COLUMBIA LIBRARIES OFFSITE ..^^EALTH SCIENCES STANDARD HX00026212 l^-.:::>- E^sS«fc-^ ■■'■ vi^)'*^'i-.*i '■ ■ QP3U ^AS\ CHolumbta Untuprfltty in ti|r (Ettg of Npuj fork (HollFgr of ^IigHtriana anb ^urgpnna Srftr^nr^ ICtbrarg Digitized by the Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/physiologyofnervOOmora PHYSIOLOGY OF THE NERVOUS SYSTEM PHYSIOLOGY OF THE NERVOUS SYSTEM BY J. P. MORAT of the University of Lyons AUTHORISED ENGLISH EDITION TRANSLATED AND EDITED BY H. W. SYERS, M.A., M.D. (Cantab.) Physician to the Great Northern Central Hospital WITH 263 ILLUSTRATIONS (66 in CO lour i) W. T. KEENER & CO CHICAGO 1906 Vi_;)^VO butler & tanner. The Selwood printing works, frome, and London. Ix- Translator's Preface The present work forms the portion of the Treatise on Physiology by Profs. Morat and Doyon which is devoted to the Functions of Innervation. In completing the Enghsh version of Professor Morat's well known work on the physiology of the nervous system, the translator expresses the hope that he has succeeded in interpreting the views of the author with fidehty and accuracy. It has been his aim to adhere as closely as possible to the text throughout the work, making use of paraphrase only when this was essential to clearness of expression. The subject treated in the following pages is a most complicated one ; but there can be no doubt that this volume embodies the latest advances in our knowledge of the nervous system and portrays the most recent views and ideas on this very intricate branch of physiology. H. W. SYERS. WiMPOLB Street, October, 1905. Preface In every living being a double current of matter and energy is present, running in a definite direction which never varies. In these two currents the transformations of energy accompany those of matter ; they are sometimes united, sometimes separated, and their union is the starting point of a cycle of which their separation emphasizes the termination. This cycle is the simplified image of vital evolution ; and in it the first traces of organization are sketched out. But in proportion as this cycle becomes complicated and elaborated we may observe the advent of fresh cycles more or less resembling it, which superpose themselves, interfere with and bestow upon it a new value. Innervation corresponds to a cycle of this nature. In fact, while the material and energetic currents proceed from the ingesta to the excreta through the intestines and the vessels, a third and an incomparably weaker current, that of the nerves, finds for itself distinct and separate channels and intervenes for the regulation of the two former, ensuring for them their most effectual employment. The nervous system does not provide force, it utilizes it ; and this duty devolves on it by reason of the perfection of its own organization. It is the nervous system which decides at what moment the energy accumulated by the living being shall be liberated, in other words shall leave matter and exert its motor functions. This point it decides with the assistance of information communicated by the organs of the senses, and by means of a sometimes extremely lengthy work of internal elaboration brought to bear on this information arriving from the exterior. In short, by the disturbances entering into it the nervous system receives impressions from the external world of which it thus obtains knowledge ; by its own activity it forms a judgment of all surrounding it from the point of view of utility ; finally, it reveals this judgment by a motor act calculated to ensure the preservation of the organism. Such is the cycle of the nervous current ; it implies successively an external phenomenon of impression, an internal phenomenon of sensation, another external phenomenon of motor response to the viii PREFACE impression, itself followed by another internal phenomenon of sensation registering the accomplished movement. In the nervous system all movement induces sensation, all sensation induces movement. This system amongst its most extraordinary attributes possesses a power of adjournment concerning the events depending on it. These events, which on a reduced scale and in a condition of representation or images, it constructs internally with the data furnished by the senses, it pre- serves until an appropriate moment arrives for partially realizing them in the form of external movements. From the fact of the introduction of sensation into the cycle un- rolled in the nervous system, events assume for it a particular signi ficance which otherwise they would not possess. According to the affective tonality (agreeable or painful) of the sensation, they are either favourable or the reverse. Obviously, and in spite of the errors which it may commit, the living being seeks the former and avoids the latter. Whether its activity is free to choose or whether it is enclosed in an inflexible determinism, is a problem which it is not the province of physiology to inquire into. But whether rigid or elastic this determinism includes a new element and factor, sensibility, which outside of the living being is either wanting, or at all events is not apparent. The relations between cause and effect which elsewhere seem so simple are here on this account extremely complicated and modified. The power possessed by the living being, and more especially by the nervous system, of the internal preservation of external events by their reduction to the condition of representations and of their later realiza- tion and enlargement in the form of visible movements, conveys to us the false impression that the end and aim of an act is the cause of this act. The cause of an act cannot be in the future, but may be in the memory of a previous act of the same nature remembered as being either useful or hurtful and which on this account determines the direction given to the movement. There must always be an aim, a general or particular tendency determined by the sensory nature of the living being, but this aim is an effect and not a cause. The past always involves the future, but in this past the living being knows how to choose, and when it recreates it it does so as much as may be to its own advantage ; whence its almost indefinite degree of perfectibihty. Thus we can see that the study of physiology gives rise to, or at any rate borders on, problems which are not in any way its special province ; and for the rest demands from psychology solutions which the latter seeks for with the aid of its own methods. A kind of neutral area, PREFACE ix common to both sciences, exists which the former endeavours to appropriate by pushing farther back the boundaries separating it from the latter. Progress must inevitably be slow, as apart from the fact of this study bristhng with difficulties of every kind, methods, in spite of the efforts of a host of inquirers, still remain crude and unsuited to the infinite dehcacy of the organs of the nervous system and their component elements. J. P. MORAT. Table of Contents INNERVATION Sensibimty and energy . Sensibelity and determinism Sensibility and organization Excitability and sensibility. Action and Reaction. Division Page 1 1 2 3 FIRST PART ELEMENTARY NERVOUS FUNCTIONS I. Static unity . ........ II. Dynamic unity ........ Organisation of energy ....... Multiple forms and ti'ansformations in the nerve Energetic cycles. Functions, their double nature ; their ciprocal influence ... Chapter I THE NERVOUS ELEMENT A. Static condition of the neuron ; anatomical data . I. External characters of the neuron ; dimensions . Form. General type ...... Signification of the parts. Synonymy . Axon. Myelinated and non-mj^elinated fibres Segmentary cells : their origin. Collaterals . Body of the cell ....... Cellulipetal and cellulifugal prolongations II. Dynamic polarisation ...... Experimental basis. Generali ration. Uncertainty . Relative importance of the prolongations. Adaptation nexiron. Avoidance ...... Objections ........ III. Individuality of the neuron ; its proofs IV. Physiological data. Specificity of the neurons . Varied forms. Amacrine cells. Nervous amoeboism Changes of form in the dendrites .... Functional dissociations ; mechanism and localization Connexions of the neiu'ons. Adhesive svipports Mitral cells of the olfactorj^ lobe, Pericellular baskets Spiral fibre ; its signification .... of the 8 9 10 12 13 15 15 17 18 19 21 23 25 26 26 27 29 xu TABLE OF CONTENTS Sutui'e between centrifugal B. Dynamic condition ; functions of the neltron 1. Functions of internal or tropliic connexion . I. Wallerian or descending degeneration Laws of Waller. Nutrition and conduction. Trophic centres and functional centres ..... Generalization. Cellular laws. Loss of excitability. Struc tural alterations TI. Ascending degeneration. Coiidition of survival. Clironiatolysis Its phases. Application ..... Alteration of fibres. Equivocal meaning of words. Ascending ncLU-ites .... III. Atrophic degeneration IV. Regeneration. Nervous suture . Non-regeneration of nerve cells. nerves of different fiuictions .... 2. Functions of external or nervous union properly so called Internal and external acts ; properties and fmictions I. Numerical relation between the exciting energy and the energy svip plied by the n^uscle . IL Excitability and conductivity Initial shock ; specific excitants and general excitants Evidence and estimation of the activity of the nerve III. Isolated conduction ...... Nervovis induction ..... IV. Propagation in the two directions V. Integrity of the structiu-e : welding of the nerves VI. Local excitability of the different parts of the neuron. Excita bility and conductivity VII. Rate of conduction. Method. Period of propagation and latent period .... Electrical stimulus C. Stimulation of the nerves Gradation of the effects. Different stimuli Preliiyiinary conceptions ....... A. Electrical energy ; its units. Electromotor force ; volt Page 29 30 30 33 33 34 35 36 37 37 38 39 39 40 42 43 43 44 45 46 47 48 49 50 50 51 52 53 olim ; ampere : Quantity of electricity ; coulomb B. Electric fluxes, discharges, currents (instantaneous, continuous, constant) .......... Polarization. Non-polarizable elements. Acciunulators. Cur- rent (oscillating, alternating, sinusoidal, dii^hasic, continuous, triphasic) .......... 54 C. Induction ; lines of force ; field of force ; induction (electric, magnetic) ; solenoids ; magnet ...... 54 D. Apparatus for stimulation. Condensers. Induction ajoparatus . 56 Cells ; accumulators. Rheotomes . . . . • .57 E. Apparatus for the verification and the measurement of electro- motor phenomena ........ 57 Galvanometers. Electrometer of Lippmann .... 58 Non-polarizable electrodes ....... 58 TABLE OF CONTENTS D. Laws of electrical stimulation I. II. III. IV. V. Problem to be resolved. Historical ; facts and opinions Old formula ...... Contradictory facts .... Conditions to be realised. Method. Result Fixity of the quantity for a given dtu'ation quantity ...... Law of electrical stimulation Methods of stimulation .... Polar influence ..... Active pole ; indifferent pole. Cathode, anode the current ..... Unipolar excitation in an open circuit . Periodic excitation. Remark Ai'rangement variation of the Dii-ection of Xlll Page 58 58 60 60 61 63 63 64 65 66 67 Chapter II ENERGIES OF THE NERVE A. Energies recognizable in the nerve ; origin and succession Initial state ......... Final state. Transmitted and localized energies. Sources of energy and stimulation ....... Intensity. Reserve of energy. Heat developed by the nerve . Electrical energy of the nerve ..... B. I. Current of repose ........ Origin. Derivation. Pre-fexistence. Alteration. Axial current II. Negative variation ; current of action. Its importance Wave of excitation or of propagation. Dii:)hasic current Form of the lines of flux. Displacement of the phenomena. In- tensity of the variation ....... Form of the variation. Unipolar method. Oscillatory variation Nature of the phenomenon. Nervous interferences. Electrical resistance of the nerve ....... Part played by the cell in the transmission of the impulse ; dif- ferent argvmients ........ Contradictory experiments. Modifications of the cell dm*ing fimctional activity ....... Chromatolysis ........ C. Consecutive effects of the stimulation ; fatig Definition. Resistance of nerves to fatigue Temporary dissociation of the muscle and the nerve. Conclusion. Mechanism of fatigue Another method D. Electrotonus ......... I. Electrotonic condition. History. Designations . 1. Electrotonic current. Interference with the ciu-rent itself Rate of propagation ...... 2. Paradoxical contraction ...... Difference from the secondary contraction. Ineqiiality the phases ........ of 73 73 73 74 75 75 76 77 79 80 82 83 84 85 85 86 86 87 88 88 89 89 90 90 90 XIV TABLE OF CONTENTS Difference between electrotonus and negative variation 3. Theories of electrotonus .... II. Electrotonus ; modifications of excitability . Proof of excitability. General formula of electrotonus Positive and negative modifications A. Medium current ...... Ascending and descending ciu^rent. Excitability and condvic tibility ....... B. Strong current ...... Anelectrotonus in inhibition C. Weak cvurent ...... Electrotonus in man .... III. Law of contraction ...... Diagrammatic scheme. Discussion Law of Ritter-Valli ..... Electrotonus in nerves of different fvmctions . E. Different employment and effects of electricity Action of the magnetic field ; electric waves and rays High frequency currents ..... Death by electricity ...... F. Nerve poisons ....... I. General poisons. Anaesthetics .... Chloroform and ether ansesthesia Cocaine ....... II. Special poisons. Curare ..... Strychnine. Atropine. Pilocarpine Page 91 91 93 94 94 95 95 96 96 97 97 98 98 100 101 103 103 104 104 106 106 107 108 109 110 SECOND PART ELEMENTARY SYSTEMATIC FUNCTIONS Fundamental data. Relations of sensibility to movement . First Section NERVOUS ORGANIZATION Its scheme .......... Chapter I SENSIBILITY and MOVEMENT ; their RELATIONS A. The roots of the nervous system ; their functions . 1. Simple facts ; general laws ..... I. Nerve pairs. Distinct functions .... Nature of these functions ..... Laws of Magendie ...... II. Current of entrance and of exit ; order of succession Comparative physiology ..... 117 120 124 125 125 126 128 128 129 TABLE OF CONTENTS xv Pagk III. Functional connexions. Reciprocal influences . . * .130 2. Organic complications ; recurrent sensibility . . . . .132 I. Tlie fact and its conditions . . . . . . .132 II. Raison d'etre . . . . . . . . . .133 III. Generalization of the fact . . . . . . .134 IV. Recurrent motricity . . . . . . . .135 3. Conventional definitions ; sensorj'' field, motor field . . .136 I. Comparison of the anterior and posterior portions of the spinal cord and of the brain ........ 136 II. Mixture of sensory and motor elements . . . . .140 III. Pliysiological proofs . . . . . . . .141 B. Spinal nerves — Metamerism ....... 143 I. Radicular metamerism ; spinal metamerism . . . .144 II. Cutaneous radicular territories ; areas of anaesthesia . . .145 III. Radicular muscular territories . . . . . . .149 IV. Mixed nerves 150 C. Cranial nerves. Functional determinations . . . .154 I. Morphological regularity of the spinal cord . . . .154 II. Irregularity of the medulla oblongata . . . . .155 Vertebral theory of the skull . . . . . .155 A. Physiological characters of the nerve pair . . . .158 B. Relations with the great sjinpathetic . . . . .162 1. Nerves of special sense . . . . . . .165 a. Olfactory nerve . . . ' . . . . . . .166 6. Optic nerve . . . . . . . . . .167 c. Auditory nerve . . . . . . . . . .167 d. Gustatory nerve . . . . . . . . . .168 2. Sensorj^ and motor nerves . . . . . . .168 a. Oculo-motor nerve . . . . . . . . .168 b. External oculo-motor and pathetic . . . . . . 16{> c. Facial . . . . . . . . . . . 16& A. Peripheral portion . . . . . . . .171 I. Deviation of the features of the face . . . . .172 II. Elements of general sensibility. Recurrence; anastomoses . .173 B. Deep portion ......... 173 I. Motor elements . . . . . . . . .173 II. Sensorial elements . . . . . . . . .174 III. Ganglionic elements. Superficial and deep petrosal nerves . .174 Filament for the muscle of the stapes. Chorda tympani . .174 C. Origins. Intra-cranial stin:iulation . . . . .175 Gustatory sensibility. Secretory action, vaso-dilatory . . 176 XVI TABLE OF CONTENTS d. Trifjemitvxl ........ A. Sensory and motor functions .... Sensory paralysis. Borrowed sensorial elements Motor paralysis. Intra-cranial stimiilation B. Connexions with the great sympathetic Secretory elements ...... C. Trophic distiu'bances ...... Ophthalmic ganglion ; its nature Classification of the elements. Vaso-motors Elements whose function is undetermined (trophic) ; posi tion of the question ..... D. Indii'ect action on the senses .... €. Glosso-pharyngeal ....... I. Section. Gustatory paralysis ..... Sensory paralysis ....... II. Stimulation within the skull. Vaso-dilatory elements . Secretory elements ; ganglia, their nature /. Pneumogastric ........ A typical arrangement. Origin. Ganglia. Distribution Protoneurons and intracentral neurons. Specific functions A. ResiDiration Laryngeal branch Recurrent nerve Pulmonary nerves B. Circulation .... Depressor branch Cardio-inhibitory elements C. Digestion .... Pharyngeal branch. Movements of the oesopliagus Gastric sensibihty. Stimulation of the origins of the vagus Secretory elements ...... D. Trophic action. Its complex mechanism ■g. Spinal accessory ....... Essentially motor function. Forcible extraction A. Internal branch. Vocal function. Hoarseness, aphonia Recurrent nerves. Asphyxia .... B. External branch. Its function in effort h. Hypoglossal ........ I. Effects of its section, motor paralysis II . Sensibility by anastomosis ..... Ill . Ganglionic anastomosis ; vaso-constrictor elements motor, inhibitory Page 177 204 204 206 206 TABLE OF CONTENTS xvii Chapter II PRIMARY SYSTEMATIZATIONS Origin of the nervous system ....... Its development ......... A. COMMXnsriCATION OF THE IMPULSES ; REFLEX ACT Historical. Extension of the phenomenon .... I. Localized reflex acts ....... Anatomical data. Connexions between elements. Elements of association ........ II. Symbolical figure. Ordinary sense of the word III. Conscious, sub-conscious, and unconscious reflexes IV. Elementary reflex. Centre of reflexion V. Experimental data. Delay of the stimulus. Intensity VI. General direction of the currents. Method for its determination Irreversibility of the reflex cycle ..... Directive organ ........ its laws .... VII. Dispersion of the excitation ; VIII. Classification of the reflexes IX. Reflex centres of the spinal cord X. Sub-cortical enceplialic centres . XI. Reflex cortical centres .... Reflexes in patholog3^ Medullary reflexes Principal centre, Goll's nucleus Cerebral reflexes B. Suspension of the excitations ; action of arrest or of inhibition I. Its scheme. Apparently paradoxical datmn .... II. Analysis of the system. Distinct existence of the inhibitory nerves Specificity of the relations. Inhibition is a phenomenon in- ternal to the nervous system ...... III. Excitation and inhibition. Reflex and inhibition combined in the same cycle ....... IV. Mechanism of inhibition ...... V. Secondary effects of inhibitory excitation . C. Conservation of the stimulus. I. Circulation of the impulse II. Avitomatic stimulation Nervous circulation D. Classification of nerves ...... I. Initial and terminal nevirons. Sensory and motor nerves II. Intermediary neurons ...... Excitatory and inhibitory neurons III. Fimctional systems of neurons ..... IV. The centres . . 