or groups of fibres) stimulated. In this case, therefore, both contacts are distinctly perceived. Bernstein's theory is clearly illustrated by the geometric diagram (Fig. 23).
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I CUTAireOUS SENSIBILITY 45
The lines pp represent the cutaneous flurface, CC the central surface, in whush are the central stations of the nerve-fibres nn. When point 1 on th« skin is touched, the excitation is conducted to the corresponding point 1 iu the cortex. The intensity of the excitatioa is expr^sed by the ordinate ab, and tbs curve led represents the spread of the excitation from the point of arrival (which correspouds with the apex) to surroandiug points. The same phenomena occur when point 2 is touched separately ; the curve fgk represents the spread of the excitation when it reaches the centre. Since points 1 and 2 he in the same cutaneous tactile circle, there is a single sensation of contact when they are excited simultaneouBly. On Bernstein's theory this is because the two excitations represented by the two curves hed, fgh summate
p. — I I I I M I M I I I I I I —
Fic M.— Dtagnm to ihow how excitation* from
BiptiMtla
geometrically, and form a single resulting curve ikl, the apex i of which is at a point equidistant from the two component apices hf. When the distance between the two cutaneous points touched is increased, the two curves intersect at a point w, which becomes increasingly lower till they no longer cross ; the two stimuli are then perceived separately.
While undoubtedly ingenious, this theory has weak points. Why do exercise and attention restrict the tactile circles ? On what does tlie enormous difference between the area of the tactile circles in the more sensitive and the less sensitive regions depend ? And even admitting that the theory explains why the two points of the aesthesiometer produce a single sensation wittiin the limits of any circle, while beyond this circle two separate sensations are perceived, on what does our power of localising these sensations at the periphery depend ?
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46 PHYSIOLOGY cbap.
In reply to the first question, BernBtein asBumes that fuDctional exercise increases central resistance to the spread of the excitations. But this contradicts the generally accepted view that exercise renders the paths of transmission lees resistant to nervous excita- tions. It seems more reasonable to assume that the spread of the _; excitations is increasingly limited, as the inhibitory powers of the centres Iwcome developed.
In attempting the solution of tlie second difficulty, it is not enough to invoke the varying number of the tactile spots in various regions of the akin, because as Kiesow showed (see pp. 38-39) the extreme differences observed are never greater than 1 to 6-35, while the maximal diameters of the so-called tactile circles varr between 1 and 67 mm. To explain this fact on Bernstein s theory it is necessary to assume a subsidiary hypothesis, according to which the central spread of excitations arising from the regions in which the power of localisation is less well developed must be enormously in exceaa of the spread of excitations from regions In which the localising power is more highly de- veloped. The improbability of this surmise is obvious.
And, in reply to the third point, ae the
hypothesis of the transmission of local signs
from the periphery to the centre cannot be
accepted, we are forced to assume that the
fh!. »4.-Ari.ioUeii «■ power of localising contacts at the periphery is
jwriment,^ t»^wh(ch purely aud Simply a consequence of the general
touFhinK two laionte law of the exccutric projection of sensations.
Bnd^ddi'e'^Hn^^"''" But this Is merely the statement of a fact, not
its scientific explanation.
In regard to this law, t^ain, it is a matter of controversy
whether the empirical or the nativistic theory should be appHed to
it. Aristotle's well-known experiment favours the former. When
the index and middle fingers are crossed (Fig. 24) and an object
is placed between the finger-tips, there is an illusory sensation
of touching two distinct objects. The illusion is so strong that
it does not vanish when controlled by sight, and it increases if
the object is rolled between the fingers. Obviously this depends
on the fact that the sensitive skin-surfaces are in an unusual
position, owing to tlie crossing of the fingers. When the two
fingers are in their normal position, we cannot touch an object
simultaneously with the outer edges of the tips of the fore and
middle fingers ; two objects are required to produce the double
sensation. Hence the illusion of two objects on crossing the fingers
depends on the experience already impressed on our brain, wliich
has become the general rule of our perceptions, i.e. of the un-
I CUTANEOUS SENSIBILITY 47
conscious judgments associated with our sensations. It is thus evident that the escentric projection and objectification of our sensations into the external world occur only according to the law of experience. Aristotle's illusion has recently been explained by Menderer and Fonzo in the above manner, but by a more subtle analysis.
They recognise the same cause for Aristotle's illusion, and use it to explain a number of other illusiona Among the latter is the converse observation discovered by Bivers, that on touching with two rods the edges of the fingers which face in opposite directions when they are crossed, there Is an impression of only one rod between them. The same illusion is obtained on putting two objects under the tips of the crossed fingers, which are then confounded into one (Ponzo).
Other similar illusions have been observed and studied by Ponzo, in which, on displacing some (tart of the iKtdy, e.g. the lobe of the ear, from its normal position, the impressions are still referred to the i^ion in space in which the displaced part is normally situated. Along with these we must group the so- called finger-exchange (Henri, Ponzo), in which when the fingers are crossed stimuli acting on one finger are referred to another.
The power of localisation at the cutaneous periphery is not confined exclusively to sensations of contact, but extends to all the cutaneous sensations. Ponzo has recently estimated errors in the localisation of tactile and pain sensations by pricking different parts of the body so as to stimulate single specific pointa He found that the magnitude of the error varies with the region of the body. The maximal delicacy of localisation is found in the tip of the tongue, end of the forefinger, and middle part of the free border of the lower Up; the minimal in the lateral aorfacee of the thorax. Sensations of cutaneous pain may be localised as exactly as tactile sensations (Ponzo) ; it is consequently aAllacy to suppose that pain sensations cannot be localised, or ^R so no less exactly than tactile sensations. Thermal sensatlonH, So, are localised, but less accurately. A systematic study of this ffnbject is wanting, but the experiments of Bauber (1869), of Goldscheider (1887), and the more recent work of Ponzo, show that cold spots are capable of more exact localisation than heat spots. The stimulation of two cold spots can be plainly appreciated at a distance of 08-3 mm., while in the same region that of two heat spots can be recognised separately only when 2-5 mm. apart
lastly, the power of distinguishing two punctiform contacts on
the skin must not be confused with the power of localising them ;
similarly, two points viewed through a prism may be distinct
^ from one another, and at the same time lie localised in a position
in space other than their real place. Thu.'i, in observations made
^iooyle
48 PHYSIOLOGY okap.
by Schittenhelm and Spearman, in oases of leeioDS of the apinsl cord, the power of locaUsing sensations in the affected limbs was ahuost normal, but the patient was unable to discriminate the points of the compass applied to the thigh.
IX. Those sensations are termed painful which are character- iaed bj an affective tone of physical or corporal discomfort, even when this is low in intensity. Certain olfactory and gustatory Btimuh can produce disagreeable sensations even at their affective threshold ; certain too vivid contrasts of colours, too hareh dissonaDOes of musical tones, offend eye or ear. No one, how- ever, speaking accurately, will apply the term painful to these sensations, in the sense that this term is applied to the discomfort produced by a wound or bum. Pain is a sensation sui generis which cannot be confused with the affective tone that sometimes accompanies the so-called specific sensations. Further, the sensation of pain is one of the simplest psychical states, and cannot be transformed into perception. We may feel pain without perceiving its cause and projectii^ it into the external world. When, on the contrary, we smell a bad odour, or taste a nauseating food, our disagreeable sensations are associated with the perceptions of something external to ourselves, which acts on our smell or taste. The same holds good for unpleasant , auditory and visual sensations.