1. The nervous system and heat. The thermic nerves i I. Cellular exciting function for the disengagement of heat II. Systematic regulating function of the temperature 2. The nervous system and nutrition. Trophic nerves I. Immediate and consecutive influence. Alteration of the muscles P. h Page 211 212 214 214 215 215 217 217 218 218 221 221 222 223 224 225 225 225 226 227 227 230 232 234 235 236 239 239 239 242 242 243 244 245 246 247 247 247 248 248 xviii TABLE OF CONTENTS Paqk II. Alteration of the skin and the cornea ; unity of function and unity of cellular excitation ....... 249 3. The nervous system and animal chemistry ..... 250 Reactions of the organism. Chemical equilibrium . . . 250 Rupture of the ec£uilibrium. Direct nervous action. Indirect action 251 Chapter III CONSCIOUSNESS AND UNCONSCIOUSNESS I. Division at the periphery .... II. Division in the cerebral cortex III. Division between the deep and peripheral systems, of the boundaries ..... The problem of consciousness ; its difficulties Variability A. Dispersion and reflexion of the excitations ; the spinal cord I. Sensibility in the spinal cord ..... A. Stimulation of the grey matter .... I. Stimulation of the centres ..... II. Determination of the reflex centres .... B. Stimulation of the posterior columns I. Total effect Endogenous and exogenous fibres. Isolated stimulation of endo- genous fibres ....... II. Conclusion ........ III. Direct and indirect prolongation of the sensory roots Cerebellar tract ....... Tract of Gowers ....... Deep lateral tract and lateral ground Jiiuidle IV. Stimulation of the lateral column .... C. Section of the medullary tracts .... I. Hemisection of the spinal cord ; preservation of sensibility Hypersesthesia ........ Crossed anaesthesia ....... II. Explanation ; decussation of the sensory tracts . Syndrome of Brown-Sequard ..... III. Sensory medullary paths of tlie deep organs IV. Important part played by the grey matter in sensory transmis- sion ......... 2. Motility in the spinal cord ...... Motor field. Pyramidal tract. Fundamental tract Cerebello-spinal motor paths. Anterior marginal tract I. Stimulation of the descending tracts II. Decisive experiment ...... III. Quantitative difference ...... 256 257 257 258 259 260 260 260 262 262 262 262 263 265 267 268 268 269 270 270 271 271 272 274 274 275 276 277 277 279 279 280 TABLE OF CONTENTS IV. Section of the antero-lateral columns V. Part played by the grej^ matter in motor transmission VI. Direct and crossed action ...... Syncineses ........ Mode of association of the neurons .... XIX Page 281 282 283 283 284 B. Animal life and organic life ; the great sympathetic 1 . The two lives ; theu- distinct representations in the nervovis system So-called cerebro-spinal systems and great sympathetic . Signification attributed to these terms .... Anatomical differences between the two systems . The limits of the great sympathetic ..... Its sensory elements. Metameric arrangement. Cranial sjanpa thetic .......... 2. The grey matter of the ganglia ; its functions Ganglia ; motor nuclei ....... Apex of the lieart ; mj'ogenic and neurogenic doctrines Periodic inexcitability ; refractory phase ; compensating repose The nervous network of the heart's apex .... Anodic stimulation ; its inhibitory effect. Anti-tonic action of the vagi . Conclusion. Functional part played by the different ganglia of the heart .......... Action of oxygen on the movements of the heart A. Tonic power ........ B. Reflex power ........ C. Inliibitory power ;...... I. Systems of the life of relation and of the vegetative life, re semblances and differences . . . . ■ II. Mechanical and chemical acts ; contraction, secretion III. Specific excitations of the sensory nerves of the deep organs IV. Experimental data ...... V. Mixtm-e of stimulating and inhibitory influences 3. Special functions of the gi-eat sjanpathetic . Historical, initial fact ...... Vaso-motor function ...... Double motor and inliibitory function Secretory function. Inhibito-secretory function Vaso-motor lymphatic fmiction. Clu'omatic fionction 4. Systematization of the great sympathetic ; its two ordei's of fibres of projection ..... Granglionic metamerism and spinal metamerism Association of the metameres . Summary .... 5. Topographical determinations A. Superior half I. Elements of medullary origin First grouj) : sympathetic iimervation of the head and necl 286 287 288 288 290 291 292 297 297 298 300 303 304 305 306 309 311 311 312 313 314 314 315 316 317 317 318 319 320 322 322 324 328 331 332 332 333 XX TABLE OF CONTENTS II. Elements of bulbar origin Second gi'oup : sympathetic nerves of the superior limb Vertebral nerve ..... Third group : thoracic visceral nerves B. Inferior half ..... First or caudal group. Sympathetic nerves of the trunk Second grouj^, or that of the inferior limb Third group, or that of the abdominal viscera Elements of medullary origin Elements of bulbar and sacral origin Abdominal viscera and those of the pelvis Pagi': 335 338 339 340 341 341 341 342 342 343 343 C. TrANSMLSSION and centralization of the stimuli ; THE MEDULLA OBLONGATA ...... Proper functions ...... 1. Sensibility and motricity in the medulla oblongata A. Motor paths ; anterior pyramids Decussation. Alternate hemij^legia B. Sensory paths. Fillet {Ruban de Reil) . 2. Local reflexes ...... I. Reflex cry ...... II. Blinking of the eyelids .... III. Conjugate deviation of the eyes Convergence for adaptation to distances IV. Sympathetic origins .... V. Deglutition ...... VI. Mastication. Suction .... 3. General reflexes ..... I. Common sensorium ..... II. Locomotive functions. Part played by the pons A. Respiration ..... I. Vital knot II. Influence of the composition of the blood B. Circulation ...... I. General bulbar vaso-motor centre • II. Vaso-motor reflexes. Cardio-inhibitory centres C. Movements of the pupil I. Dilator reflex of the iris II. Irido-constrictor or photo-regulative reflex D. Secretions ...... i Diabetic puncttu*e . . 347 347 349 349 350 352 354 355 355 355 356 357 357 359 359 359 3G0 361 361 362 363 364 364 365 365 366 366 367 Chapter IV SUPERIOR SYSTEMATIZATIONS A. Orientation and equilibrium ; the cerebellum Historical ....... Comparative anatomy ..... 372 373 375 TABLE OF CONTENTS xxi Page 1. Conditions of equilibriiun ........ 376 I. Reciprocal relations between motor effect and sensory excitation . 377 II. Sense of equilibrium . . . . . . . .378 III. Automatic action ......... 379 IV. Sources of stimulus ......... 379 V. Progress of the impulses ........ 380 The cerebellum and reflex movements . . . . .381 Anatomical data ......... 382 Connexions of the cerebellum studied according to degenerations . 384 2. Experimental data and those of observation .... 385 A. Cerebellar peduncles ........ 385 I. Inferior cerebellar peduncles . . .385 II. Middle cerebellar peduncles ....... 386 III. Superior cerebellar peduncles ....... 387 IV. Olives and nuclei of the pons ; their connexions .... 388 V. Movements of rotation . . . . . . . .389 B. Effects of destruction of the cerebellmn ..... 391 I. Unilateral destruction. Consecutive effects .391 11. Total destruction ......... 393 III. Destruction of the vermis ....... 394 IV. Predominating direct action ....... 394 C. Electrical stimulation of the cerebellum ..... 394 I. Agreeinent of the results ........ 395 II. Functional relations of the cerebellmii and the brain . . 396 III. Cerebellar vertigo ......... 396 B. The emotions. Optic thalamus and corpora striata I. Reflex and conscious voliintary nervous acts .... 398 II. Emotional acts ......... 398 1. Affective tone.- — Pleasui-e and pain ...... 400 I. Their link with the different orders of sensibilitj- . . .401 II. Determinative condition . . . . . . .401 III. Non-specific nature ......... 402 IV. Habitual field .......... 402 V. Stimulation of the nerve trmiks ...... 402 VI. Stimulation of the central inasses ...... 403 VII. Stimulation of the cerebral cortex ..... 403 2. Expression of the emotions ....... 404 I. Language of the emotions ....... 404 II. Innateness of the emotional mechanisms ..... 405 III. Emotional expressions on the hiuuan face. The muscles of expres- sion ........... 406 3. Anatomical data ......... 409 The cerebral cortex and the optic thalamus . . .410 4. Functions of the optic thalamus . . " . . .410 A. Clinical facts . . . . . . . . . .411 I. SuiDerior and inferior facial . . . . . . .411 II. Peripheral and deep facial . . . . . .411 XX u TABLE OF CONTENTS III. Paralysis of the voluntary function ..... IV. Voluntary jDaralysis, preservation of the emotional expressions V. Paralysis of the emotional expression, preservation of the voluntary movements ...... B. Experimental facts ..... I. Motor tracts special to the optic thalamus II. Functional relations between the cortex and the optic thalamus III. Emotional reactions of the deep organs 5. Functions of the corpus striatum I. Experimental data ...... II. Supposed function ...... Nucleus of Luys ; corpora quadrigemina ; mammillar 6. Instinct in man and animals .... Characters of instinct ..... C. Intelligence ; the brain ..... 1. Anatomical data ; structure and connexions A. Cortex. Structure ...... Human brain ....... Brain of the primates ; monkey Brain of the carnivora ; dog .... y bodies internal capsule, crura cerebi-i, cortical fillet B. Corona radiata ; (Ruban de Reil) .... Crura cerebri ..... 2. The psychical functions of the brain . A. Ablation of tlie cortex in mammals I. Scheme of experiment State of the senses. Nutrition. Instincts. Emotions II. Earlier experiments ..... III. Cerebral cortex and sensation . IV. Sensations which have undergone reduction B. Ablation of the cortex in birds C. Ablation of the cortex in the frog . 3. Exchanges between the brain and the blood I. Indirect calorimetry II. Heat and thought . III. Cerebral circulation . IV. Vaso-motors of the brain Stimulation of the cervical sympathetic Hypoj^hysis . Epiphysis D. Cerebral localizations 1. Localizations in consciousness I. Initial fact II. Discord in the interpretation III. Number and situation of the excitable areas Extension of the excitable area. Motor and sensory centres Page 412 413 413 414 415 415 416 417 419 420 421 421 422 424 427 427 432 434 435 440 445 445 446 446 447 447 449 450 451 451 451 453 454 454 456 456 456 457 457 458 458 459 460 461 Relative characters. Artificial excitation and normal activity . 462 TABLE OF CONTENTS IV. Localized ablations ..... V. Disturbances of sensibility VI. Comparison of motor and sensory distm*bances VII. Evolution of the question VIII. The tactile system IX. The sensorial systems. Localization. Differentiation Opposition of the points of view. Difference in the criteria 2. Localizations in unconsciousness .... A. Respiration ....... Movements of the larjmx .... B. Circulation. Vaso-motor area Basal ganglia. Heart. Thermo-regulating function C. Digestive functions. Mastication. Swallowing. Movements the stomach ...... Cardiac orifice. Body of the stomach. Pylorus Movements of the intestine .... Sphincter of the anus. Rhythmic contractions . D. Secretions ....... Lachrymal secretion . . . . E. Retraction of the bladder .... F. Genital organs ...... Trophic influence of the brain ... of XXlll Page 466 466 467 468 468 469 470 471 472 472 473 473 475 476 476 477 478 479 480 480 481 Second Section SPECIFIC INNERVATIONS Specific activities ........ I. Sensory field, its divisions ...... II. Specificity of the excitant ...... III. Specificity of the sensation ...... Uniformity of function. Specificity of the neiii'ons Definite relation between the imioression and the sensation Seat of the sensation ....... Sensation considered with regard to time. Specificity of the sensorial systems ........ 494 488 488 488 489 489 491 493 Chapter I TACTILE INNERVATION A. Anatomical data. Sub-dermic apparatus Intra-dermic apparatus. Distribution Intra-epidermic ramifications B. Data of physiological observation I. Natixre of the excitants II. Experimental dissociation III. Persistence of the impression . 497 498 499 500 500 501 502 XXIV TABLE OF CONTENTS ingomelic dissociation of the IV. Primitive element of tactile sensation V. Stereognostic sense , A. Projection on the grey axis I. Passage through the spinal ganglia II. The posterior radicular fibres . III. Dispersion of the excitation IV. Short circuit ; reflex action V. Long circuit ; conscious action. Syr sensations VI. Localizing hypothesis VII. Bulbar reflexes B. Cortical tactile area I. Cortical localization of sensibility. Sensitivo-motor area II. Imperfect superposition of the sensory and motor areas III. Another localizing formula. Critical examination IV. Apparent dissociation of sensibility and motricity . V. Multiiale connexions ...... VI. Isolation ......... Primary and secondary sensations VII. ■ Exteriorization of the sensation. Illusions of those who have under gone amputation of a limb .... VIII. Localization ........ C. The tactile area from a motor point of view A. Limitation . . . . . . . . Divisions. Subdivisions ..... B. Stimulation ....... Excitability of the grey and white matter comj^ared I. Delimitation of the motor areas determined by stimulation Cerebral metamerism ...... II. Characters of the movements. Primary and secondary move- ments ...... . . III. Motor centre of language ..... Corpus callosum. Co-ordinated sensori-motor systems IV. Crossed and direct action ..... Movements of the eyes. Movements of the tongue Mastication. Swallowing. Laryngeal movements Bulbar and pseudo-bulbar paralyses. Epileptiform crises D. Muscular sense. Cin^esthesic impressions Synonymy A. Muscular sensibility I. Its proofs ..... II. Irritability and muscular consciousness III. Sensory nerves of the muscles . IV. Independence of the muscular contractility and sensibility V. Different modalities of muscular sensibility . Page 502 5oa 503 504 505 506 507 508 509 510 511 511 514 516 518 519 520 521 521 522 523 524 524 525 526 526 528 528 531 533 534 535 536 537 541 542 544 544 544 545 546 546 TABLE OF CONTENTS XXV Page 54C 547 B. Osseous sensibility .....•• C. Articular sensibility .....•• I. Conceptions furnished by the different sensibilities. Position of the limbs 547 II. Cortical localization of the muscular sense, and the cinasthesic im- pressions .......... 548 III. Feeling of effort. Muscular effort. Psychical effort . . .549 IV. Voluntary excitation . . . . . . . .551 Sensation and movement in the fcetus ..... 552 Chapter II VISUAL INNERVATION A. From the retina to the cerebral, cortex I. Morphological signification II. The retina is a nervous centre III. Optic radiations .... IV. Progressive transformations of the nervous act V. Dispersion of the impulses in the system B. Impression on the retina ; rods and cones I. Functional differences ...... II. Macular tract and peri-macular tract III. Direct and crossed tracts ..... IV. Corresponding areas ; identical points of the retina . V. Homonymous hemianopsia ..... VI. Persistence of the retinal impressions. Consecutive images VII. Unity of sensation in binocular vision C. Cerebral visual sphere ...... I. Former experiments and observations II. Cuneus and calcarine fissm-e. Lesions of the angular gyrus III.. Surface of projection. Geometrical projection. Compound pro- jection .....••• IV. Luminous sensation and mental vision V. Different kinds of blindness. Conditions of their production VI. Conceptions of form, space, locality, orientation VII. Physical image, psychical image .... VIII. [Empty space, occupied space. Exteriorization of the sensation IX. Physical and psychical blindness X. Verbal blindness ...... D. Motor effects. Paths of return I. Localities for reflexion of the impulses II. Different functional associations III. Retino-pupillary reflex ..... IV. Associated movements ..... V. Dextrogyral and levogyral hemi-oculo-motor nerves VI. Elevator and depressor nerves of the axis of vision VII. Movements of the eye in their relations with the muscles and the motor periijheral nerves VIII. Inhibitory paths .... 554 554 556 556 558 558 560 561 562 562 562 562 564 565 566 566 567 568 569 570 571 571 572 573 574 575 576 577 579 580 581 583 583 586 XXVI TABLE OF CONTENTS IX. Other co-ordinated movements of the eyes X. Movements of the eyelids. Protective reflex XI. Movements of the head .... XII. Niimerous motor areas for the eyes . XIII. Cerebellar influence ..... Signification of the centrifugal fibres liaving their terminations in the sensory elements ....... Page 590 590 591 592 595 597 Chapter III AUDITORY INNERVATION Comparative anatomy and j^hysiology Baresthesic, mantesthesic, seisa?sthesic A. Labyrinthine stimulation Different appropriations. functions ....... 1. The sense of space ...... 1. Stinauli of extra-somatic origin .... II. Stimuli of intra-somatic origin .... III. Link between the muscular tonus and the motor power IV. Analysis of the perceptions of space V. Superposition and synthesis of the notions of space furnished by each sense ...... Objective and subjective orientation. Vertigo 2. Specific sensation or that of auditory tonality . I. Auditory field ....... II. Fusion of the impulses in the sonorous sensation III. Damping of the vibrations special to the ear IV. Rate of the development of the sensation B. Transmission from the ear to the cerebral cortex Vestibular nerve .... Cochlear nerve .... C. Auditory cortical, area Physical deafness ; psychical deafness Verbal deafness, or that for words The ideas of space ; their relation to the cortex and the different senses D. Paths of return Muscles of the external ear Muscles of the middle ear (iOl G03 604 607 608 608 609 610 611 611 613 613 614 614 615 615 616 617 622 623 624 625 626 627 627 Chapter IV OLFACTORY AND GUSTATORY INNERVATIONS A. Olfactory system ........ Field for reception of the im^jressions . . . . 628 629 TABLE OF CONTENTS 1. Bulbar portion of the system .... Its organization. Connexions between elements. relations Functional varieties. Other connexions 2. Cerebral portion ..... Limbic convolution of the osmatics Osmatics ; anosmatics .... 3. Evolution of the olfactory system and of its function a. Inferior vertebrata. h. Superior vertebrata 4. Constitution of the olfactory system Absence of experimental and clinical data 5. Motor olfactory jiaths ..... Reflexes of adaptation. Relation with general sensibility B. Gustatory system ....... 1. Field of impressions. Papillfe of the tongue. Taste buds 2. Nerves of taste ....... Unknown cortical localization. Reflexes of adaptation Numerical XXVll Page . 629 6.30 631 632 633 634 636 636 638 642 643 643 644 644 646 647 • Chapter V LANGUAGE AND IDEATION Emotional expressions A. Formation of words and of ideas L Representations in consciousness I. Method of analysis. Physical and n II. Multiple operations of the niind Primordial element, sensation oral world III. Analysis and synthesis, formation of the images of objects 2. Motor acts of spoken and written language Deaf -mutism. Part played by the muscular sense Automatic language 3. Internal language .... Internal resonance. Motor tendency I. Intelligence and consciousness . II. Attention ..... III. Fatigue ...... 1. Language and cerebral localizations Pathological dissociation of the elements of language 1. Aphasias known as cortical ..... A. Motor aphasia, agraiahia ..... B. Sensorial aphasia. Verbal deafness, verbal blindness Sensorial types. Amusia, Amimia 648 648 649 649 650 650 652 654 654 656 656 657 658 659 659 661 662 663 663 664 665 XXVUl TABLE OF CONTENTS 2. Aphasia by default of conduction Schematic constructions Seat of ideation .... Automatism and intelligence 3. The area of language ; its constitution Fibres of association Fibres of projection C. Processes and Organs of association Anatomical hyjoothesis. Contestations. Experiments Moderating functions of the frontal lobe. Inhibition D. Sleep Dreams .... Supposed Mechanisms. Necessity Artificial sleep. Hypnosis. Personality : its disaggregations Spiritualism Page 665 666 668 .668 669 671 671 671 671 672 673 673 673 673 674 674 Innervation In the living being all the phenomena appertaining to crude matter are observable, but the converse does not hold good. It is obvious that a being endowed with life possesses characteristics and presents manifestations for which in dead matter we can find no parallel ; and the most marked feature distinguishing the one from the other is that of sensibility. Here is brought before our notice a fact of a purely internal nature, eluding observation as it is generally understood in science, but which common sense constrains us to attribute to beings resembling ourselves, while at the same time denying it to all objects in which this resemblance cannot be discerned. Sensibility and Energy. — This attribute, sensibility, cannot in the living being act as a substitute for the energetic phenomena of matter ; it is merely superposed to these phenomena, and connected with them by a double reciprocal link. They preside over it in the sense that a subject gifted with feeling must, of necessity, require an object to be felt ; and, on the other hand, sensibility exercises a control over these phenomena of energy, inasmuch as, though incapable of modifying them as a whole, it ca.n still regulate and control them in their execution of functions directed towards an end of which the living being itself is conscious. — This reciprocal link not only controls the relations of the living being with all surrounding objects ; it is also, and simultaneously, the distinctive feature of its organization. In its development, as much ontogenetical as phylogenetical, it is the living being which is at once both artificer and final cause. — From this double link, so frail in itself, and yet so intimate, proceeds the iinity of beings endowed with life, and in this organism, where each part depends on the whole, and the whole on each part, a synthesis is effected which confers upon it its individnality. This prodigy of complexity is also a prodigy of unity. Sensibility and Determinism. — A science having for aim the study of a being so constituted should never lose sight of this double character, and more especially when appealing to the methods and general prin- ciples of other sciences. Dissociated and brought back to the crude state of common matter, the primary elements constituting the li\ang being reveal to us in their reactions the same inflexible constancy that charac- p. ^ B 2 INNERVATION terises the laws known as physico-chemical ; yet, associated in the individual, their grouping and organization display that infinite variety and contingency whence individuality is derived. How can this pro- ceed from that ? How can that which is invisible in the element become apparent in the whole ? To these questions we can find no answer ; but, in science as elsewhere, it is always imprudent to run foul of the information given by common sense, and a problem is not solved when one of its terms has been omitted. The mind, desirous of being logical, is in fact at first offended by this contrast, and endeavours to annihilate it by evading one of the two points of view. The rigid determinism of purely energetic sciences has been transported, without restriction or selection, into biological science. In the past, and even at the present time, physiology has overlooked, and still overlooks, the fact of the being which it studies possessing sensi- bility ; and has in every case refused to acknowledge this sensibility as a causal or conditioning influence in the determinism of vital phenomena. It has carefully arranged the balance-sheet of the forces of the organism, while taking no interest in the function which regulates their em- ployment. As physical science finds no place for sensibility, neither has physiology accorded it one. The time seems to have arrived for a reaction against these exaggerations. In the living being, just as movement depends on sensation, so does sensation depend on movement. In both cases the nature of the link is unknown to us ; but none the less does this link exist, and is in biology the foundation of all that distinguishes it from pure physics. Sensibility and Organization. — In the living world sensation presents extremely varied degrees, and its development proceeds on a line parallel with that of the organization itself. It is only strongly marked in beings provided with the differentiated system known as the nervous system ; it increases in importance and elaboration with the progressive development (phylogenetical and ontogenetical) of this system. In such beings, of whom we ourselves form a class, a division of attributes is effected between the tissues, some of these employing the efficient energies which take part in the execution of organic actions, while another, the nervous tissue, watches over this employment, co- ordinating and regulating it. This latter is pre-eminently the sensory tissue, and is in a high degree both excitable and capable of causing excitation. It is this tissue which receives the stimulation and re- turns it, but transformed by the progress through its paths ; and again it is this tissue which ensures the reciprocal dependence and subordina- tion of the elements to the whole and the whole to the elements, and so confers on the organism its individuality, its unity. INNERVATION 3 Excitability and Sensibility. — All Living matter is excitable ; or, to put it otherwise, it responds to actions directed against it, by an expendi- ture of the special energy which it constantly accumulates internally. This motor reaction is never hap-hazard, but— and this fact is demon- strated by experiment — is always directed with the definite aim of preservation of life in the susbtance stimulated. Excitability is there- fore not merely a motor manifestation, but is duplicated by an internal fact of rudimentary consciousness. It should therefore be considered as either a degraded form or a first rough sketch of sensation. The elabor- ated organization of the superior animals, by giving to it its highest development, permits of our analysing the conditions of its existence ; fundamentally these conditions are everywhere the same ; they are located in the links of reciprocal dependence of the portions composing the organism. The more simple and homogeneous is the latter, so much the more do its reactions resemble those of ordinary movement ^ and so much the farther are they removed from those which characterize genuine sensibility. But in proportion as the organism is complex and differentiated, so much the more ^Adll its movements possess the contingent characteristics of sensible and intelligent beings. Action and Reaction. — In other words, the living being reacts against actions reaching it from the external world, and in so doing obeys a general, universal, and indeed fundamental law, one of the first inscribed in the physical code, a law, obedience to which no living body in nature can escape. Only, from the fact of organization itself, tliis law has assumed a new character, of which it may be said that it imphes in the living being a remembrance of the past and a prevision of the future. The more elevated is the organization, the more promi- nently does this character stand forth ; on the other hand, the nearer we approach the purely physical elements entering as components into this organization, so much the more is this character effaced, nothing being left but the simple reaction strictly and solely answering to the action of the present moment. Vital reaction, practically so different from physical reaction, proceeds from it by successive halting places and elaborations, just as the living being itself is evolved from progressively organized crude matter. Division. — The nerve tissue is, like all other tissues, originally formed of cells ; but while other cellular structures are usua.lly merely composed of duplicated and juxtaposed elements, it, thanks to the connexions established between its component parts, displays a genuine systemi- tization. Its study may therefore be carried on from two different points of view ; one in which the functions common to all its elements are considered {cellular functions), the other, in which the functions E* 4 INNERVATION special to the groups or systems formed by these elements are taken into account {systematic junctions). In the study of nerve tissue the distinction between these two orders of functions is a fundamental one, and the obscurity stiU enveloping numerous questions connected with this study is partly due to the fact of tliis distinction being so frequently ignored. The first of these studies completes the history of the ceUular functions arranged in unison with the principal types of living elements. The second permits of our penetration into the aggregate functions to which the mutual association of these elements gives rise, and it is in the nervous system that we shall find the connexion where these aggrega- tions are brought into being and their functions organized. The study of the nervous system is a kind of nodal point in the exposition of physio- logical science. PART I Elementary Nervous Functions The complexity of the living being and of its smallest organs is such as to compel us to study them from two points of view : the one static, or that of absolute repose, that is to say, death ; the other dynamic, or that of activity, that is to say. hfe, in its different mani- festations. Anatomy is concerned with the static condition, taking account, as it does, of vital forms in their fixed state ; the dynamic condition, or that which physiology investigates, is concerned with movement. The two sciences are mutually related and sometimes their special methods are interchanged. A summary review of the principles of the one always aids in the elucidation of those of the other. By mutually borrowing they tend to reciprocally fill up their lacunae. 1. Static Unit. — A static unit of the nervous system exists which is commonly known by the name of element : this is the neuron. This unit is of cellular order and is indeed a symbiosis, in the sense that to the fundamental cell of which it consists others are added which in a way form one structure with it. It is an element in the relative sense of the term only, because a cell is a being of complex organiza- tion, but this unit is very well defined and thus acquires great import- ance. It is necessary therefore to briefly recapitulate its most essential characters. 2. Dynamic Unit. — Corresponding to this static unit there exists a dynamic unit in the same way as for the muscular element, the glandular element, etc. In the nerve this dynamic unit is less easily recognizable, because it does not declare itself, as do the preceding elements, through externally visible phenomena (contraction secre- tion), but only by the excitability which is more or less the heritage of every cell, or rather by the reaction of this excitability on that of the other tissues which are connected with the nerve. This djmamic unit, which we can only describe by the unsatis- factory name of elementary nervous irritability, is the aggregation of the energies which take their origin in the neuron, in order to preserve its existence, its composition, its structure, its internal and external manifestations, the whole being co-ordinated to a definite end. 5 6 ELEMENTARY NERVOUS FUNCTIONS Organization of Energy. — Just as in the nervous element the struc- ture is not homogeneous, but is made up of comphcated structures which nevertheless form a coherent whole, so in this element, as in every cell, there is an organization of energy, which, by the depen- dence which it creates between its multiple forms, causes the latter to take part in the conservation of the element, and guarantees its social action in the economy. Like every living individuahty, the nervous element, the neuron, is at the same time both single and multiple, and it must not be forgotten that it is so both from the dynamic and static point of view. The Multiple Forms and Transformations of Energy in the Nerve. — If it be asked what is the energy which circulates in the nerve, the question is badly expressed, because it suggests that one sole force occupies its substance (as does electricity a conducting wire), and, a priori, it is obvious that such a comparison is inaccurate. The utmost that can be done is to investigate the nature of the final energy which the nerve makes use of at its point of contact with the muscles, or of the organs which it excites. But before arriving at this last phase, it is certain — for proofs thereof exist — that energy has undergone many transformations of which we only know those which are the most striking. (a) Chemical Force. — In the nerve, as in every tissue, force is present in its chemical form ; because, as in every other tissue, it is constantly produced by exchanges with the blood through the vessels which irrigate it (gaseous exchanges and those of soluble substances). (6) Caloric Force. — In masses of nerve tissue of considerable size it is now admitted that there is a slight disengagerhent of heat ; because the temperature of these masses may be slightly higher than that of the blood which circulates in them (Mosso). (c) Electric Force. — In the nerve, as in all the elements ha\'ing a definite orientation, electro-motor phenomena have been discovered which give rise to currents passing in a definite direction ; so that a place must be given also to electricity in the transformations of the energy employed by the nervous element. It is only necessary to be aware that the assimilation of a nerve to an ordinary electric conductor is radically inaccvu*ate. The conception of these currents is complicated and one, so to say, special to the nerve, and their circulation probably takes place in particles in size approaching that of the molecule. Cycles of Energy. — Those chief forms of force wliich arise by transfomiation the one from the other enter into cycles of energy, which sketch tlie first out- lines of that oi'ganization of force in the element, without which all would be confusion, and, thanks to which, order and miity become paramount in it-. We are but imperfectly acquainted with the detail of these cycles ; but every- thing shows that in the nerve element (as in every cell) they are numerous, giving rise to varieties and infinite gradations. But that which chiefly charac- terizes them, in the living being, is their mutual penetration, their superposition. THE NERVOUS ELEMENT 7 their convergence towards a definite end. Not one is absolutely complete in itself ; but each on the contrary expends a part of its force on neighbovrring cycles, botli parallel and successive. Hence it is that insurmountable difficulties often arise in the analysis which endeavours to isolate them in order more accurately to study them. Functions. — The cycles of energy which are essentially simple, and are concerned with the performance of what we call nerve functions, are what form the founda- tion of that dynamic nervous unity which our intelligence is not yet accustomed either to see or to investigate, but which in oijr science is just as necessary as the cellular conception (which is the concrete form thereof) is in anatomy. If the details of this organization of forces were better known it would lead us without transition to the knowledge of those complex acts which are the functions and which we at present only recognize through their results. Their Double Nature. — These fvuictions are in a nervous element (as in every living element), of two orders ; both make use of the energy which penetrates the nerve and quits it after transformation ; but from it they evolve a different order of result. Some are concerned with organization and, when once this is established, the conservation of the organization of the neuron : for this reason they are called organotrophic ; they are internal to the neuron itself. The others affect the social aspect of the cellular individuality so far as tliis concerns the total nervous system and the individual itself : hence these functions are those properly called nervous ; they are external to the neuron, and while the first establish connexion between its constituent parts in order to preserve its static and dynamic identity in the midst of perpetual renewal of its substance and its energy, the second form amongst all the neurons (and the elements in connexion with the neuron) a systematized tie which gives unity to the nervous system, and hence to the individual of whom it forms a part. Their Reciprocal Dependence. — The internal and external functions of the neuron, both trophic and nervous, however well defined they may be in their object, have necessarily very close mutual inter-relations. The second are clearly not possible luiless the first are in existence ; the functional activity of an element, whatever it may be, presupposes its nutrition : but in their turn, the first assume with regard to the second a more indirect dependence, but one which is equally real ; nutrition languishes when function is in abeyance. CHAPTER I THE NERVOUS ELEMENT The nervous element is the neuron. The neuron is essentially a cell which is differentiated for the performance of a special function, that, namely, of innervation or of stimulation of other cellular elements. This cell {the nerve cell) is provided with prolongations (nerve fibres) which proceed to a greater or lesser distance, but have a free termina- tion, so that the structure is limited in a definite manner. To the nerve cell thus furnished with its prolongations other structures are superadded, and these are of a more or less cellular nature and origin (myelin sheath, and the sheath of Schwann with its nuclei). It is this cellular s?ym620sf 5, regarded in its totality, which forms the neuron. ELEMENTARY NERVOUS FUNCTIONS A.