So, too, the specific cutaneous sensations (of contact, cold, or heat) are quite distinct from sensations of pain. It is true that on exciting any point of the skin by stimuli of excessive strength or duration we can easily provoke painful sensations, which with increased strength of stimulus may produce reflex cramps, mental disturbances, fainting, etc. But it would be a mistake to interpret this fact by assuming that there is a painful component inherent in sensations of pressure, warmth, or cold which increases dis- proportionately when the stimulus and the reaction to it become violent. In a moderate tactile or thermal sensation there is no trace of pain. A painful sensation aroused by too powerful compression of the skin never appears to any one on introspective examination as an excessive tactile sensation, although the stimulus is only a stronger application of the contact. The pain caused by a burn is not felt as a sensation of excessive heat. Both violent compression and excessive heat when applied to the skin produce intense pain that dominates the specific sensations of pressure or heat, and suppresses them altogether.
This brings us to the question whether the nerves and the specific end-oi^ans for pressure, heat, and cold are also capable of giving rise to sensations of pain when excited by excessive stimulation (as assumed by Hageu, Lotze, Wundt, Eichet, and others), or whether the skin contains specific nerve-endings and the central nervous system distinct centres for pain sensatioos.
I CUTANEOUS SENSIBILITY 49
considered aa a special modality of cutaneous sensation (as held by Brown-S^uard, Funke, Miinaterberg). We have already Been that the more recent work of v. Frey and Kiesow is decidedly in favour of this view, and must now investigate the arguments on which it is founded.
Funke (1880) was the first to call the attention of physio- l<^i8te to the interesting clinical observation that a dissociaitd paraiy»is of pain sensibihty is possible while other specific modal- ities of cutaneous sensatioa remain oormaL Special forms of dissociation of the different forms of cutaneous sensibibty have been described since Weber's time in a number of coses of spinal disease (spinal compressions, traumatic spinal lesion^, syringo- myelia, tabes dorsalis, etc.) Analgesia with integrity of sensi- bility to contact, cold, and heat is not uncommon; it not merely involves the akin, but may also extend to the deeper tissues, the muscles, the bones, and the mucous membrane. Tbe case cited by Weber of the Swiss physician Viessaux (18X8) deserves mention. He was attacked by spinal disease, and noticed with surprise that the fingers of bis right hand could be wounded or crushed without producing any pain, although he was able to detect all tbe clinical characters of the pulse with them. In this case of dissociation of cutaneous sensations the post-mortem examination showed a lesion confined to the dorsal horn of the spinal grey matter, which agrees with the view of Schiff and Budge.
How is such isolated analgesia to be explained, if we assume that the peripheral and central oi^ans for pain sensibility are tbe same that subserve tactile and thermal sensations ? As Funke correctly remarks, it would be paradoxical to assume that those peripheral and central organs which subserve tactile, thermal, and pain sensibility could become iuexcitable to the strong stimuli that are necessary te induce pain, and at the same time preserve their excitability to the slight stimuli that suffice to produce tactile and thermal sensations. To account for the phenomenon of isolated analgesia it is necessary to admit either that from the spinal cord up to the brain the pain paths are separated from the tactile or thermal paths, as assumed by Schiff, or that the former are already distinct from the latter at the periphery, and that the skin contains specific nerve-endings for pain, other than those for tactile and thermal aenBlbility. Funke leaves the question open.
In his investigations with point stimulation, BUx observed that every here and there the point of a needle could be pushed deep into the skin without producing the least sensatioa of pain, while in other parts a shght prick with the needle did cause pain. But his investigations into pain sensibility did not furnish facts to justify the assumption of pain sense-organs anatomically
VOL. IV E
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50 PHYSIOLOGY chap.
distinct from those of the other modalities of cutaDeous secsa-
tiOD.
Goldecheider succeeded in proviug that the cutaneous spots for heat and cold are normally analgesio ; that pressure spots, on the contrary, when excited with strong stimuli give rise to intense pain ; and that the area surrounding the pressure spots (and provided, in his opinion, with nerves of common sensibility) reacts to tactile and pain stimuli, but far more feebly than the . pressure points.
Von Frey obtained difTerent results from his wider and more accurate researches. He showed that with suitable mechanical stimuli it is possible to demonstrate the existence of well-circum- scribed spots, which do not usually coincide with the tactile spots, in which sensibility to pain is maximal To obtain pain sensa- tiens unaccompanied with sensations of pressure or contact, it is necessary to use sharp {>oint8, to moisten the epidermis previously, and to excite the skin where the touch spots are far apart. Even with chemical stimuli, and under certain conditions with electrical stimuli, it is possible to produce isolated sensations of pain.
Pain spots are distinguished from tactile spots by a longer latent period, and by being four times as numerous (on an average more than 100 to 1 sq. cm.).
There is also a marked diSerence in the minimal value for mechanical stimulation between tactile points and pain spots, according to the area of the excited surfaces. On stimulating surfaces of 3-12 sq. mm. the sensibility of the nerve-endings to pressure is a thousand times greater than that to pain (v. Frey). But as the excited surface diminishes, a given mechanical stimulus becomes gradually more effective for the pain spots, till with a minimal surface the threshold for pain may be lower than that for pressure.
It was formerly believed that there could not be pain unless the skin were excited with stimuh strong enough to act directly on the subjacent nerves (Weber). But more careful investigation has proved that it is possible to excite pain with such weak mechanical (v. Frey) and thermal (Thunberg) stimuU that all direct excitation of the nerve-Hbres must be excluded. It is further to be noted that when the skin is excited by effective instantaneous stimuli (meclianical or thermal) the sensation of pain has a very long latent period.(0'9 sec). While on the one hand this excludes the hypothesis of any direct action on the nerves, which never have this enormous latent period of excita- tion, it shows on the other hand that the stimulus acts on nerve- endings which are capable of transforming weak stimuli into neural excitation by some physico-chemical process (v. Frey).
The topography of the pain sensibility of the skin (cutaneous algeaimetr;/) has been the subject of much research, mainly Irom s
I CUTANEOUS SENSIBILITY Bl
cliiiical point of view. But owing to the imperfect methods employed, the results are very scanty and do not seem worth mention. Taken as a whole they show that the ditwimiltr sensi- hility to pain of different regions of the akin depends largely upon the varying depth of the homy layer. Not improbably it also depends on the varying number of the pain spots in different r^ons, but methodical investigation of this difficult and delicate subject is still wanting.