— STATIC CONDITION OF THE NEURON ; ANATOMICAL DATA 1. External Characters ; Dimensions. — The dimensions of the neuron are very various, especially as regards length. Some are of micro- scopic dimensions (for example, in the retina) ; others may (in man) attain and even exceed a metre in length (for example, the nerves of the posterior root, going from the sole of the foot to the medulla oblongata) ; between these extreme dimensions many intermediate lengths may be found. Form. — Not only the dimensions, but also the shape of neurons may vary extremely. These variations are in rela- tion, both the one and the other, with the particular connexions which it is the duty of these elements to establish be- tween the nervous system and the organs, as also between the different parts of the nervous system itself. Nevertheless, in spite of this apparent diversity, they may be brought back to a common type in connexion with their most general func- tion. General Type. — The neuron presents a swollen portion, which is its cell of origin (formerly known as, and still often called, the nerve cell) from which prolongations in different directions are given off. These prolongations are of two species : the first called protoplasmic, ramified con- siderably, without, however, anastomosing with one other ; the second, thinner and paler, called the prolongation of Deiters, or axis cylinder, proceeds to a variable distance from the cell origin and gives off branches, partly on its course {collateral branches), and partly at its termination {terminal branches). A tree with its roots, its trunk and its branches would thus sufficiently clearly depict this arrangement if there were added to it at the point of expansion of the roots, a swollen portion which would represent the body of the cell. Signification of the parts. — From the physiological point of view the feltwork of the roots and the foliage of this miniature tree give rise in Fig. 1. — Diagrammatic represen- tation of an ordinary neuron. N, nucleus of the nerve cell ; Pp, protoplasmic prolongations ; Pa, axis cylinder ; Gm, sheath of myelin ; Sch, sheath of Schwann ; Ea, annular node ; T, terminal ramifications of the axis cylinder. THE NERVOUS ELEMENT K4I- [^ 71- the neuron to two parts which are functionally opposed, and which are united by the stem. The felt work is, taken as a whole, an organ for the reception of impulses ; the branches would form an organ for the distribution or rearrange- ment of these same impulses to the nervous or non-nervous organs in which they ramify. The trunk gathers them together and transmits them from one of its extremities to the other. As re- gards the cell, we shall endeavour to define the part which it plays a little farther on. Synonomy. — In the new terminology the pro- longations of the neuron formerly known as pro- toplasmic are called dendrites ; the prolongation of Deiters, or the axis cylinder, is known as the axon or neurite. The axon is nothing more than a nerve fibre, of the same kind as those which form the peripheral nerves. Its essential portion is the axis cylinder, around which are placed the myelin sheath, then the sheath of Schwann, with its nuclei and its regularly arranged nodes. From its initial to its terminal extremity the neuron thus presents three morphologically dis- tinct parts, namely : — 1, the dendrites or proto- plasmic prolongations ; 2, the body of the cell, formerly known as the nerve cell ; 3, the axon or neurite, which is the axis cylinder covered with its double protective sheath. , (a) Axon. — Ranvier has sho\\^l the cellular natui'e of the envelopes of the axon. The myelin sheath is in- terrupted from point to point by the nodes of the sheath of Schwann, which divide it into regular segments (about a millimetre long in ordinary nerves) of which each bears a nucleus in the middle of its length. With its myelin contents (phosphorised fat), each segment would be com- parable to a fat cell of cylindrical form, crossed in its axis by the prolongation of the nerve cell which is rightly known as the axis cylinder. Inasmuch as after death coloured or penetrating sub- stances pass into the axis cylinder by the nodes, it has been thought that this was the normal rovite for the passage of nutritive material during life. But this idea is based on an inexact appreciation of the nutrition of cells, as it represents the cell as merely absorbing certain substances through the most delicate portion of its structure. Absorption, or, to speak more accurately, the metabolic exchange, is effected all over the sur- face, as is the case in every cell whatever. On the other hand, these exchanges I Fig. 2. — Interannular segment of the axon. Ca, axis cylinder ; n, nucleus of the sheath of Schwann ; p, pro- toplasm of the same ;: e, node of Ranvier. 10 ELEMENTARY NERVOUS FUNCTIONS >p imply successive and nvimerous operations, n^utations and transformations, in whicli the cellular contents (in this case myelin) take a part just as do all the rest. The functions of the sheath of Schwann and of the myelin are often considered to be piirely mechanical, or as comparable to those of an electric insulating substance. Without denying the part which the myelin plays in this con- nexion, it is certain that the jarincipal functions of the cells which envelop the axon are not linaited to so simple a role. These functions have essentially for their foundation a chemical evolution, which we can enounce in prin- ciple, but which our actual means do not permit us to accurately define. Myelinated and non-myelinated fibres. — Near the termination of the axis cylinder, the myelin ceases at the level of a last node, which is called 'preterminal. The axis cylinder is thereafter bare. This portion deemed denuded is present in all nerves. In the nerves of the life of relation it is extremely reduced ; in the branches of the great sympathetic it is sometimes of great length. These non-myelinated fibres, still called fibres of Remak, nevertheless do not form, as is obvious, a particular and indepen- dent species, but only the continuation of the my- elinated fibres. In the locality where it quits the nerve cell (pro- longation of Deiters), the axis cylinder is equally denuded for a certain length. The cell itself is enclosed in a capsule, whose cellular nature has long been recognized. In the spinal ganglia of the frog, yellowish drops are found in the winter season, representing stores of fat (and not of myelin), form- ing a season's reserve, which disappears at the approach of smnmer (Morat). These stores belong to the cells of the pericellular capsule (Bonne). Although of a different natm-e, they have some analogy with the myelin reserve (this being perman- ent) of the peri-axile cells of the axon. Segmentary Cells ; Their origin. — The axis cylinder, which is uncovered at its two extremities, is thus bare throughout the whole of its length when first developed. It starts from the nerve cell, like a length- ening branch, until it attains the organ for which it is destined (Rouget and KoUiker). Mesenchy- matous cells (cells of conjunctive nature) are ar- ranged from point to point along its length, and mould themselves on it (Vignal). In their mode of appearance and their development they follow the same order as does the axis cylinder in its growth ; that is to say, they extend from the proximal to the distal end of the axon. In their interior they secrete myelin, and thus become the segmentary cells of Ranvier. In the extra- rachidian, or peripheral, nerves they become invested with membranes which, being continuous, form the sheath of Schwann. In the deeply situated nerves of the spinal cord and of the brain, this sheath is wanting, and the myelin is limited externally by the protoplasm of the investing cells. Pig. 3. — Non-myelinated fibres, or those of Remak. n, nucleus ; p, proto- plasm surrounding it ; b, constituent fibril. THE NERVOUS ELEMENT 11 Collaterals. — The axon or neiirite exhausts itself by ramifying in the organs, nervous or other, to which it transmits the impulse ; but in its transit it fre- quently gives off fine fibres, which are very appropriatelj- called collaterals. It is frequently the case that these fibres are not distributed along the length of the axon in a regular manner, but leave it only when it passes through some portion'of grey matter, as the fibres of the great sjmipathetic in the gangUa, or those of the pyramidal tract in the pons. Thus these collaterals belong to the transmitting polar field, to which they give a particular extension and a special aspect, not coinciding with what was formerly supposed to hold concerning the relations between the nervous elements. All cells which are at a great distance from the nerve cell Fig. 4. — Season's reserve of the cells of the spinal ganglia of the frog in winter. Drops of fat, single or multiple, coloured by osmic acid, are seen in the capsule of the cell,'and stand out more or less prominently in its interior. In the centre of the diagram a drop has been displaced by the manipulations, and the resulting drawing by Bonne). einpty space is seen in the capsule (froma are considered as fulfilling the same function as the strictly terminal ramifica- tions ; but there are some which take origin from the axon, sometimes at a short distance from the cell itself, almost at the origin of this axon. There has been hesitation in giving to these collaterals the same signification as the terminal branches of the nem-on, and many hypotheses have been brought forward con- cerning these singular formations. And at first sight it seems indeed strange that the nerve element, which has just received the impulse by its dendrites in some area of grey substance, should there and then distribute it to this same field before leaving it. But if we reflect that in this field itself associating cells exist (cells whose dimensions are relatively limited, and which do not leave its territory), which transport to it in this way an impulse from a short distance, the fact will appear to us less surprising. Neurons of great length, as the radicu- lar neurons of the antex'ior horns of the spinal cord, alone combine the fmiction of associating cells with that of elements of projection. Of the impulse which it has just received from its dendrites, the nem-on thus built up yields a portion to the dendrites of neighbouring neurons, while it carries away the remainder to 12 ELEMENTARY NERVOUS FUNCTIONS a long distance. It may be supposed on the other hand that these collaterals, if neighbours of the nerve cell, perform the function of dendrites, and belong to the receiving pole ; but although up to the present time no method of settling the question experimentally has been discovered, the resemblance of the colla- terals to the terminal rainifications makes us inclined to support the first explana- tion ; the analogy of form, in the absence of the experimental proof, prejudices in favour of the analogy of function. (b) Body of the Cell. — The body of the nerve cell presents itself in the form of a protoplasmic mass provided with a large nucleolated nucleus. In the interior of this mass the details of its structure can be made out. There may be distin- guished in it a substance which is known as chromatic, which can be coloured by methylene blue, and a non-chromatic substance which is known as the enchyleme. CO- Fig. 5. — Xerve tubes (axons) of the spinal cord without the sheath of Schwann. mg, myelin sheath ; g, peripheral en- velope ; c, nucleus and protoplasm observable on the surface of some slender nerve tubes. Fig. 6. — Neviroglia cells in a human foetus. A, superficial cell of the neuroglia ; B, neuroglia cell of the grey matter. The chromatic substance is looked upon as a sort of nutritive reserve, distri- buted in the cell in the form of gi'ains, and visible even at the point of origin of the protoplasmic arborizations. It is this substance which changes its appear- ance, its distribution and its quantity, in the principal conditions of the cell, as the result of long continued stimulation or after section of the axon. Golgi, Verrati and Nelis have demonstrated in the interior of the cell a net- work which Avould seem indeed to be double, occurring in two different planes, the one intracellular and the other pericellular. Bethe, and Apathy have also described networks of this description in the nerve cells of invertebrates. The connexions of these networks have not yet been definitely determined. The latter authors describe them as continuous with the constituent fibres of the axon. On the other hand, the body of the cell (and not only the dendrites) receive the contact of the terminal fibres of the nevirons which enter into relation with them ; it may be that the pericellular network serves to establish these relations. THE NERVOUS ELEMENT 13 Cellulipetal and Cellulifugal Prolongations.— The body of the cell displays, in the neuron, a locahty towards which the currents transporting the impulse tend to converge and to condense ; after this, according to the direction of the axon, Pig, 7. — Short neuron of the cerebral cortex with its dendrites (varicose), its axis- cylinder prolongation (very fine) and the collaterals of the latter. cy, axis-cylinder prolongation and its collaterals ; t, terminal varicose ramifications ; p, proto- plasmic prolongations. they begin to start again, diverging by the collaterals, which take their origin from the axon, sometimes but a short distance from the cell. Ha\dng respect to the progress of the impulse, the naine of cellulipetal has been given to the converging prolongations and to the currents wliich they convey, and that of cellulifugal to those which proceed from the cell, and in tMs way diverge. When these two terms are considered as being equivalent to centripetal and centrifugal, the ceil is compared to a centre of association of the neurons and of the transfor- mation of the unpulse. As a matter of fact, the associ- ation really takes place at the reunion of the terixiinations of the axons with the origin of the dendrites ; the transformation of the impulse can only be possible at the identical place where tliis association occiirs. (c) Dendrites. — It would seem that the dendrites are the ex- tended and ramified body of the cell, assmning the form of ar- borescent prolongations. It is only since the apphcation of the method of Golgi that their real form has been recognized, and that it has been possible to ex- jTiG. 8. — Short neuron with its two orders of pro- longations seen as a whole. The cellulipetal prolongations (protoplasmic) as well as the cells are in black ; the cellulifugal prolongations (axon and its collaterals) are in red. 14 ELEMENTARY NERVOUS FUNCTIONS plain their extension, often veiy considerable , throughout the grey matter, and even sometimes the w^hite The peculiar aj^pear- bordering matter, ance of these prolonga- tions had formerly caused them to be con- sidered as possessing a protoplasmic nature analogous to that of the body of the cell, and a function, rather trophic than nervous, in the exchanges between this latter and its surround- ing medium. But their function as organs which are receptive and conductive of the imiDulse can no longer be contested. However, their functional is not a -priori exclusive of their tropliic role ; it is only necessary that tliis latter be effected in a slightly different manner to that which was formerly current. It is scarcely admissible, as Golgi thought, that these prolongations act as conducting channels of the juices which they take up by their extremities in contact wdth vessels towards the cell ; but it is credible that the differentiated protojjlasm, the conductor of the impulses, which is present in them, is steeped (here as elsewhere) in a gangue of primitive orchnarily troi^Iiic protoplasm, wliich,by its exchanges with the medium in wlaich it is immersed, maintains its composition and its structure, and therefore its ner- vous function. Hence it may be asked if the modifications of form which ensue in these prolongations as the result of different conditions (stimulation, repose, fatigue), are anything more than the changes correlative to the nutritional requirements of these parts. The jSTeuron and its Different Constituent Segments. {Succession, Different and Synonymous Forms.) Fig. 9. — Dendrites (protoplasmic prolongations) of a root cell of the anterior horn of the spinal cord. cy, its axis cylinder (prolongation of Deiters) proceeds to form the axon, which extends the whole length of the motor nerve. Prolongation of Dei- Terminal ar- \ ( ters continued by b oriza- the axis cylinder tions (at with its sheaths of the extre- Nerve cell. myelin and of mity of the Protoplas- Schwann. axon. mic pro- — o 6 longations. — Ph "o — ' 00 ^ Dendrites. Body of cell with Axon or Neuraxon. Collaterals > { nucleus and nu- M on axon (single on the m ■43 \ cleolus. axon) course of 1 Inaxon (long axon). the axon. — Dendraxon (short axon). — Neurite. Diaxon (double axon). Folyaxon (multiple axon). Telodendro- non. \ Schizaxon (divided ) axon). THE NERVOUS ELEMENT 15 2. Dynamic Polarization. — By this term is described the functional opposition which is attributed to the two extremities of the neuron. When apphed to the neuron, the term dynamic polarization expresses a principle which is established by all experiments to which the nervous system is submitted ; namely, that the waves of the impulse which pass through it traverse it in a definite direction, which is invariable. Thus the neuron has two poles ; the one receptive, whose duty it is to receive the impulse ; the other distributive or emissive, which transmits it to other organs, or to neurons other than itself. Between the two poles is an axial portion, the axon, which carries the impulse from one pole to the other. It is possible that the axial part possesses the power of propagating the impulse in either direction, but the poles themselves are incapable of inverting or exchanging their function. There would be no inconvenience, and it would probably be advan- tageous, to describe, as Brissaud has suggested, the receptive pole as the positive pole and the emissive pole as the negative pole, it being under- stood that to these expressions, borrowed from the science of elec- tricity, only a comparative signification, and in no sense that of iden- tity, be ascribed. As has been described above, each of these poles possesses ramifica- tions which are frequently numerous and sometimes very widely ex- tended. Both ramify in a territory or area whose size varies and in which each, according to its nature, receives or distributes the impulse. In a given neuron when examined microscopically it is generally possible to say which of the two poles is the subject of examination. The receptive pole is that whose ramifications closely approximate the cell, so closely indeed that they appear to originate in the latter. The respective designation of the poles is not inferred from a structure which would explain their function, but is based on experimental observations which in a large number of cases have shown that the propagatio7i of the impulse proceeds frotn the dendrites to the axon, and passes through the cell body, and not inversely. The Experimental basis of the Polarity of the neurons. — The func- tional polarity of the nervous elements has been proved to demonstra- tion by physiology. It lays down in principle that in each of these elements, possessing normal connexions, the impulse is transmitted in a definite direction, invariably the same, and that it neither returns nor oscillates, and this physiology proves in a very considerable number of such cases. And, on its part, anatomy having succeeded in defining in a number of instances the structure and the limits of these elements, it has been possible to establish certain relations between these ana- tomical data and the direction of the physiological conduction. 16 ELEI^dENTARY NERVOUS FUNCTIONS Generalization of this datum. — A neuron being given (or even a recog- nizable fraction of this element), it is usually possible to say in which direction the current of excitation is transmitted ; but there are also cases in which it is impossible to predicate anything on this point. This is because the morphology of the neuron (form and situation of the cell and of its prolongation) is not in any way determinate, but, on the other hand, lends itself to the thousand exigencies which the recep- tion and distribution of impulses require according to the order and the special nature of these functions. Uncertainty in certain cases. — Whenever the neuron in which the problem of the direction of the conduction arises resembles those in which this direction has been ascertained by direct experiment, the reply is relatively easy ; it becomes uncertain whenever this criterion is wanting ; further, it may be only partial, that is to say, possible as Fig. 10. — Conriparison of the neurons of the posterior roots with those of the~anterior roots (after M. Duval). CM and CG, their cells of origin, one in the spinal cord, the other in the .spinal ganglion ; A, bare axis cylinder ; B, portion of the same covered with a sheath of myelin without a sheath of Schwann ; C, portion covered with a myelin sheath and sheath of Schwann ; D, portion with a sheath of Schwann without myelin ; E, bare terminal arborizations. According to Dogiel, the cell CG of the spinal ganglion bears jirotoplasmie prolongations or dendrites analogous (though less visible) to those of the cell CM. regards certain of the prolongations of the cell, and indeterminate as concerns other prolongations. Discussion of a particular case. — One of the most aberrant and most difficult forms to classify in the empirical law formulated above, is that of the posterior radicular neurons of the spinal cord. The cell of each of them is situated in the spinal ganglion ; the axon is morphologically represented by the prolongation which arises from it, and itself divides in the form of the letter X, to jaroceed on the one side to the spinal cord, and on the other, to the cutaneous investment. The dendrites, for a long period ignored, have been discovered by Dogiel : these are the prolongations which arise from the cell, and which terminate in the ganglion Itself, in contact with terminal arborizations belonging probably to the great sympathetic. THE NERVOUS ELEMENT 17 If such a neuron were obedient physiologically to the law of polarity, under- stood in its ordinary morphological sense, it should receive impulses only in the ganglion, and should distribute them thence to the spinal cord and to the skin simultaneously. And it may even be added that, if such a neuron were met with in any other locality less accessible to the isolated stimulation of its branches, there would scarcely be hesitation in attributing to it a conductive capacity in the sense which has just been alluded to. But experiment shows us here that the two branches of the axon conduct the impulse, not merely as the formula would require, away from the cell (in the cellulifugal direction), but the one impulse approaches the cell from the skin to the ganglion (cellulipetal), while the other proceeds from the cell, from the ganglion to the spinal cord (cellulifugal) in such a manner that it traverses the two prolongations placed end to end, just as would be the case with a single fibre proceeding from the skin to the spinal cord. Concerning the exact connexions of the dendrites and of these remarkable neurons, and the nature of the exact direction of their current of excitation, experiment is silent, becaiise it is impracticable under given conditions with the methods which are available to us. It is thought, however, with some prob- ability, that the principal current is that which, proceeding fron:i the organs of touch situated in the skin, is conveyed in the grey matter of the spinal cord. This physiological argument is looked upon by some as of such importance that it dominates all the others, and they call " dendrite every celluli])etal prolonga- tion and axon every cellulifugal prolongation " (Van Gehuchten). But then, obviously, these designations lose their morphological meaning in order to acquire one which is purely functional, and which depends exclusively upon experiment. Relative importance of the different prolongations at different ages. — It has been long known by certain definite indications, such as relative size and pre- cocious development, that spinal ganglia have, at a certain period of intra-uterine life, an important function which progressively diminishes with the develop- ment of the nervous system, and which is relatively wanting in the adult. The new anatomical methods require that embryos or young subjects be studied ; it is necessary to determine in the adult if there exist intra-ganglionic con- nexions which can be revealed by the use of these methods. Adaptation of the Neuron to successive functions. — In assuming that the traces of these structures may be definitely persistent, the example of the posterior radicular neurons shows us how the functional evolution of a nerve elenaent may adapt its several parts to functions altogether different to those which are dis- charged by the equivalent parts of other similar nerve elements, to such a degree indeed as to invert the direction of the conduction ; a new proof, if such were wanting, that structure does not predicate function, and that conclusions must be drawni from the fii'st concerning the second only with the gi'eatest caution. The n^orphological law would appear to have its own grounds different from those which initiate the physiological or functional law ; it is not possible, therefore to substitute the one for the other by confusing them in one common statement. The Mixed Axon, or the Axon with a double inverse Conduction. — Avoidance. — The progress of the impulse in the nervous prolongations, its concentration in the axon, its dispersion in the collateral and terminal ramifications, its passage through the body of the cell, the manner in which it enters or quits the latter, are so many questions which experinaent cannot resolve in a direct manner, and on which only tentative opinions can be exjDressed. As regards the sensitive radicular neurons, a cjuestion of this kind arises concerning the median prolongation of the T, which laterally connects them with their cells of origin. According to P. C ELEMENTARY NERVOUS FUNCTIONS Cajal, tlie impulse transmitted from the skin to the cord avoids this lateral pro- longation, and thus saves itself tlie journey to this cell. According to others this prolongation, more voluminous than its two divisions, would represent the association of their fibrils (Ranvier), which are thus, in the same axis cylinder, some cellulipetal, and others cellulifugal. Thus a mixed axon would result, if by this qualification mixed is implied elements (here fibrils) which possess the facility of conduction in opjDosite directions. It is impossible to give a decisive opinion for or against these two modes of regarding the question. For those who maintain the possibility of the propaga- tion front one neuron to another by simple contact, there would seem to be no impossibility in the passage of the impulses from one to another gi'oup of fibrils in the interior of an axon ; but it must be observed that the special nature of this so-called contact is unknown to us in these two circumstances, and that we are reasoning on resemblances or crude analogies. However, Bethe, in experimenting on the crab, in which these median prolongations have all the same orientation, has been able to divide them with one cut, and thus to sejiarate the cells of origin from the sensitive fibres which appertain to them ; and he has observed that after this separation cutaneous stimula- tion still gives rise to reflex movements, although the latter are weakened. The neuron ramifies at its two extremities ; it re- ceives impulses by numerous and sj^aced jsrolonga- tions ; it distributes them by its branches which are generally very widely dispei'sed. On this subject a problem arises which we cannot resolve, but which must not be ignored. The impulses which the several dendrites receive converge on the axon ; do they mix in it, as does the blood in the venous trunks which arise from the capillaries ? Or do they follow parallel and indepen- dent paths in the axis cylinder ? In other words, do the collaterals, each individually, enter into a deter- minate connexion with the dendrites ; or are they all connected with each dendrite, and reciprocally ? — The network which is observed in the interior of the cell, and which is found on the passage of the den- drites to the axon appears to furnish the re^^ly to this question. Whether this network is limited to the cell, or whether the reticulated arrangement is con- tinued over the whole extent of the axis cjdinder, a formation such as this is able to give to the disti'ibution of the imj^ulses a special character which may be described as neither absolute independence nor complete mixture, but a re- arrangement which obeys special laws and which may vary according to the function of the neuron. The specific nature would depend in a certain measure upoii the latter. Objections. — The theory of the neuron, on its first appearance, rapidly gained gi'ound, not only with neurologists, but with the whole scientific world ; and it may be said that it has not, speaking generally, lost favour, either with the former or with tlie latter. However, if it has preserved its supporters, it has met with determined adversaries, who do not despair of finding it wanting in some essen- tial point. Their niost serious objection is that the limitations of the neurons, rendered so obvious by the chromate of silver method (isolated impregnation of Fig. 11. — Cell of a spinal ganglion in the rabbit. Its single prolongation is. united to a fibre of the pos- terior root at the level of an annular node, whence its T- shaped appearance ; E, node of the T-shaped tube ; N, nucleus of the first segment of the cellular branch of the T ; E', first node of the cellular branch ; M, nuclei of its capsule. THE NERVOUS ELEMENT 19 some neurons in the middle of others), may be an artificial result which is due to the action of the re-agent employed ; thus has arisen once more the discussion on the continuity or the contiguit>/ of the articulated prolongations of these neurons. It does not enter into the object of this work to take part in these technical discussions : the question wluch has thus arisen, whatever may be its impor- tance, must be fought out by anatomists. But it seems that, in pursuing tliis detail, the essential point which this large and admirable collection of observa- tions has so vividly brought out, the essential point of view has been lost sight of. Tliis point is the individuality of the nerve element, of the neuron. For a long time this was unknown to us. It was generally admitted that the nervous system is formed of two species of elements, fibres and cells, and the point of departure between them was placed in a locality where it is clear that there neither was, or had been at any period of development, any discontinuitj*. We now know that in the nervous sj'stem there is no fibre which is not the prolonga- tion of a nerve cell, and when this fibre passes from one cell to anotlier, we can affii'm to which of the two it belongs. Thanks to the new methods introduced by Golgi, the nerve element can now be demonstrated as regards its general structure, its exact boundaries and its principal details. 3. Individuality of the Neuron. — Its proofs. — The two following facts estabhshed, one by embryology, the other by the method of degenera- tion, are convincing on this most important point. (1) The nervous elements, the neurons, arise from independent cells ; these cells, as His has observed, give out prolongations in different directions, in order to be mutually connected and to associate them- selves with the fixed elements of the organism. (2) When the prolongations are separated from the cell by section they hecome the seat of a degeneration ichich terminates precisely at their extremity. In leaving undetermined the question whether these prolongations have contracted connexions which establish their continuity by means of welding, or ^^hether they remain in simple contact, these two facts ]3rove to demonstration that each nerve cell, each neuron, has a definite territory, instead of the indeterminate distribution which the hypo- thesis of a vague network connecting the cells formerly held to be the case. Further, we can recognize the limits of this territory. Hence it follows that the individuality of the nerve elements is no longer a contestable matter. His has remarked that the nerve elements pro- ceed from cells definitely separated the one from the other, and that these latter give rise to polar prolongations with free extremities, of which one especially is of long extension, in order to form the different connexions of the nervous system. Ranvier had also seen that the regeneration of nerves, after section, was effected, just as is their origin, by a progressive budding of the axis cyhnder towards the localities to be innervated. These facts, however, had not attracted attention, and all the conclusions to which 20 ELEMENTARY NERVOUS FUNCTIONS they point had not been deduced from them. But when, by Golgi's method, it was possible to demonstrate by preparations in which the detail of the prolongation was clearly visible ; when, above all, Cajal had given a correct interpretation of them, then only a light was thrown on the subject, and the nerve element appeared in its true limits, very different, it must be admitted, from those which Avere formerly attri- buted to it. Whether these elements have their ultimate fibrils simply in contact, as many believe ; or whether there is between them a con- tinuity by welding, as others maintain, is certainly a detail of import- ance, because in science nothing is a matter of indifference. But, from the special point of view of the individuality of the neuron, it is a secondary question. The limits of the latter are certainly as has been described. Starting from cells separated, and sometimes widely separated from one another, the prolongations which thus proceed to meet each other were in the first instance necessarily discontinuous. Whether they are free or whether they are welded, they do not cease (as degenerations show) to belong each one to a different cell, instead of forming an indefinite network as was formerly supposed. In studying the nervous system of invertebrates bj^ the aid of the new colour method. St. Apathy and Al. Bethe have discovered in the neurons of these animals a fibrillary network which is present in the cell, and is continued into the prolongations of the latter. To this network they ascribe a fundamental role in the production of nervous phenomena strictly so-called, reserving to the cytoplasm of the cell a purely trophic function. As a result of these observa- tions, the theory of the neuron is once again plunged into discussion. The arguments which have been ch'awn from these observations are in reality of two kinds, and quite independent : the validity or inaccuracy of the one does not imply the validity or inaccuracy of the others, or reciprocally ; hence it is necessary to examine them separately. These arguments are as follows : ( 1 ) The neurons being, so far as regards their really nervous portions, formed of fibrils, if it can be proved that these fibrils are continuous from the one to the other, the individuality of the neurons must be compromised, and this the more because, in the opinion of these authors, the connecting fibrils interposed between the cells would appear to effect a sort of independence as regards the latter. In this way a return would be made, under a somewhat new aspect, to the old division of nerve elements into fibres, on the one hand, and cells on the other. As a matter of fact, neither of these two authors clainis to have proved this con- tinuity of fibres from one neuron to another ; it is a supposition which appears merely probable to them. The arguments given above in favour of the individu- ality of the neurons remain unimpaired. (2) The fibrillary portion being that which is alone essential to the performance of the strictly nervous functions, and the cytoplasm being reduced to a merely trophic function, the nerve cell would no longer play the preponderant part which has been conferred upon it in nervous acts properly so called, in physical functions for example. This second question is altogether independent of the preceding one ; it concerns the internal organization of the neuron, while the first involves the organization of the ner- vous system by the connexions established between its elements. It is capable of a solution which, whatever it may be, predicates nothing for or against the individuality of the neuron. THE NERVOUS ELEMENT 21 Trophic and functional Protoplasm. — In taking into consideration all the data of observation and of experiment which are available concerning the nerve element, I arrived at this conclusion ; viz., that two portions, two different protoplasms, niust be distinguished ; the one, primitive or organotro'phic, is that which is visible in the nerve cell, in which the nucleus of the latter is immersed ; the other, junctional or nervous strictly so called, is that whose properties we are able to observe in the axis cylinder, when we have separated this latter from its cell of origin. 1 Without any doubt the plane of separation between the two is not marked out by the cut of the scalpel, executed by the experimenter in order to render apparent either the trophic role of the one or the functional role of the other. This surface of separation seems to be accurately defined in the preparations of A path J- and Bethe. Having thus localized (in the interior of the neuron) nutrition on the one hand and fvmction on the other, it is necessary to understand that these pheno- mena are mutually interdependent, and that the structures which represent them are, hence, in a constant condition of exchange and of mutual dependence. In the neuron, in every cell, as in the entire organism, nutrition and function are but very distinct points of view of an assemblage of functions, all of which tend to the same end : the conservation of life. 4. Physiological data. — Before the doctrine of neurons had been suggested, physiology had already furnished proofs that certain nervous elements terminate in a definite manner in certain locaUties which experiment pointed out. These localities correspond exactly to certain of those in which anatomy really establishes the points of contact between the neurons, namely, • the ganglia of the great sympathetic. In order of precedence, these are the facts on which this opinion was founded. (1) If the chain of the great sympathetic be stimulated below the first thoracic ganglion (with reference to the vaso-motor nerves pro- ceeding to the ear), the auricular vessels contract. If this chain be stimulated above this ganglion, these vessels dilate. The dilators of the auricular vessels end in these ganglia (Dastre and Morat). (2) If the sympathetic chain be stimulated either above or below the lumbar ganglia (with reference to the inferior extremity), the hair stands on end in certain areas of the latter. But if these ganglia be impregnated M'ith a solution of nicotine, stimulation applied below continues to cause erection of the hair, but when applied above has no effect. There is a spino-gangliojiic element ichich possesses the function of erecting the hair and ivhicli terminates in the ganglion (Langley and Anderson). The specific nature of the Neurons. — In accordance with the law of division of labour, which ^^'e find is applicable to the nervous system, as to the whole organism, the neurons should possess specific functions. 1 This is clearly the idea which has been i-eproduced under slightly different names, of distinguishing in the neuron a tropho-pJasma (tropho-protoplasm) and a kineto-plasma (functional or nervous protoplasm strictly so-called). 