We saw above that the cornea is rich in pain spots and contains no true touch spots ; the conjunctiva of the eye and the glands are riph in cold and also in pain spots; the mucous membrane of the cheeks, the posterior part of the buccal cavity, the posterior part of the tongue, have little senaitiveDess to pain. Acccffding to Kiesow's observations, in some parts of the mucous membrane of the cheeks {eg. those corresponding to the second lower molar) pain spots are entirely absent : pain is not produced here by the strongest mechanical and electrical stimuli
Comparison of the results obtained on exciting the pain spots and touch spots respectively shows the following differences : —
(a) The threshold of sensibiUty to punotiform mechanical stimuli is, generally speaking, higher for pain spots than for touch spots ; but the relation between the two thre^olds varies in the ififferent regions and may be reversed (v. Frey).
(&) The threshold for electrical stimuli (faradic currents applied by the unipolar method) is higher for touch than for pain spots.
(c) Faradisation of the pain spots at a frequency not exceeding 20 shocks per second arouses a continuous sensation, while fara- disation of the touch spots up to a frequency of 130 per second produces discontinuous sensations of vibration.
(d) The latent time for pain is always much longer than for contact. The after-effect also is incomparably longer (Riobet).
(e) The sensation aroused by the stimulation of a tactile spot is projected to the surface of the skin, and may be confined to one spot; the sensation of pain aroused on stimulating a pain spot seems to spread superficially as well as deeply, and has no precise local sign. But, according to Ponzo's latest work, pain sensations too are projected to the cutaneous surface and localised there.
(/) Cooling of the skin produces hyperaesthesia, followed by loss of sensibility. The paralysis of the pain spots invariably precedes that of the touch spots. Cocaine applied to the tongue abohshes first tactile and then pain sensibility.
When we refiect on the teleological importance of sensi- bility to pain, it is readily seen to be one of the most effective weapons of defence of the organism ; but it appears to be unequally developed at different degrees of the animal scale. We have no
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82 PHYSIOLOGY chap.
proof that it exists in the lower auimals. By a niinute analyelB of the motor reactione of Lumbricus, Nonuann has proved that thej 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 Oammarus the stomach can be cut away during copulation without interrupting it. Bethe noticed that the abdomeu can be cut off a honey-sucking bee without diaturbii^ its occupation. The frog reacts violently to electrical stimulation of the sciatic nerve, but whether it feels much pain is doubtful, as the same reactions take place after decerebration. Herbivora are less sensitive to pain than carnivora. Veterinary surgeons know that horses continue to eat while undergoing an operation, and the labbit eata directly after serious operations. These and other observations show that the development of sensibihty 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.
Fain, then, is a function of the inteUigeuce, a psychical element superposed upon the subconBcious 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 alt^r. 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 be 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-ordiuation of function to a given end takes place witliiu definite limits, and that the sense-organs, like all the other organs, are adapted to function teleologically duriug normal relations with the environment, and not in ex- ceptional circumstaDces.
Whether the sensations of tickling and itching are to be con- sidered as specific sensations in tlie 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 witli 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 a desire to scratch to remote the atmojaaca In these parts it ia not neceseary to excite with light and deUcate stimuh ; coarse mechanical etimiilatioD will arouse such a tickling that it causes Woleut reflex movements spreading to ahnost 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 morbiil variety of cutaueous sensibihty 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 sensibihty to the pressure end-oigan, refers tickhng also to a special mode of excitation of the same organ. Von Frey and Eiesow, 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, hut as secondary sensations, caused by reflexes acting from the nerves of touch and pain upon the vaso- motor nervea 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 Kicsow. 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 cutaueous 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 sensibihty in parts insensitive to tickling. He further cites a case of hyperalgesia communicated by Gofdscheider in which there was hyperaestheaia for sensations of tickhng 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.
PHYSIOLOGY
EDITORIAL NOTE
This chapter would not be complete for English readen at least without a reference to the inveetimtions ol Head and Rivers on the mechaniun of peripheral sensibility. Their conclusions were drawn mainlj from a studj of the sensory changes pnjduced by section of a small cutaneous nerve in Head's arm and tne obe^vation of the seneoir phenomena that occurred during its regeneration, but they were supportea oy numerous clinical examinations made in con^nnction with Sheiren.
In addition to those fibres concerned with cutaneous sensibility, they described a s^tem that subserves deep »mitibility (see Chapter IL) 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 this deep sensibility run mainly with the muscular nervee 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 reiacting to all painful cutiineouH stimuli of every nature, and to the more extreme detpees of beat and cold, that is, to thermal stimuli above 40° C. and below 24' C., but the sensations produced are difiuse, unnaturally intense, and unaccompanied by a definite recognition of the locality of the spot stimulated.
Through the second system, styled " ^ncritic," light contact and the inter- mediate degrees of heat and cold are appreciatetT and on it, in addition, depends cuteneous localisation, the discrimination of the compass points, and the appreciation of size.
Tne 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 probaUy on nerve-endings that lie between the sparsely scattered heat spots, and the appreciation of coolness, as contrasted with cola, may be due te end-organs other than those of the cold
It is suggested that these three peripheral systems were developed at
~ "' ■ o-called
separ- of the protopathic elements is usually less extensive than the epicritic, and during regeneration and recovery of function, since the " protopathic " sensibiUtiee reappear first. On the glans penis, too, there is only "protopathic"
different phylogenetic periods The two cutaneous systems, the Bo-ca "epicritic and the "protopathic," can be, according te Head, studied se] ately on an area of skin after section of a sensory nerve, as the loss of
These ingenious and ela1iorat« observations have not been, however, verified or supported by authoritative independent investisations, and the researches of Trotter and Davies, especially, have thrown much doubt on them and on the accural^ 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 deteut«d. 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 stetcfl 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 56
point in Head's theciy, &nd thej eonld not (xmfirm the Biniultaueous return of tenaibility to touch 4nd to moderate dwreee of temperature in the same ueu, on which Head's hypotheaia largely depends.
BlBUOOHAFHT
For JobkiinBa Muller's thmry of the Specific Energtea of the Sense OrgUl^
the studeDt maj refer to the two followiag monographs, in which all the earliest
literature is githeted Dp : —
GoLiMCBiiDBS. Die Lehre tod den speiifischen Energisn dn Sinnesneiren,
Berlin, ISSl. Weisshann. Die Lehre von den EpetifisoheD SinneaeneTgien. Hamborg and Leipiig, 189G.
The following are the moat important mon<%rapha of the Physiology of the Cataneoua Sciuea ;^-
E. H. Webis. Annotationes anat. et phys. Lipaiae, 1831. K. H. Weber. Wagner'a Handworterbuch d. Physiol, iii. Part 2, p. *81.
Bmnswick, 184S. Mbissneb. Beitr. i. Anat u, Physiol, der Haut. Leipzig, I8G3. FECBHKa. Elemente der FsTDhophyaik. Leipzig, lS6u. HbBIDO. SitzniiKaber. d. Wien. Ak. Iiit., 1877.
Fthkb. Hermann's Handbuch d. PhyaioL iii. Part 2, p, 289. Leipiig, 1880. BlIZ. Zeitachrift fiir Biologic, u., ixL, 1844-85. GoLDSCHUDiB. Arohiv fiir Anat. n. Physiol., Snppl. Vol., 1S35. GoLDBCBBiDKB. Cber deo Schmcrz. Berlin, 1894. GoLDSCHEiDES. Gessmmelten Abhandlungeo. Leipzig, 18BS. Von Frbt and KiBaow. Zeitachr. t Psychol, n. Physiol, des Sinnesorg. xx., 188B. Na.aEL. Pfliiser'a Arch, lii., 18SG.
Von Fbbt. Bericbte d. k. sitchs. Gea. d. Wisa. iiiii., IBM. Von Fret. Abhandlunsen derselben Oesellschaft, ISM. EtBaow. Arch. ital. de bid. xxivi., 1901. Kinow. Wondt's Philoa. Stud, xix., 1902. KiKsoir. Zeitaehr. fiir Paychologie, xxir,, 1904.
EiBSow. ArcbiT fnr di«ges«mte Psjchologie, i., 1907 ; iviii., 1910 ; ixii., 1911. Albctz. Skandin. Arch. f. PhyaioL xvii., 1905. Alkutz. Atti del V Congresso int. di Psicologia ■ Eoms, 1906. Tbi;»bkbo. Kagel's Handbuch d. PhyaioL iiL 647, 1906. (This is a complete
mouogrsph which reviews and quotes the whole literature bearing on this
subject) H. PoHzo. "Uber die Wirkung des Stovains auf die Organe des Qeschmacks,
der Hautemphndungen, dee Oerucha und des Gehiira etc" Archiv fiir die
gesamta Psychologie, iii. and tv., 1909. H. SnoAB. " ThennoaDaestesia cutis paradoia." Orvoei Hetilap, No. 9, 1910. EiESOvr and Ponzo. Archiv ftir die ges. Psychologie, ivi., 1910. Po.tzo. Archiv f. die ges. Psyohologie, xir., 1909 ; ivi., 1910. PoNlo. Uemorie della E. Ace delle Scienze di Torino, Series II. h., 1909 ; Iki.,
1810. PoKzo. Arch. itaL de biol. li. and lii., 1911. Ponzo. Atti della R. Ace. delle Scienze di Torino, Ixri., 1911. A. STKOMPBtL. Trattato di patologia apsciale medica e teraiiia, ii. Part 2.
Halattie del aistema narvoso, p. 3. G. Lerda. "Sot I'^volutioa de fa sensibility dans lea uicatriceg, dans \ea auto-
plasties et dans lea greffes." Arch. ital. de biol. iliv. Part 1, 1905. A. FoNTAHT. Contribuzione alio stndio delle sensibilita nei coiidilomi aciiminati.
Communication to the International Congress of Dennatology and Sipliilo-
gtapby. Borne, April 9-13, 1912,
Cataueoas Nervous Apparatus ; see the latest hiatological memoir : — BovriBI. Bevue ginirale d'hiatologie, pub. Renaut et Kegaud. Lyons, Paris, 1M6.
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56 PHYSIOLOGY chap, r
Becent English Literature : —
Head and Bivrrs. A Euma.ii Experiment in Nerve DlTisioo. Brain, IMS, xxxi. S23.
TROTTBit and Daties. EiperimenUl Studies in the Innerration of the Skin. Jonrn. of Phjaiol., 1809, mviii. 1S».
Tbottbr and Da vies. The Peculiaritiee of Sensibility found in Cutaneous Areas supplied hj Regenerating Nerves. Journ. f. PsychoL u. Neurol., 1913, xx. 102.
BoRiNO. Cutaoeous SeusatioD after Nerve DivisioD. Quart. Journ. of Experi- ment Physiol., 191S, x. 1.
Mi'BRAT. A Qu«lit»tivfl Analysis of TickUng. Amcric. Jnuni. of Psychol., 190B,
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CHAPTER II
8EK8IBILITY OP TBI INTKBNAL OBGANS
. . _. . -1. ClwsiGutiOD of iDternal wnsatioiw. 2. Common senMitiaQ of th« bodj or totnaalhena. S. Pain in tlie internal organi and tUauei. 4. Ali- mcntatr needB (hunger and thint). 5. Sexual desire. 6. The mascular senu : «eimbi[itj of moscles, tendons, and joint*. 7. iDnervatian genae in the centrea of Toluntar^ movsmsnt. S. Active tactile perceptions and theitcomponeata. 9. The DiikoAn^AtAt>o adT.bA rtf >n.t...»>>^ •"»\o iiid its vafiationa in reference to the fnnctione
All internal organs and tieaueB provided with afferunt nerves have a greater or less degree of senitibihty. The sensations aroused from the peripheral terminations of these nerves are almost always independent of external stimuli, and depend ae a rule upon the somatic conditions inherent in the oi^nism. They are accord- ingly grouped together under the name of Internal or Bodily Sensations.
While the specific sensations aroused hy the action of the outer world are the basis from which our intellect is developed and perfected, the internal sensationH do not normally give any clear indications of our internal world. Nevertheless they are of great importance from the peycholi^ieal and philosophical point or view, as was well brought out by Calianis at the l)eginning of the last century in his famous hook Rapports du physique tt du moral de fkomme. 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 band we know that they exercistt reflexly. along the efferent nerves, a controlling influence upon all the functions of the vegetative and animal life.
I. The physiolc^cal study of the internal sensations of the organs has progressed very little because their indefinite char- acter usually makes a strict application of experimental methods impceaibla Fhysol<^sts have iieen content to hand over the study of this category of phenomena to clinicians, who have frequent opportunities of investigating them in tlieir patients, in whom they are often ex^^rated and become more conspicuous,
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58 PHYSIOLOGY chap.
or are, on the other hand, suppieased so that the effects of deficiency can be studied. Few physiolo^sts have attempted to classify them accoiding to rational criteria. Mt^ndie seems to have been the first who divided the internal senaations 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, aud instinctive appetites that originate from a too protracted a])Btinence. Such are hunger, thirst, desire to mictiu^te or to defnecate, sexual desire, etc.
(b) The second appear during the activity of the organs. They are oflen obscure or quite subconscious sensations ; but may be urgent, as the aenaations 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 oue of the Imst studied, is that of tension, constrKtion, and e^ffbrt felt during muscular activity, by which we judge of the range, speed, direction, and energy of our movements.
(c) The third group includes the feehngs that arise after protracted or energetic action of the organs. Such is the sense of fatigue that succeeds after too prolonged or excessive activity of the mviacles, drowsiness after long waking, the feehng of exhaustion and languor after sexual indulgence, of satiety after a full meal, etc.
(i^ 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 leas important in their effects although vague and indefinite in character, so that it is doubtful whether they normally cross the threshold of consciousnesa 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 ]«)8ition 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 speolGo external senses.
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11 SENSIBILITT OF THE INTERNAL ORGANS 59
Many of the bodily feelii^ thus classiiieit escape phyaiolwioal analyraa owing to their TSgue and obscure character. Of otnera we know Uttle, and that httle claimB qo special mention here, either because it falls within the domain of common observation, or because it comes into the special department of neuropathology and psychiatry. We must here confine ourselves to the more important physiological points that have been cleared up.