22 ELEMENTARY NERVOUS FUNCTIONS It was at first thought that this specific action was connected with certain external morphological characters of the neurons, that, for example, all those which experiment proves to be sensory would corre- spond to a definite, externally recognizable, type ; all those which are known as motor, to an equally definite type, but differing from the preceding one, etc. . . . This induction has not been verified. The study of morphological characters, in proportion as it is carried out by the aid of more perfect methods, has rather tended to collect to- gether the neurons discharging all functions under a common type, recognizable among a great variety of forms, but of which not one corresponds to any function with which we are acquainted. This rebuff is due to the fact that the reasoning on which these obser- vations were based was fundamentally erroneous. The functions, of which up to the present time the connexion with the individual form of nerve elements has been investigated, are not localized in these elements, but in the systematized groups of which they form a part, and whose function they regulate in a certain manner. By itself the neuron is incapable of causing either movement or sensation, far less ideation ; but it originates these several functions in the organs or the complex systems with which it is itself united. In other words, motion, sensation, ideation are not cellular functions, but systematic functions. They are not simple facts, but synthetic expressions imply- ing the co-operation of a large number of elements, whose individual function is not perceptible in the perfected whole. The existence of specific systems, corresponding to specific functions, nevertheless postulates that the order of their elements is different in each of them. There is then, on passing from one system to another, a specificity of connexion amongst those elements which causes one to react in a different fashion from the other. Finally, a system, considered apart, is not the indefinite reproduction of the same element, but necessarily consists in an aggregation of parts or of differentiated elements, with- out which its function would be merely that of this element, magnified indeed or multiplied, but without other alteration. It must be then that the nervous elements present a certain individual specificity, in the aggregations which they form ; but this specificity must be sought for in other ways and on other foundations. The fact of the name of system being sometimes applied to cell groups which are in reality in no sense systematized, has contributed to bring about this change. The muscular structure, for example, consists merely in the repetition of a great number of elements, whose cellular function is obvious (irritability manifesting itself by the definite con- traction of its protoplasm) ; by itself it is not a sj^stem, but its elements. THE NERVOUS ELEMENT 23 mutually attached and connected with the organs of sense by nervous elements, make up functional systems which are more or less complex, such as that of respiration, of phonation, of speech. In systems thus built up, the initial elements (organs of sense) and terminal elements (muscles), have very obvious cellular functions, while those of the intermediate elements (nervous) elude direct analysis. Hence the cellular function of the muscles has often been transferred to the ner- vous system (motricity), and hence also the total function of this system (sensation) has been compared to a cellular function of its elements. These inaccuracies in the use of terms must be rectified, otherwise error and confusion will result. Varied forms. — A large number of nem-ons appear at tlie first glance to depart widely from the general type described above ; it is possible, however, to bring them back to it without much difficiilty. The body of the cell is not necessarily situated in the vicinity of the receptive pole, but may occur also in the course of the axon, as in the ganglia of the acoustic nerve. Further, certain cells (for example those of the spinal ganglia) are called unipolar because they give off but a single prolongation ; but it is known that this single prolongation bifvircates like the transverse branch of a T, in order to send a fibre to the skin and another to the spinal cord, both terminating by ramifications, in such a way that the two poles can be found there without difficulty. The reason of this singular arrangement is not knoAAOi to us, and the function of the median prolongation to which the cell is suspended is equally obscm'e. It is possible that the impulse avoids it wholly or partially, as Cajal maintains ; it is possible that an inverse double root may exist in this prolongation, which assures its circulation in the cell, as Renaut holds. In certain neurons the axon, instead of being single, verj^ soon divides into two branches, which sometimes follow different directions for a gi-eat distance and sometimes even ]iroceed in opposite directions. Amacrine Cells. — Certain cells which are entij-ely embedded in the grey mat- ter, or in the membranes which recall its structure, as the retina, are furnished with numerous arborescent prolongations, amongst which none is detectable which morphologically represents an axis cylinder or axon. It would not be right to maintain from such an arrangement that in these elements the direction of condviction is a matter of indifference : it can only be said that this direction is imknown to us. In none of the localities in which it is available, does physio- logical experiment, made in situ, prove the existence of any instance of indefinite conduction ; but, on the contrary, this latter appears to be everj^where effected in a perfectly definite manner. Nervous Amoeboism.- — The mode of termination of the nervous j^rolongations, whether free or fixed, is connected with a problem different from that of the individuality of the neuron. If tlie neurons are fixed tliey are necessarily mechanically immobile ; if they are free from attachment they are capable of receding and approaching one another under certain conditions which are not yet ascertained. Rabl-Ruckard. Lepine, Tanzi, M. Duval have appealed to displacements of this character in order to explain the dissociations, variations and functional paralyses which are observed in healtli and in certain maladies. To these supposed movements M. Duval has given the name of nervous amoeboism, a very definite expression, but one which, by comparing the extremely differentiated prolongations of the nerve cell to the pseudo-podia of an amoeba. 24 ELEMENTARY NERVOUS FUNCTIONS clearly goes far bej^ond his meaning. As a matter of fact, do movements of this kind (analogous to muscular contraction) take place in the nerve terminations ? Fig. 12. — Different types of nerve cells coloured by the rapid method of Golgi. A, nerve cell of the superior cervical ganglion of a human embryo of 25 centimetres (after Van Gehuchten). B, cell of the molecular layer of the cerebral cortex in a rabbit aged eight days (after Ramon y Cajal) : cy, polar or principal axis cylinders ; a, supernumerary axis cylinders starting from different protoplasmic branches ; h, ramifications of tlie axis cylinders. C, cell of Purkinje, from the cerebellar cortex of a cat aged fifteen days (after Ramon y Cajal). D, large pyramidal cell of the cerebral cortex of a mouse aged one month (after Ramon y Cajal) ; Pp, peripheral spiny protoplasmic prolongation. E, two root cells of the anterior horns of the spinal cord of a fowl at the eiglith day of incubation (after Van Gehuchten). In all the diagrams, cy indicates the axis-cylinder prolongation. THE NERVOUS ELEMENT 25 Changes of form in the Dendrites. — Demoor, Stefanovvska and Manouelian claim to have demonstrated that, independently of the slow movements of growth of the prolongations during their development, there are other extem- poraneous movements, which give to these prolongations variable appearances and dimensions, following two conditions, the one of activity, and the other of repose. In examining the dendrites of the cells in the brains of animals killed in these two conditions, these authors have found that they differ in the following points. In the one case these dendrites bear at their extremities and laterally pyriform prolongations ; these are met with in brains investigated in their normally active condition ; in the other case tliese prolongations have disappeared into the trunk which supports them, which has assumed a varicose aspect in conse- FiG. 13. — Pyramidal cells of the marmot in two different conditions (after Querton) On the left, pyramidal cell of the marmot awake ; on the right, that of the marmot asleep. quence of their absorption : this condition is met with in morphia poisoning, anaesthetic sleep, and sleej) succeeding prolonged fatigue, itself the result of violent stimulation. The preceding authors are unanimous in regarding these changes of form as explaining the two conditions, the one of functional activity, the other of abeyance of function, of nervous groups : the first of these two conditions being rendered possible by the establishment of connexions between the elements which help to make up a system, the second resulting from a temporarj^ rupture of these connexions. — Whether these changes are definitely connected with special states of the nervous system, may be decided by experiment ; but that they explain these conditions, is niore difiticult to adnut. Kolliker has brought forward the following argunaents against nervous amoeboisra : the axis cylinder on which we are able to experiment is not contractile ; the nervous prolongations 26 ELEMENTARY NERVOUS FUNCTIONS wliich can be followed into the tissues in transparent animals show no visible movement. Functional dissociation ; Mechanism and localization.— Concerning the exis- tence of a nervous amceboism projierly so called, we have just made express reserves which are rendered necessary, we consider, by the want of precise infor- mation regarding the functional mechanism of the neuron, and the difficulty of attributing a causal signification to the anatomical and experimental facts by which it has been attempted to explain this mechanism. Having made these reservations, we consider, nevertheless, that the fundamental idea on which this conception is based deserves to be weighed, and that there is something in it which is worthy of discussion. It is this idea which it is now necessary to evolve. The study of the nervous system displays it to us as presenting during its functional activity, dissociations and associations of its different parts, which are sometimes isolated the one from the other, sometimes united, according to the natiare or the complexity of the act to be accom- plished. It being admitted as proved that such separations and recon- nexions arise in tlie nervous system, it is natural to suppose that a rupture and a renewal of the connexions between its several parts should be effected, especi- ally in that locality where its elements (nerve cells), in giving off their opposed prolongations, have first encountered each other in the course of their develop- ment in order to build up the system in its totality and the sub-systems which compose it. — Thus, in so far as these phenomena of dissociation and association are localized at the extremity of the neurons (at their points of contact), which the study of nervous functions renders apparent, no risk of self-deception arises, whatever may be the modifications or the facts which the future may reveal as necessary to our p)resent conceptiors with regard to the constitution of the nerve element. It seems, indeed, that it is here (at the point of junction of the neurons) that the principal transformations which the impulse undergoes in passing through the grey matter take place. Fundamentally, what is generally described as a centre is merely a locality tvhere the neurons are able to organize themselves into a definite system (partial) in order to perform a definite function. But to this problem of iGcalisation another is added, concerning the intimate mechanism of the transformation brought about in the grey matter every time that its organization adapts itself to the performance of a si^ecial act. Do the breaks and the union between neurons consist in mechanical and visible displacement, or in molecular movements which our optical appliances do not permit us to recognize ? At the present time it is impossible to express a definite opinion on tliis C|uestion. Dissociations and associations between nerve elements certainly exist ; in all probability they may be localized in the grey matter at the points of junction of the nerve elements. No definite state- ment can be made concerning the mechanism by which they are carried out. Nevertheless, as mechanical phenomena properly so called are those which are most easily comprehended, as they are those which, in the study of every function have always contributed to furnish the first intelligible ideas, the doctrine of amceboism, by clearly defining the question of the connexions between nerve elements, indicates progress in the study of nervous physiology. From another point of view, it has led to the production of works and to the determination of facts which, however obscure their signification may be at the present time, are yet of great interest. Connexions of the Neurons. — The individuality of the neurons is proved ; it is founded on the idea of their exact limitation and of their independent life. It remains to ascertain the mode of connexion which exists between them and the other tissues. On this point it may be said that scarcely anything is known. Whether in the purely static condition, or whether in the dynamic condition, the data we possess concerning these connexions are very incomplete. THE NERVOUS ELEMENT 27 The terminal and collateral arborizations of certain neurons come into con- tact with the initial arborizations or dendrites of other neurons. The collaterals and terminals of the neuron form no connexion amongst themselves, any more than do the dendrites. Such is at least the view generally maintained. Xever- theless, Renaut, studying the retina by means of methylene blue, has some- times observed two cells luiited by a large protoplasmic expansion ; each one has its dendrites, one only gives off an axis cylinder ; he calls these Fig. 14. — Termination of neiu-ons in the twin neurons^ On other occasions two secretory elements of glands, neighbouring cells, possessing their A, cell of the parotid gland in the rabbit ; axis cylinders and protoplasmic pro- B, cell of the mammary gland of a cat during •^ -11 i- 1 gestation. — The ternimations have no con- longations, are united by one of these nexion with the nuclei of the cells, but only prolongations ; these he calls coupled with their differentiated protoplasm. neurons. Adhesive supports. — Even by leaving aside these exceptions to the general law, it is very difficult to gather together the connexions between nem'ons in a single formula. Contact of the collaterals and terminals occiu"S, not only as regards the dendrites, but also often with the body of the cell which receives the transmitted impulse. On the other hand, this contact is effected not only by the extremities of these prolongations (axial or protoplasmic cylinders) between themselves, but also for a certain extent of their length. The two orders of prolongations form networks and felting, which render it possible for the same order to enter into contact witli several others, or several times with the same. These are the multiplied contacts which Renaut terms adhesive supports. Observing, on the other hand, that the protoplasmic prolongations present, under certain conditions, a headed aspect which he regards as connected wdth the state of activity of the neurons, this author explains, by the apj^earance and disappearance of this beaded state, the occurrence of these adhesive supports, which enable the transmission of the impulse from one neuron to another to be effected. It is umiecessary to fm-ther criticise these explanations and other similar ones, each of which is based indeed on some special anatomical fact, but whicli, in its totality and according to the admission of its author, remains hypothetical. Apart from the fact that the impulse is transmitted from one nem-on to another in a definite direction, we are almost entirely in a state of ignorance witli regard to this matter. We know neither tlie nature of the transmitted movement, nor the niedium in which it is transmitted, nor the conditions of its transmission. We can, it is true, detect after death changes of form corresponding to such or such a condition which we have induced in the animal during its life ; but we must remember that the movements of cellular protoplasm are of diverse nature, in connexion with the different functions of the life of the cells ; there is no guarantee that the changes thus observed are of a pvu-ely functional nature, the modality of the function being itself unkno^^^l to us. Under the influence of the impulses which are communicated to it, the pro- toplasm of the body of the nerve cell experiences analogous changes of form, of volume, of coloration, or of the situation of its parts, which may be considered as being of a trophic rather than of a functional natiu-e. Mitral Cells of the Olfactory Lobe. — The cells of the olfactory lobe are disposed in a typical manner, which is of great value as regards the interpretation of the connexions between neiu'ons. Eacli one of these cells has a special cellulipetal prolongation whose dendritic arborizations are (in the glomerulus) comiected 28 ELEMENTARY NERVOUS FUNCTIONS with the terminal arborizations of the axons of the nem'ons proceeding from the sensorial portion of the nasal mucous membrane. In the opposite direction it gives off a cellulifugal prolongation, which is an axon proceeding in the direction of the brain. Lastly, these cells emit lateral prolongations, arranged at I'ight angles to the preceding ; these last are connected, either directly or by associat- ing cells, with the terminal ramifications of the axons of the centrifugal elenients which are contained in the olfactory tract, centrifugal elements which are met with again in the optic nerve and other analogous sensory structvires. ' The excitations of different origin arrive therefore at the nem-on, thus built up, by different routes ; none of the elements which are in connexion with it Fig. 1.5. — Connexions of peripheral neurons with the deep neurons in the olfactory system, at the level of the glomeruli of the olfactory bulb. The mitral cell receives the impulse in the glomerulus by an elongated prolongation which ramifies in it. The glomerulus is a functional nervous centre. directly touches its cell. This is a proof that this contact witli the cell is in no sense necessary, and that the impulse is received by special apparatus adapted to the purpose. Pericellular Baskets. — However, there are a considerable number of cases where the connexions between neuron and neuron are effected by the contact of the terminal ramifications of the one with the body of the cell of the other. It is clear that there is here, under these apparent diversities, a general arrange- ment which enables them to be collected together under a common rule. The body of the cell, more than the nvicleus and the other parts which make it what it is, contains the expansion of the fibrils which, in all the other portions of the neuron (dendrites, axon, collaterals and terminals), convey the impulse. When the cell is on the course of the neuron, it is simply passed through by this latter ; when it is at its origin, in such a way as to itself form its receptive pole, it necessarily contains the initial extremities of these fibres, which renders them capable of gathering together the impvilses, when it is, as often happens, placed on a chief point of bifurcation, or of the ramifications of prolongations, the fibres which pass through it adapt themselves to this distribution, varying THE NERVOUS ELEMENT 29 according to the circumstances of the impulse. It is clearly the fibrillary part which, in each one of these cases, assumes the performance of the fimction which is demanded of it according to its situation. Fig. 16. — Elements of the cortical matter of the cerebellum amongst which is a basket cell (M. Duval). CC, basket cells ; CP, pj-ramidal cell ; co, several baskets ; cp, the basket cell supported by the pyramidal cell. Spiral fibre ; Its signification. — Tn the batrachia are found, in the ganglia of the great s;y'Tnpathetic, pyriform pedunculated cells on an axon which turns towards the periphery ; around this pedicle, as round an axis, is rolled a fibre called spiral, of w^hich the connexions with the cell are noticeable. This fibre is the terminal prolongation of another neuron whose cell is situated higher up, and which has just come in contact, by its ultimate ramifications, with the pyri- form cell, tlirough a varicose pericellular network situated between the cajisvile and the nervous protoplasm (Xicolajew). B. DYNAMIC CONDITION : FUNCTIONS OF THE NEURON Each system, individual cell, or organized group, necessarily pos- sesses two orders of functions : the one internal to the system itself, for the conservation of this system (or its development if it is in process of organization and of growth) ; the other having an influence at the exterior of the system, and which connects it with its surrounding medium, or with a more complex system of which it forms part. Bio- logically, the first are commonly called trophic functions, that is to say, those of conservation or nutrition ; while the second are known as social. 30 ELEMENTARY NERVOUS FUNCTIONS Tig. 17. — Cells furnished with a spiral fibre in the frog. On the left is seen the origin of the spiral fibre in a fine pericellular network. that is to say, those func- tions which are related to cell elejnents of more or less equivalent value. I. Functions of Internal or Trophic Connexion The neuron is a living unity ; but this unity is made up by the associa- tion of complex parts, maintained in a condition of mutual dependence, by which its conservation is assured. These ties be- tween its component parts represent the func- tions which are internal to the neuron itself, and which, in order to conform to the cus- tomary mode of expression, w^e shall describe as trophic or organo-troiJihic. When, by any means whatever, we break these ties, disorder reigns rampant in this co-ordinated whole ; the equilibrium which main- tained its form and its existence is destroyed ; this form and this exist- ence are both simultaneously compromised ; the element, to adopt the usual expression, degenerates. This degeneration may entail its total or partial death ; in this latter case, when its essential portions escape destruction, reconstruction commences in order to restore the form of the neuron as well as its dimensions and its primitive integrity : this is regeneration. At least three forms of degeneration are distinguished, viz. : Wallerien or descending degeneration ; ascending degeneration ; atrophic degen- eration or that resulting from loss of function. All these varieties have this in common, that a local limited lesion, produces alterations which are propagated to a distance in the nerve element or in another element consecutive to it, but each form of degeneration is met with in determinate conditions. 1. Wallerien or Descending Degeneration. — Fontana had already remarked that section of a nerve is followed, after some days, by the loss of its excitability, but to Waller is due the discovery of the laws which regulate the degeneration in its most typical and common form. THE NERVOUS ELEMENT 31 descending degeneration, which nov>' bears his name. The following are the conditions of its j)roduction. Experiment. — Let the anterior and posterior roots of a pair of spinal nerves in an animal be cut simultaneously^, and after some days exam- ined from the point of view both of their excitability and of their modifications of apjDearance and of structure ; the peripheral end of the anterior root and the central end of the posterior root will be seen to have assumed a greyish appearance, which contrasts with the normal whitish colour of the two ends. Microscopic examination shows the Fig. 18. — Experiments of Waller, on which the laws of degeneration are foiuided. Above on the left, section of the posterior root between the ganglion and the spinal cord ; below on the right, section of the anterior root ; below on the left, section of the posterior root between the ganglion and the peripherj- ; above on the right, section of the mixed truiik formed by the union of the roots. In each ease degeneration attacks the segment which is separated from the nucleus of origin of the nerve (spinal ganglion for the posterior root, spinal cord for the anterior root). presence in this end of the nerve of extreme disorganization. At the same time it is noticed that the two degenerated segments are no longer excitable. Stimulation of the peripheral end of the anterior root, motor in function, as we shall see further on, no longer causes the muscles to contract ; and the central end of the posterior root, sensory in function, no longer excites pain when it is pinched or faradized. Of the four ends or segments produced by the section, two have not 32 ELEMENTARY NERVOUS FUNCTIONS degenerated : these are, as regards the anterior root, that which re- mains connected with the grey matter of the spinal cord and, in the posterior root, that which is still connected with the ganglion of this root. Two have degenerated : namely, those which have been separated respectively from one and the other of these two organs. Waller called them their trophic centres, and this term is still used. These two examples appear to show that the degeneration occurs strictly according to the direction in which the nerve conducts the impulses, towards the spinal cord as regards the sensory nerves, towards the muscle in the case of the motor nerves ; as a matter of fact this is not so, as the following experiment proves. If the posterior root be cut, no longer between the ganglion and the spinal cord, but between the ganglion and the periphery, it is the cen- tral end which is preserved, and the peripheral end which degenerates. To sum up, the end which remains intact is invariably that which has preserved its connexions ivith the cells of origin of the nerve fibres which have been cut, and the other, in whatever direction conduction may be effected, is doomed to destruction. In current language, these facts are in- terpreted in the following manner : the anterior and the posterior root are formed by the prolongations of the neurons whose cells of origin are, as regards the first, in the spinal cord, and, as concerns the second, in the root ganglion. Every prolongation separated from its cell is destined to be destroyed ; whether axon, dendrite, or the equivalent of any one of these parts its lot will be the same. The cell, on the contrary, which was originally the germ of the neuron, and which possesses the power of preserving it, as it formerly had the power of creating it, is maintained intact, both it and the fibres or segments of fibres which remain in connexion with it. Remark. — The neurons of the posterior roots, so accessible to analy- sis in consequence of the arrangement of their prolongations in two directly opposed directions, do none the less form a particular case, which is somewhat rare as regards all the arrangements which the neurons affect. Speaking generally, the body of the cell is found at Fig. 19.— Section of the anterior and posterior roots near the spinal cord. Appearance of the fibres after several days. On the left, posterior root of which the fibres, having preserved their connexions with the cells of the spinal ganglia, have remained healthy ; on the right, an- terior root whose fibres, sepa- rated from their cells of origin, have degenerated (segmentation of myeUn). (Augustus Waller.) THE NERVOUS ELEMENT 33 the origin of the neuron, in such a way that, from the experimental point of view, a section can only be performed on the axon or the axis cyhnder prolongation. Under these circumstances, which are those usually present, degeneration ensues in the direction of the conduction. Hence the method of degeneration may be employed in order to ascer- tain in what direction impulses are conducted in certain nerve bundles (especially as regards the spinal cord and brain). Laws of Waller. — 1. Nerves (Nerve Fibres) when separated from their trophic centres (Nerve Cells) degenerate. 2. The direction of the degeneration is independent of that of nerve conduction Nutrition and Conduction. — The attempt has often been made to connect the direction of degeneration with that of the conduction of impulses, in such a way as to include the two phenomena in one and the same law expressed by a single formvda. The example of the sensory nerves in the posterior roots proves that there is no necessary connexion between them. The process by wliich the neuron preserves its existence and that by which it conducts the impulse may indeed be related, being mutuallj- dependent, but they are fundamentally and essentially distinct. It is merely necessary to be reminded that, from the mor- phological standpoint, the case of the posterior roots is a special one. The cellu- lipetal and cellulifugal prolongations of the spinal cell have therein the same organization, that of axons or fibres covered with myelin ; and thus we see that the myelinated fibres are dependent on the cell whatever position they may occupy with I'egard to the latter. On the other hand, we are not aware of what passes in the protoplasmic prolongations, or dendrites, after they are separated from the nerve cell. It is probable that they degenerate and finally disappear by absorption of their substance. Trophic and functional centres. — From the distinction thus established between the process of conservation (or of degeneration) and that of nervous function, another arises wliich is adso of importance. Usually " centres " imply localities in which the connexions of similar or different parts are organized and united in order to form a common force. The experiments of Waller prove that the connexions which the different jDortions of the neuron gather together are centred in the nerve cell, whence the name of trophic centre is given to this latter. In their turn the nevu'ons associate and collect thenxselves together in more or less independent systems, in functional centres (or nervous, properly so-called). This association of nerve elements is effected by their prolongations in the compli- cated connexions contracted by the latter in the interior of the grey matter. Thus the functional centres are distinct from the trophic centres, with which they have been confounded. Generalization, — The laws of Waller are general laws. They may be verified in all nerves, in the tracts found in the spinal column and in the brain and the great sympathetic system. Laws of cells. — I consider that these laios are not only applicahle to the elements composing the nervous system, hut to every cell element. Every cell contains an original, germinative, constructive and con- P P 34 ELEMENTARY NERVOUS FUNCTIONS servative portion as regards form and structure, and a differentiated part as regards its functional relations with other elements. The latter is dependent on the former and cannot exist without it ; yet the converse is possible, at all events for a certain time, and in certain conditions. Loss of excitability. — After section of the nerve, the latter does not immediately lose its characteristic properties. The rupture of equili- brium which results from isolation from its nutritive centre does not exert its full effect until after some days. During this period the nerve preserves a local life, thanks to its vascular connexions. After twenty-four hours it is still excitable ; it may be, indeed, that its ex- citability is increased. In the dog, according to Longet, this excitability completely disappears after four days. In the rabbit, according to Ran- vier, it disappears after forty-eight hours ; in the pigeon, after two and a half to three days (Waller). In the frog and cold-blooded animals it continues much longer than in mammals. In the first, especially, it varies very greatly according to the season ; in other words, accord- ing to the temperature. In the frog, in the winter, the excitability may persist up to thirty days after section (Brown-Sequard). Further, this persistence of excitability varies even in the same animal according to its nutritive condition and its vitality. Structural alterations. — The neuron is a symbiosis. The superadded cells form a sort of sheath for its axon (sheath of Schwann), which is separated from the axis-cylinder by an internal layer of myelin. In the cut nerve, the unity between these parts is broken ; the myelin becomes segmented ; the protoplasm of the cells of the sheath becomes thickened ; the axis-cylinder falls in pieces and is re-absorbed ; the nuclei increase in number ; and the sheath persists as the sole relic of the nerve thus destroyed and deprived of its functions (Ranvier). In the spinal cord and the brain, in which a genuine sheath of Schwann is wanting, these changes are carried out in a slightly different manner, but the essential result, loss of excitability and disappearance of the axis cylinder, is the same. The neuroglia, by filling up the cavity left by the degenerated fibres, gives a peculiar appearance to a section of the latter which is characteristic of degeneration in a slightly advanced stage. 2. Ascending degeneration. — If that part of the neuron which is separated from its cell of origin is doomed to certain destruction, the other portion which has maintained its relation with this cell also experiences the counter effects of this amputation (Nissl). It may be that it degenerates wholly, including the cell ; it may be that it survives and, in this case, regeneration of the destroyed portion THE NERVOUS ELEMENT 35 may more or less completely ensue ; but even when this happens changes in its appearance will be present, indicating internal alterations of its structure. Conditions which are necessary for survival. — We have seen that degeneration occurs indifferently in the upward or downward pro- longation of the cells (as regards the conduction of impulses), whenever these prolongations are separated from them ; but in order that it may survive, it is not a matter of indifference to a cellular element, mutilated or not, whether it receives impulses, or is completely deprived of them. If the section is below, it receives impulses although incapable of transmitting them ; if the section is above, it is deprived of them, and, in the second case, the cell, having for a time survived its amputated member, finally atrophies and disappears (Van Gehuch- ten). Thus it is easily understood that section of a posterior root between the spinal cord and the ganglion permits the continued exist- ence of the cells of the latter, while section of the sensory nerve between the ganglion and the skin necessarily causes their destruction (Lugaro). And it can also be understood that section of a motor nerve permits the continued existence of its centre of origin. This persistence is doubtless due to the reflex or voluntary impulses which are transmitted to this centre by the connexions which it has preserved (Marinesco, Goldscheider). Nature of the change ; Chromatolysis.— Wliether the cell is to survive or to die, it will present at the coiixnienceinent the same modifications. These consist essentially in a dissolution of the chromatic substance in the enchyleme {chromato- h/sis) which is clearly obvious after a day and a half or two days, but more mark- edly so after iour or five days, and whicli attains its maximum towards the fifteenth day. This chromatolysis, commencing near the nucleus, extends to the body of the cell in order to attain its prolongations ; it is accompanied with a swelling of the cell and with a displacement of the nucleus. If the process stojjs here, the cell will survive and there will be regeneration. All this disorganization is the result of an effort for the reconstitution of the type of the mutilated element. On the other hand, if matters go further, there will be destruction of the fibril- lary network of the cell, disorganization of its protoplasm, and disappearance of the latter (Van Gehuchten). Its phases. — The alteration v.-hich ensues in the cell of the neuron after the separation of its prolongations is only then a true degeneration in certain cases ; it is limited in other circmnstances (defined above) to a temporary reaction, which is seen to be inevitable, after such an important nnitilation of the neuron. Hence the change will present phases whose result differs according to circum- stances. The first phase will be that of reaction ; should the process continue it will lead sometimes to a true degeneration or destruction of the nerve cell ; on the other hand, it may terminate in a reparation of the mischief wliich has been ^\Tought in the latter. This reparation is manifested by a very marked hypertrophy of the cell, and by its deepened colovu" due to the abundance of its chromatophile elements ; sutiu^e of the ends of the cut trimk facilitates this reparation (Marinesco). 36 ELEMENTARY NERVOUS FUNCTIONS Application. — Tlie reaction of Nissl has been made use of, especially by Marin- esco, in order to ascertain the nuclei of origin of nerves after total section of Fig. 20. — Chroniophile substance (granulations in black) uf se\'eral types of nerve cells (after Van Gehuchten). On the left, bi-polar cells of the cornu ammonis in the rabbit ; on the right, three multipolar cells of the nucleus of origin of the oculo -motor nerve. In the degeneration of Nissl the chromatic substance is dissolved from the nucleus towards the prolongations. these latter, or of merely a part of their bundles of distribution, in order to discover the situation of these nuclei, or rather of the portions of them which individually corres])ond to these bundles of fibres. Alteration of the fibres. —Not merely the cells, but the fibres themselves in their journey to them, present modifications consecutively to section, modifications which in the aggregate are very different from those of Wallerien degeneration, which occurs in the fragment cut off from the axon. They are slower than these latter and less deep, and they permit the persistence of the axis cylinder. This degeneration, known as retrograde, which, after section of the nerve, ensues in the extremities of the fibres which adjoin the cell, should be clearly distin- guished from the reaction which occurs in this cell contemporaneously with the Wallerien degeneration. The reaction of Nissl and Wallerien degeneration are the immediate manifestations of the mischief which ensues in the neuron in con- sequence of its mutilation, naanifestations whose progress is different in the cell and in the cut axon. Retrograde degeneration is a consecutive manifestation, following this mutilation and the disorder which ensues upon it. All these alterations may be observed in the peripheral nerves and their cells of origin (spinal and medullary), in the elements of the great sympathetic, in the special nerves of the spinal cord and of the brain. Retrograde degeneration is most frequently met with in these latter organs, so that their regeneration is but seldom spoken of, contrary to what is noticed in the peripheral nerves. Equivocal Sense of the Word.