II. Heole gave the name of "common sensation" (OemeingffUhl or coeiuustfusia) to "the sum, the confused chaos of the sensations which are incessantly transmitted to the brain from all the parts of the body." Normally we have no cl^ar and distinct consoious- nesa of the functions of the internal oi^ns and tissues, but we undoubtedly have a dull and oliecure 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 body, which Condillao termed the "fundamental sense of existence," and which is the link between psychical and phy8i<i- Ic^cal 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 consoiouBness, and is prevented from becoming a distinct sensation with special characters and specific locahsation. But when it reaches a certain intensity it is perceived as a vague sense of general well-lraing or the reverse. The former, known to clinicians as euphoria, is the expression of an exaltation of the physiolt^cal functions of the organs, the latter of their disorder, transmitted to consciousness by the cerebro-epinal sensory nerves, or by the' afferent nerves of the sympathetic system.
" It is probable," Foster writes, " that sensory impulses, not <)f the character of pain, are continually, or fruin 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 exaimne them psychologically iu 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 wliolly or partially suppressed. In some forms of mental disturbance, in certain cases of anaesthesia or partial paralysis, the jiatients have no sensation in one part of the body (e.g. in one limb, tlie stomach, the brain, etc.), or from some cerebral disease sensation in one part ifl abnormal — e.g. the i>atient fancies he has a glass or wojMlen arm. More rarely there is a total abolition of coenaesthesia. It is said that the olwtetrician Baudelocque in the last days of his life lost
60 PHYBI0L06T ch*p.
conaciouBneBa 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 Ribot gave a brilliant analysis in his Maladies de la personnaiiU, come into this group of phenomena, since coeuaesthesia is the physical basis of individual personality.
Can the sensibility of the internal organs be so extended and iuteusi&ed that the work of the organs of vegetative life, which is normally carried on unconsciously, may reach consciousness ? la the threshold of consciousness at a fixed and coostaut level, or does it oscillate, and can it under certain extraordinary or abnormal conditions drop bo much that its range is correspondii^ly increased and widened ? This question is as important aa it is delicate. Beliable authors who have concerned themselves with hyp-nosia and other similar states (Beaunis, Idebeault, and others) afhrm 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 haemorrh(^ can bo 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 organa In order, says Liebeault, to explain the su^pstive 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 Personahty " contains a fimd 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 SENSIBILITY OF THE INTERNAL ORGANS 61
most important modality ol' exaltation and perversion of the aenaibility of the internal organs and tissues is certainly repre- sented by pain, in its various forms and varieties.
In the last chapter we examined pain as one of the distinct modalities of cutaneous sensibility, having nerves and nerve- endings different h-om those of the otlier sense-organs of the skin. But onlike the senses of pressure and of cold and heat, the sense of pain is not specific, but belongs to the group of internal senses that give rise to sensations that are incapable of transformation into perceptions. It further differs from the other cutaneous sensibilities in certain important characters ; it is not excited by special adequate stimuli, hut can be aroused by any kind of stimu- lation (mechanical, thermal, electrical, chemical) that is capable of acting on the nerve-fibres 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 direction 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 earhest 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, bat that, in the animal aeries, 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." In such a case, if there really
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62 PHYSIOLOGY chap.
were special organs destined exclusively for sensations of paia, we should be " driven to conclude that such ... a mechanism of pain has been preserved intact but unused througb 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 correBpOD(£ag 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 sensibihty 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 ot 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 narcosia 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 sensibihty 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 vnih nerves are sensitive to pain stimuU: the epidermis, the horny tissues in general, the cartilages and fibro-cartilages are totally insensitive, because they have no nerves.
(h) 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 excliMively by the nerve-fibres of the sympathetic system are httle sensitive to pain stimuli under normal anatomical and functional conditions, but in a state of inflammation they may acquire an exquisite sensibihty 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, hgaments, tendons, and aponeuroses have, under normal conditions, an indefinite sensibihty to pain. The periosteum is very painful, as shown on scraping the bones in certain surgical operations ; but bone itself, particularly the
II SENSIBILITY OF THE INTEKNAL ORGANS 63
compact eubstance, is inBensitive, as proved in amputations without chloroform. The pain sensibility of bone-marrow under physio- logical couditiooB is douhtfuL
The muscles in the normal state are but little sensitive to pain. During amputations without anaesthetics they give no pain. Strong oompresBion gives rise to a specific duU pain ; intense faradisation is very painful This seneitiveneBS to pain is not due to excitation of the cutaneous nerves, because Duchenne observed it with direct electrical stimulation of the peotoratis major muscle exposed during excision of the breast. The feeling of muscular fatigue presents every gradation Jrom a ample 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 oeaophf^us. 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 laryngeal tractK, but inBensitive through- out the bronchial ramifications.
The mucous membrane of the ureto-genital system is very
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64 PHYSIOLOGY chap.
sensitive along the uretlirftl canal, particularly in the prostatic or membranous part ; that of the bladder, on the contrary, has little Bensibility. Even large calculi may remain unperceived tor 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 out or cauterised without producing pain. Fain in these parts undoubtedly depends on compression or traction of tlie 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 coUc is well known.
The heart, arteries, and veins are insensitive to pain in the normal state. The same may be said of the hepatic parenchyma, spleen, pancreas, kidneys, and lymphatic glands. Tlie genital glands, the testicles, tlie ovaries and their appeud^es are, on the contrary, highly sensitive. Compression 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 liigh 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 patholi^cal 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. Tbia is sensitive under both normal and abnormal conditions, especially to mechanical stimuli (traction, dilatation) ; while the whole of the intraperitoneal 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 Tnete&rism or abnormal distension of the intestinal canal, by the traction due to inilammatory adhesions, or lastly by the produc- tion of toxines or irritative chemical products. The hyperalgesia of the parietal peritoneum eaixsed by these products fully explains the fact that in acute alKiorainal diseases the weakest stimuli may provoke very intense paia
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II SENSIBILITY OF THE INTERNAL ORGANS 65
Accordiog tu Lenuaader, ihe sauu; huldfi for t)ie thoracic aud cranial cavitiea The liiugs and visceral pleura are iusonsitive, and the pains felt in the chest in certain illnesses are caused by the transmission of excitations to the parietal pleura, which is sensitire under normal conditions alsa
The brain is insensitive, and the pains in the head so frequently felt are dae to transmission of excitation to the dura mater.
Generally speaking, Lennander holds it probable that all the or^ns innervated by the sympathetic alone, and by the branches of the vagus after the separation of the recorrent nerve, are LQsetiBitive 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 snch a general form. How can we deny the aeneitiveness to pain of the bile duct, the ureter, and the intestinal tract in cases of gall stones, of hernia, and of other forma of obstruction of the digestive canal ! On the other hand, the work which Daceeacbi 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 obvioosly painful reactions (general agitation, disturbed respiration, cries, characterktic movements of the tail, similar to those made by the cat when it is hurt by stimulation of the cutaueouB sensory nerves). The reactions are seen even after section of both vagi, or both aplauchnics ; they only cease when both have been cut. " It is interesting," writes Ducceschi, " to note that the atomach 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 stomacb, cutaneous aenaibUity dechnes. At the end of the experi- ment, after about two hours, a alight tap on the wall of the stomach caaees strong general reactions, while pinching the ear, paw, or the skin of the abdomen does not cause even the slightest reaction. There is evidently ahock of the peripheral aensory apparatus, accompanied by gastric hyperaesthesia."
From Lennander 'a latest communications it appears probable that the mucous membrane not only of the rectum, vagina, and uterus, but also of the ovary, oviduct, and ligamenta lata are insensitive to pain. All theae 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 vf^nalia ia highly sensitive. We must reserve our opinion on these theories also.
In opposition to and parallel with the chnical observations of Lennander, the clinical theory of referred pain has recently
VOL. IV F
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66 PHYSIOLOGY chap.
assumtid great physiological importance. Laage was the first who conflidered the pains and cutaneous hyperaesthesia that accompany certain diseases of the internal organs that are Kttle sensitive, or insensitive, to be reflex. Since the work of Boss (1888), of Mackenzie (1892), and specially the more detailed
Fill. 3!>.— I^^iminarianesiuid Imu of hypmlgeBla. (lUr the cllnicul rexsarchM ot HMd.
observations of Head (1898), the theory of referred pain has acquired great interest, and in view of the modem segmental theory is of marked tlieoretical importance.
According to Head, certain morbid conditions of the internal organs ai-e capable of provoking painful sensations, but these are almost always falsely located, i.e. in a part other than that afifected. In the diseased organ, too, abnormal sensatiouB
II SENSIBILITY OF THE INTERNAL ORGANS 67
are oflt;u felt, but these aiv not so much true imii as au ubscuro feeling of heaviness or strain, while true, sharp, stabbing pain is projected to the surface of the body. Accon^ng 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
Fui. id.— OUgnm of con«fl ind iiwu of b; penlgesiii, aft«T the cLInlcm] rexearchefl of ITrad. Eiplumtlon on p. tt.
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- Itility in close central connection with a part of much greater sensibility, the pain produced is felt in the part of higher sensi- fnlity rather than in the part of lower sensibility to which the atimolus was actually applied." '
■ Brain, 1893, vol. xvi. p. 127.
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6S PHYSIOLOGY chap.
The iuttirnal organs are, gi^uerally 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 ohserved 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 tliat 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 were strengthened by Head's mapping out of the hyperalgesia 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-
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 Sherrington, do overlap to a large extent. But Sherrington's later work showed the overlappii^ 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 Rynberk on the central area of dermatomes has thrown new light on this subject. And it is probable that Head's hyperalgesic zones represent s^mental zones of Sherrington's pain areas, or of the central areas of the dermatomes of Winkler and van Rynberk.
Head's clinical investigationB liave audi great practical importance tliat it ia desirable to reproduce the following diagram and table, which aum 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) bf mapping out the areas in a number of cases of cntaneoiis liy]>er- aestheaia with coincident visceral atfections ;
(6) from the topography of the eruptions in 52 cases of AwpM 'toiler ;
(c) by mapping out the analgesic areiia in organic diseases of the epinal cord and rQol&
The 8 cervical segments arc indicated by CI, G2...C8; the 12 doreal or thoracic segments by Dl, D2...Dl2i the 6 lumbHr segments by Ll, L2...LQ ; and the 4 sacral segment^ by Sac 1, Sac 2. ..Sac. 4.
The areas of the head are indicated as follows : N =naBal or rostral area ; FN=fronto-nasalarea ; MO^niedio-orbitalarea; FT =fron to- temporal area; T = temponil area; V = vertical area; P = parietal area; 0 = occipit«l area; NL=:niiso-labinl area ; Max. = maxillary area; M&n.=mandibular area;
n SENSIBILITY OF THE INTERNAL ORGANS 69
M = ment&l area; I£ = superior ]&ryngea1 area ; LT = inferior laryngeal ai«a ; TO = ]ijoid area.
The following table showa the relations between the cutaneouB areaa and the intemal ot^ns :
|
^1. ...,„..„« .,.,., |
Aim Id »< Hwd. |
|
Heart . . C3, C4 - D2 - D8 . . . . Laoga . . C3. C4 - D4 - D9 . . . . StouiKh . . . . D7 - D9 . . . . |
fV«iitride»»iidaorl»,N, FN, MO, FT. \ Auricles . . . . FT, T, V, P . , . . N, FN, MO, FT, T, V, P. . . . . FN, MO, T, V. P |
|
U»M . . C3, C4 - D7 - DIO . . . |
. . FN, MO, T. V, P, 0 . . . . |
|
Testicle . . DIO |
0 . . . . |
IV. Of interual senaations summed up under the generic name of " deairea," that for food is certainly one of the moat im- portaot from the teleological point of view, because it is directed to satisfying one of the conditions indiapensable to life- — the supply of nourishment. In its milder etages this desire is not unpleasant and is even an agreeable feeling, commonly known aa " appetite " ; when more inaistent 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, "Eegnlarity of meals," said Beaunis, "csnses the aensationa of hunger to recur with tlie precision of clockwork," Chango of habit in the hours of meals is able to modify the rhythm of hunger : if the meal is delayed 1-2 hours the appearance of hunger is delayed by a corresponding time.
The degree of hunger varies conapicuoualy in different in- dividuals, and in relation to age, to the rate of metabohsm in ■ different constitutiona, in different seasons, different professions, and so on.
Cienerally speaking, hunger is an unpleasant sensation at the level of the epigastric region, which disappears and ia replaced by
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70 PHYSIOLOGY chap.
a pleasant sensation aa 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 aurrounding parts, until aensations of constriction, cramp, and atraiu 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 aponeurosis, 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 metalioliam and increase the loss from the oi^nism — such as muscular exercise, cold air, mountain climate, sea 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 ^e, narcotics (opium, tobacco, cocaine, alcohol).
Under abnormal conditions hunger may reach a morbid level, clinically known as bulimia. In this case hunger sets in ^^in 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-waete 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 durii^ hypnosis. In the insane dtiophobia (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. l>ad news, drives away the feeling of hunger. If the attention is keenly
II SENSIBILITY OF THE INTERNAL ORGANS 71
attracted by an iDtereflting book ot intellectual preoccupation witb some important problem, the senae of hunger disappears, and the hoar 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. Id forced inanition hunger is present from the first in abnormal intensity, and is complicated later on by a peculinr 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 days of abstinence, and may decrease and entirely disappear after that. Succi, in one of his many fasts of thirty dkya which we investigated (Florence, 1889), required a narcotic to allay his hanger only in the first two days ; in the remaining twenty-eight he only ingested mineral waters, and showed no sign of suSering. 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 Shh 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 wonis ;— " 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 intestioes are quiet ; my head is untroubled, my sight clear. The few remaining momenta 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 fote of Tantalus, which is the most appalling the haman organism can endure — may induce delirium, which soon brings death in its train.
Thirst increases more rapidly than hunger with the duration of the fast, and becomes even more intense. But here, again, we must distinguish between forced and voluntary abstinence. As we Bald above, in Viterbi's case thirst was painful and lasted much longer than hunger, but it, too, decreased, and finally dis- appeared in the last two days of Ufe.
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72 PHYSIOLOGY chap.
All causes that reduce the high percentage of water in the composition of the body are able to produce the sensatioii of thirst. The heat of the atmosphere which increaaee cutaneous and pulmonary perspiration, and muscular exercise which excites secretion of sweat, accentuate thirst. Hydropic effusionfl, diarrhoea, diabetic polyuria, haemorrhage, etc. promote the desire to drink and produce polydipsia. Ingestion of highly spiced or salted 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 origin of hunger and thirst have not led to anyi 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 aniiual. By means also of intravenous or intraperitoneal trans- fusion of defibrinated blood, hunger can be relieved in dogs, but the starvation deficit cannot be arrested (Luciani and Bufalisi, 1882).
In certain cUnical cases in which ingestion of food by the stomach becomes impossible the pangs of hunger may be reheved liy 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. Sehiff 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 himger and thirst are localised in the first place to definite regions of the digestive system? Are these sensations central or peripheral in origin ? Various physiological theories have been propounded in I'eply to these questions, all of which appear to UB 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 sejisO' tions. This fact in no way contradicts the preceding observation that the fundamental quaUty of hunger and thirst, on which their teleological value as "desires" depends, is more or less
II SENSIBILITY OF THE INTERNAL ORGANS 73
diffused over the whole of the living tissue elementB. 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 paiu ; in these the liminal stimulus that causes paiu is normally much lower than that of the aenaory nen'es of the internal organs. They ate 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 moat 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 aJl observations made on patients uith 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 to 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 memhTane, 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 H. Figa 40, 41, pp. 120, 123). It is possible that the gastric turgescence excites the peripheral endings of the sensory nerves to the mucous membrane ; but it seems to us more probable that the excitation depends on the chemical changes in the epithelial protopUsm.
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' stimulus to hunger.
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74 PHYSIOLOGY chap.
So, too, it may be held that the peripheral stimulus for thirst consists in the dryness of the mucous membrane of the mouth and pharynx, which causes physico-chemical ohai^ee in tha epithelia, which again excite the terminations of the corresponding sensory nerves.
By what patlis are 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, since it is — so far as we know — unique in the whole hterature of physiology. Two young dc^, each weighing 4500 grma., 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 theu), so that the nerves could easily be drawn out and divided ; the edges of the wound were then sewn together. 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 water only. After 48 hours' starvation for the one animal and 72 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 suatch 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 which the auimals devoured meat and bones.
These ex[)eriments, which complete the too long neglected work of Brachet, seem by their simplicity to be of no little value to the theory of the geue.sis of hunger. They show, not (as Brachet tliought) 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 SENSIBILITY OF THE INTERNAL ORGANS 75
afTerent fibres of the sympathetic are leas excitable to hunger- iiDpalses, and only become active some time after the vagi have be^ cut, or when on prolonged lasting hunger becomes more acute.
The centres for hunger and thirBt are certainly, even if not excluBively, localised in the bulb and pons. This is proved by anencephalons human monstetB, which, though they have no cerebrum 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 Goltz 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 c^e. When a dish of nulk and big pieces of meat were broi^ht near its nose it lapped and chewed and swallowed with evident satisfaction, like a normal dog.
Scbiff 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- Btilution of the blood, be held that this must directly excite the nervous centres. Local sensations of hunger and thirst are, be 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 aud thirstiug 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 stoucB, chalk, and indigestible vegetable remains are often eaten. Thirst can be temporarily reUeved — e.<;. 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 difiScult of control, we have at hand, within the reach of every one to verify, a vahd argument against SchiiTs theory — viz. that the sensation of hunger disappears rapidly on in^oducing food into the stoiuacb long before it has been digested and al^rbed, and therefore before the alteration in the blood, which Schiff held to be the direct stimulus to the centres, can have been corrected. The same may be said of the introduction of bever^es and the sensation of tliirst, the more bo 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 uecessary to prove that the nerve-centres were more excitable to Btimnli than the peripheral nerve-endings. Any such attempt.
76 PHYSIOLOGY chap.
however, is superfluouB, seeing that the exact oppoeite is upheld by every phy8ioI<^iet ; the centree, that ie certain parts or nuclei of grey matter, are totally iiiexcitable to direct stimuli, and have in other parts (as the eo-called excitable area of the cerebral cortex) and other nuclei of grey matter, like the nervea along their course, a much higher Hminal 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 tliat of its lociiJ 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 iiidividuala This objection, bow- 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 tlie gastric mucous membrane; and that in any case the sensory nerves of the stomacli, while nornially 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 lieeome 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 aliiu«utary wants are teleologically co-ordinated with the preservation of the individual, so sexual dedre is corre- lated with the preservation of the species. This desire is felt vaguely and indefinitely irom 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 e^ioch of puberty. The whole organism then under- gites a crisis; the genital organs become the starting-point of new sensations, til! 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 SENSIBILITY OF THE INTERNAL ORGANS 77
lu uiuet aoimals, other than niaD, the xexual <lesire appeals with puberty, and is only felt at certain seasODs, 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 to old age ; in women it lasts till the climacteric, when the OMtries cease t(» function, except in certain eases of retarded sexuality. In animals tbe female, after fertilisation, obstinately refuses to consort with the male; in the human race and the higher apes the female has no repugnance to sexual intercourse, even after impregnation. This distinction is not, however, absolute. In the domestic animala, in which the two sexes are continually in contact, the periods of sexual excitemeut are more frequent, and there is. particularly in the male, a tendency to persistence of a«xual 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 womeiL
The most interesting manifestations of sexual appetite in the higher animals are tlie stru^le of the male to possess the female, and tbe 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. Piffer- ences of individual temperament, of climate, of social surroundings, of moral aud 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 erotoviania or nymphomania. The perversions of sex 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 aud external
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78 PHYSIOLOGY chap.
causes uf the excitatiuii whiuh, if transmitted to thtt centres, proiJuceB the consensus of pleasant and voluptuous sensations that tinally lead to completion of the sexual act.
The sex impulse is essentially connected with the presence of the male and female genninal elements, the spermatozoon and the ovule. This is the fundamental fact by wliich the indispensable internal conditions of sexual inipiJses 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 liarems 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 ad\'anced childhood or adolescence.
Another interesting fact may be observed in eunucha Although they lose the reproductive desire properly so-called, the voluptuous seasations of sexual affection are not wholly abolished, viz. such as are furnished by sight, hearing, the tactile and muscular sense, and the oll'actory 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 sufficient 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 iiersist even after castration.
Animals exhibit practically the same phenomena. Experi- ments have been made on them to determine the relative import- ance of the respective senses in regard to sexual desire, and the results ate to a large extent applicable to man alBO.
In the first place there is the work of Lazzaro Spallanzani, wlio 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. Tlie following experiment is particularly interesting:' — "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 theui ' QuoUd from the Genevan edition of 1876, by Siaebier.
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II SENSIBILITY OF THE INTERNAL OKGANS 79
developed into tadpoles. . . ." Two iuteresting cuiiclusiouu cau be dxawQ from these experiments :
(a) The sexual impulse in toads and frt^ is more potent than the most painful sensations these animals can undergo.
(ft) Kemoval of the most sensitive parts and of the whole hrain, includii^ of course the olfactory and visual organs, does not inhibit the se-x;ual clasp nor interrupt it if already in progress.
(joltz 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 hy which the male embraces the female depend, and by what paths is this centre excited ?
On these points Goltz came to the following concluBions :
(o) 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.
(h) 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 hy 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, froiii the 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 do longer takes place.
Tarchanoff continued Goltz' experiments on the frog and sncceeded in isolating the stimulus that produces sexual desire ia the male ; it is due to the tension in the seminal vesicles when the spermatic fluid collects there. Wliile no otlieL 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 oi>ened and emptied, the clasp ceases at once, or does not occur if not already licgun. On the
80 PHYSIOLOGY chap.
othei' liaud, uiuri; dilatation of the vesicle with an iudiil'ereut fluid, Buch 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 vesiclea, and is transmitted by 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. Durii^ copulation, the whole of the senses with their respective nerve-centres are active, and it is necessary to extirpate them all before the clasp can be inhibited.
Ko doubt much the same process takes place in the higher animals. Every one knows timt in mammals, e.g. in dogs, the odour guides the male to find the female, aud 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 iu 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 by Lussana and of late years hy Bunge — is the centre of the reproductive 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 : bitches have periods of fieat, in which the whole mucous mem- brane of the genital organs is congested aud secretes a viscid, bloody iluid which excites the olfactory sense in the male, whose advances are received with evident pleasure by the female — two, three, or even more suitors being accepted.
Ou the other hand, Goltz' researches on the effect of succeasive 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 exhibit traces of sexual impulse, since they sniff at the genital organs of other dogs, even if only momentarily. The " braiuless d<^," on the contrary, never gave the slightest sign of sexual attraction during the eighteen mouths in which it was under observation. So that there can be no doubt that the centre which is particularly active before aud duriug 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 cau be observed in man as
II SENSIBILITY OF THE INTERNAL ORGANS 81
in animals, althoogfa in difTerent degrees, inasmuch as they are subordinated to the higher development of the intellect and the evolation of the aesthetic and moral senae.
One farther phyoological problem must here h« taken into consideration. Is the sense of pleasure, which is localised especi- ally in the mucous membrane of the internal gt^nitat organs of both sexes, a special modification of the sense ot contact, or is it a special sense, served by specific corpuscles or nerve-endings ?
Huch morphological research has been directed to this 8ahji<ct, but no conclusive solution has at present been reached.
Krause( 1866-81) first studied the nerve-endings in the external genital organs of both seses, and described special corpuscles in the form of end-bulbe, which he termed " genital corpuscles." Of the many other histologists who have studieil this subject, Retzius (1876-90), Aronson (1886), Dogiel (1893), Timofeew (1891), and Sfameui (1904) deserve special mention.
Betzius 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-fibree 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 Erause's end-bult» or spherical corpuscles, large and small, he also found Meissuer's corpuscles. He further discovered that filaments ran ont 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 Itetween the uer\'e corpuscles and the epithehal 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 myelinated fibres, but also the fibres non-myehnated from their origin, ])articipated. 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 meduUated, which lose their myelin sheath as soon as they emerge from tlie capsular sheath, and then expand into the form of a l^iid with dentellated edges, uid terminate at the opposite pole of the ramified or simple, pcunted or roiinded corpuscle; the other, much finer, which aJso lose their myelin sheaths, and terminate after branching repeatedly in deUcate varicose fibrils which form a network. He confirmed the presence of Pacinian corpuscles on the external genital organs of both sexes, as already described by Schweiger- Seidel, Elein, Kauber, and others.
Sfameni's more recent observations were made upon the genital organs of the cov, sheep, mare, ass, bitch, and woman. In all these species the differences in the nerve-endings are insignificant. In any one animal the different types of corpuscles ]iresent an
vou IT G
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82 PHYSIOLOGY chap.
endleas Beriee of transition&l formB which pasB imperceptiblj from the typical Pacinian corpuscle to more elaborate and ditfuse 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 losii^ their medullary sheath (if myelinated) expand within and around a granular and nucleated substance."
Three points here deserve consideration : —
(a) 8fameni 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
and another ; in the chtoris, 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 m3'ehnated and non-myelinated fibres, which arise in the super- ficial plexus. Both kin(k 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 Meissner's corpuscles, as well as forms analogou.'i to the papillary nerve plexus which Kuffini described in the finger-tips (Fig. 28). Within the network certain cellular formations 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, tlierefore, these must be true nerve-cella, and not connective tissue -cells, as some have asserted.
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II SENSIBILITY OF THE INTERNAL ORGANS 83
(b) The reticular layer of the cutis contains a great variety of nerve^ndingB. Tlie simplest and most sttperficial form is Erauae's end-bulb, whieli Doriel 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
Pin. ».— CUUnii at tbenp. BKtIon obUiiTie «.r., granular ntlcnldni ; rt, lupcrncial «
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 Kuffini in the finger-tips. Lastly, there are corpuscles which appear to be transitional forms between Golgi-Mazzoni corpuscles and Pacini's corpuscles. These are lai^Iy represented in the female genital organs.
(c) There are comparatively few nerve-endings in the loose subcutaneous tissue. Ruffini's end-oi^ans 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.
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84 PHYSIOLOGY chap.
The general morphological conclusions of Sfameni from his own observations and those of Dogiel upon the genital organs are shown in his dif^am (Fig. 35).
Without pausing to discuss and analyse the hypothesis, by which, according to Sfameni, the difTerent 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-epitheual fibres), we will con- fine ourselves to stating that ac- cording to Sfameni the whole of the nervous apparatus which he re- presents must be 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
V which physiolt^istB have adopted
from minute researches into the
lo. 31. — Spherical gflnlUl corpun i-litorls. (Sbin«Dl.) aj., my
ditl'erent modalities of sensation at distinct parts of the body- surface. The skin, in whicli physiologists distinguish four dif- ferent senses, possesses, according to Sfameni, only one of the five senses recc^nised by physiology from all time, i.e. tactile sensi- bility. This enormous disparity proves the vast superiority of phyBiol<^cal methods of research over the anatomical methods of analysis of the sense-organs.
The topography of the different kinds of sensibiUty in the huDian 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 SENSIBILITY OF THE INTERNAL ORGANS 85
true touch spots. On exciting with iwinted mechanical stimuli an exceptionally high threshold of excitation is found, corre-
V^n. tfi. — CompuDDd geoitvl corpiiiclft tron libto mLnom. (Sbinflnl.) b.c., blood upllLuin ;
sponding to that of pain, but not to that of contact, which is Qormallj much