— The words " descending and ascending," used in the sense just pointed out, are ill ciiosen. In the first place they are faulty, inasmuch as degeneration does not -progress by successively attacking different por- tions of the length of the nerve, hut affects all portions simidtaneously, and progresses equally in all portions at once. Secondly, they are equivocal, inasmuch as they have no connexion with the real position in space of the degenerated segments. THE NERVOUS ELEMENT 37 It would be better to speak of proximal degeneration as regards the segment which remains in connexion with the cell, and of distal degeneration for that which is separated from it. On the other hand, when it is merely a question of Wallerien degeneration, by far the most easily recognized, in the case of section of the spinal cord, the de- generation will be descending for the fibres whose cells are situated above the section and ascending for those whose cells are below it, without, however, ceasing to be distal. Ascending neuritis. — The description which has just been given only includes those alterations which follow the interruption of continuity of nerve fibres and the ruptiu-e of their nutritive equilibrium, which is the direct conse- quence thereof as regards t)ie nem-on. Under different toxic or pathological influences the nerve may present various alterations, among which the preceding may occur, but which do not concern in a more direct manner the physiological study of the nervous tissue. 3. Atrophic degeneration. — Like every cell element, the neuron is susceptible of undergoing, through prolonged default of functional activity, an atrophy or diminution of all its component parts without any notable change in its structure. This atrophy is the consequence of the isolation which reaches certain neurons after a time when those which come before, themselves destroyed by degenerative changes, no longer furnish the impulse which is normally supplied to the former. Considered ^ith regard to the union Avhich exists between the elements of the nervous system , section of a bundle of nerves may, in certain conditions, bring about the three orders of degener- ation : Wallerien degeneration in the fibres separated from their cells ; ascending degeneration in fibres united to cells ; atrophic degeneration in elements which are deprived of impulses by the degeneration of the preceding. Remark. — By these experiments of merotoniy, we tluis are enabled to demonstrate the existence in the neuron of internal functions which establish a dependence between its parts. We are also able, in a certain degree, to locaUze the fiuictions, by proving that the portions of the neuron which have preserved their relations with the nucleus of the nerve cell are capable of survival, the nucleus being appar- ently an organ which is essential for nutrition to be main- tained. But we know nothing of the reason of this, the intimate detail of these relations being imdetermined. And this point is ec^ually unknown to us as concerns every kind of cell as well as the nerve cell. 4. Regeneration. — When the nerve cell escapes destruction (which is the rule in ordinary conditions), it undertakes the reconstitution of the segment of the degenerated fibre. The extremity, or the distal end, swells, Fig. 21. — Regen- eration of nerve fibres, several montlis after sec- tion (after Ran- vier). From the cut ex- tremity of the old nerve fibre one, and often several, new smaller fibres bud out. 38 ELEMENTARY NERVOUS FUNCTIONS sometimes even divides, and in this way supplies one or several growing points, from which axiscyhnders take their rise, these latter re-establishing more or less completely the former connexions. When these fibres have again found their way, either within or between the empty sheath of Schwann, regeneration follows its regular course. Vanlair considers that the rate of growth is about a millimetre a day. The degenerated fibres whose connexions arc re-estabhshed again assume their functions, and thus their local excitabiUty. Conductivity re-appears before excitabihty (Duchenne, Erb, Ziemsenn, Weiss). Nerve suture. — Hence suture of the two ends of a cut nerve favours the speedy regeneration of the cut and degenerated segment. In certain cases the reappearance of the functions of the injured nerve has been so rapid that it has been thought that immediate reunion of the cut fibres has ensued (Schiff, Herzen). It is clear that it is not possible to directly negative the feasibility of such a reunion. But, apart from the fact that it would remain an exceptional case, it is advisable to be suspicious of supplementary phenomena of all sorts which are capable of restoring more or less completely disturbed or vanished nerve functions. The re-establishment of continuity of the nerve should be affirmed only when direct proofs thereof are available : such would be the stimulation of a previously cut nerve, which has been then isolated for a certain portion Of its length and stimulated above the suture, and in the case when this stimulation should produce its ordinary effects before the delay necessary for the regeneration of the cut fibres. Non-regeneration of nervous cells which have been destroyed. — Duval and Laborde, Brown-Se.quard, Vitzou have concluded, from their experiments, that regeneration of the nerve centres after partial ablation of certain portions of the same is possible. Juliana, Magini, Schiefferdecker, Cohen, and many others, have protested against these results, and hold with Bizzozero that the nervous tissue is one ivhose elements are persistent, incapable of multiplication and of renewal after destruction. A tendency to regeneration is alone observed ; cary- ocinesis does not extend to a division of the protoplasm. The tissue which supports the structure, whose power of multiplication is very considerable, attacks the nerve cell and fills up the lacunae left by its destruction (Marinesco). Suture between centrifugal nerves of different functions. — Calugareanu and V. Henri, after having divided the hypoglossal and lingual nerves, have sutvu'ed the central end of the first with the peripheral end of the second. After the lapse of some months they have noticed an exaggerated function of salivation on the part of the sub-niaxillary gland of the corresponding side every time that the animal masticated, as if the motor impulses proceeding from the nucleus of the THE NERVOUS ELEMENT 39 hypoglossal, instead of going to the tongue, proceeded to the sub-maxillaiy gland. Having exposed the hj'poglossal nerve above its point of union with the lingvial, they have ascertained that its stinuUation causes secretion in the corresponding gland of a large quantity of saliva which was capable of trans- forming starch into sugar. The lingual nerve contains not only centripetal, but also centrifugal fibres, wliich come to it from the chorda tj^mpani nerve, and certain of wliich go to the sub-maxillary gland. The fact discovered by these authors is explained, not by assmning that the fibres of tlie hypoglossal have become welded with those of the chorda tympani, but that these fibres (at the point of departvu'e of its central end which has been imited to tlie lingual) have swollen out and occu- pied the empty sheaths of the degenerate chorda tympani. and have come into contact, through their new terminations, with the cells of the sub-maxillarj' gland wliich, by their irritation, are excited to functional activity. The motor impulses of the hypoglossal nerve (when the animal eats) become, through this change of route, secretory impulses. The fact is interesting because it proves that a nerve usually inotor in function may become a secretory nerve. The stimulus which the first conimunicates to the muscle should be of the same natiu'e as that which the second supplies to the gland, since the one may in a functional sense take the place of the other. In other words, the process (whose intimate nature is unkno^\^l) of the stimu- lation of organs by their nerves would appear to be the same in every tissue. Langley has also observed that the central end of the vagus nerve may, when it is united to the peripheral end of the sympathetic, give rise to jDhenomena of the same natxire. Stimulation of the vagus above the reunion of the two nerves produces the ordinary effect of that of the sympathetic. 2. External connecting functions or nervous functions, properly so called. — The neuron, Avhen once its personal existence is assured, assumes, like every cell in the organism, a special function in virtue of Avhich it is differentiated. We have already said that this function is distin- guished from others inasmuch as it is not, properly speakitig, a function initiating energy, but a junction ivhich directs energies developed by other cells. The methods by which this function is exercised are almost unknown to us, but the aim of the function we are acquamted with. In the nerve something is transmitted in a definite direction ; the nature of this something we are ignorant of ; on the contrar^^ its objective is clear : it is to cause the organs to pass from a state of repose to one of activity ; in present-day language, to set free their tensions, to expend their potential energies. To this something we apply the name excita- tion, an expression which recalls its end, but says nothing as to its modality, and which, for this reason, has no equivalent in physics, where the end of phenomena is not taken into consideration. The nerve receives impidses at one of its extremities and transmits them at the other. Internal and external acts, properties and functions. — The external manifestations considered apart, by which the cellular elements of the 40 ELEMENTARY NERVOUS FUNCTIONS organism manifest their functions, proceed from internal actions of these elements, which are sometimes called their 'properties. For example : the movement which is amplified by the osseous levers, and which manifests the functional motricity of the muscles, proceeds from an act internal to the muscle, contraction, or, if it is thought pre- ferable, from a specific property of the muscular elements, contractility . Hence, in order to characterize the dynamic aspect of the muscle, we can refer equally to its property or to its function, because we are well acquainted with both, and the relations of both are perfectly clear. When we wish to specify the nerve in actii, we are much more em- barrassed, and the expressions " animal spirits," " nervous influx," " neurility," etc., which have been successively applied in physiological language, without one indicating any superiority over the other, suffi- ciently prove this. Contractility is the expression of a definite change in the form of the muscles, a change towards which all the special acts of muscular tissue converge. Neurility merely expresses the idea of a change localized in the nerve, a change which we know must exist, but which it is impossible for us to define. As we are incapable of defining the internal nervous act, it remains to consider the function of this act, that is to say, its utilization apart from the nerve element itself , in order to give it an appropriate name by which it may be recognized as often as we shall have occasion to speak of it. This function is only knoAvai to us from a wholly general point of view, but, in this sense, this knowledge is of extreme importance. It is the function which we call stitnulation. From a functional point of view, the currents which pass through the nervous system in such a diversified manner are currents of excita- tion. It is the essential function of the nervous system to be htimu- lated, and to transmit this stimulus to the organs. This function, then, is that of each of its component elements which are excited successively or contemporaneously, and which finally give rise to the stimulation of organs which make use of energy under all its forms. 1. Numerical connexion of the exciting energy with the energy furnished by muscle. — The notion that the energy given out by the muscular tissue is furnished to this tissue by the nervous system is so widespread that it will be useful to refute it by an experimental fact. Let the gastrocnemius muscle of a frog, maintaining its connexion with its motor nerve (sciatic), be prepared ; and let an artificial stimulus of electrical nature be applied to this nerve ; in this way a definite quantity of energy will be expended ; as the result of this stimulation THE NERVOUS ELEMENT 41 the gastrocnemius muscle executes work which is also definitely known. In this way we can compare the quantity of energy which our exciting apparatus supplies to the nerve with that which our myographic apparatus receives from the muscle. Energy supplied. — The energy supplied to the nerve by a condenser is less than 0001 Erg (millieme Erg). Recujterated energy. — The energy expended by the muscles to raise a weight of 200 grammes 0-5 of a centimetre eciuals 100 grammes- centimetres, which makes 100,000 Ei'gs (one hundred thousand Ergs)- According to Weiss, who has worked out the elements of this calcula- tion, the ratio of work produced to work expended is 100.000 ^ ,00,000.000 0001 The recuperated energy is 100,000,000 times greater than the energy supplied to the small system which is experimented on. When it is remembered that the mechanical work of the muscle represents merely a fraction of its total energy, a considerable portion of which is carried off as heat, it will be obvious that the preceding figure is still much below the reality, and hence that the ratio sought for must be still greater. When, further, it is borne in mind that electric stimulation, however powerful it may be, produces a result which is probably inferior to that due to the stimulation of the nerve by a muscle, or of one nerve by another, it is clear that, as regards the total energies expended by the organism, that of the nerves is quite negligible, without ceasing, however, to be real. Artificial stimuli, compared amongst themselves, yield also strik- ingly different results. According to Tigerstedt, when mechanical stimulation is made use of, the ratio of the resulting work to the force of the excitation is about 320 ; a result which is very weak when compared with that of electrical stimulation. Conclusion. — The idea (for a long time indeed questioned by physi- ologists, but still admitted without discussion by physicians) must be abandoned, that the nervous system is a route for the conduction of power in the organism ; as a matter of fact it is a pathway followed by an infinitesimal portion of the external energies, which is utilized for the purpose of organizing the emplo^anent of force, and which we call exciting energy, to distinguish it from the efficient energies which we take in with the ingesta, which, circulating in the vessels with the blood, are stored up as reserves in the tissues, and leave the body with the excreta. 42 ELEMENTARY NERVOUS FUNCTIONS 2. Excitability and conductivity. — By one of its poles the neuron receives stimuli ; by the other it transmits them to that which follows- it. There is clearly a transjDort of these impulses in the space between, the two poles. These three phenomena, reception, conduction and transmissio7i or emission, which take their origin and play their part in the intimate structure of the nervous element from its dendrites to its intra-muscular terminals in passing through the axis cylinder, obviously suggest to us that they are dependent on the movements of the nervous substance. These movements are invisible ; they cannot be detected even with the highest microscopic powers ; doubtless the}^ are molecular, but this should not be considered as an invincible obstacle to our acquiring a knowledge of them. In the present state of science our information concerning them is too insufficient for us to form even an imperfect idea on the subject. Initial shock. — The initial receptive phenomena which communicate the shock to the dendrites of the neuron are unknown to us ; the pro- gression of this shock in the axon is also unknown, as is the manner by which it leaves the terminations of the axon in order to reach the element (nervous, muscular, glandular or other) which follows it. We have, however, reason to believe that all these phenomena closely resemble each other, whether taking place in the different neurons compared amongst themselves, or whether in the extremities and the continuity of the same neuron. The infinite variety of nervous actions depends less on the individual variety of the elements composing the nervous system than on that of the connexions and the relationships which exist amongst these elements themselves. In this connexion, that which passes at the origin and at the termina- tion of the nervous system, taken in its entirety, should not mislead us. It is not luminous waves which pass through the optic nerve, nor sonorous waves through the acoustic nerve, nor a mechanical pressure through the nerves of touch. All of these have been originally trans- formed by special apparatus (the organs of sense) which have gathered them together under that modality which we regard as uniform, and which, for the sake of convenience, we call the nerve wave. The same reasoning applies to what passes at the other end of the nervous system ; the cells, fulfilling such varied functions, which receive this wave must individually possess some sort of apparatus which adapts the uniform impulse, coming from the different nerves, to the special structure and function of each of them. The molecular and unstable equilibrium which the nerves possess the power of destroy- ing in it probably requires to be attacked in a slightly different manner according to the special conditions of each case. THE NERVOUS ELEMENT 43 Specific and general excitants. — A specific excitant h that which only acts on a certain class of element which is well defined : for example, light in the case of the retina, sound in that of the internal ear. General excitants are those which act indifferently on every living element and every organized substance. Every destructive action, such as a prick, traumatism, burn, chemi- cal change, etc., sets up reaction in the substance which it tends to destroy ; there is a general cause of excitation with which it is useful to be acquainted, but which is of small value as a method of study. Electricity in the form of currents, abruptly penetrating the tissues (or even by its distant effects, when the electric perturbation is very strong) is, on the contrary, a very good excitant. This excitant is general, that is to say, adapted to arouse all the cell activities. Further, its destructive action may be rendered practically nil when the condi- tions of its employment are properly chosen. ^"-'^-^K Fig. 22. — Comparative stimulation of a muscle and its nerve, the resulting contraction being recorded on a rotating cylinder. Z, electric element ; K, rheotome or interrupter ; I, primary coil ; II, secondary coil ; MX, commutator carrying the stimulation at will to the muscle by wires connected witli its two extremities, and to the nerve by wires connected with a i^ortion of its length (borrowed from Waller). Electric stimulation has been the means of causing great progress in nervous physiology, because this procedure has rendered the analysis of the different systematic and cellular functions much easier ; the little that is known concerning the analysis of the nerve itself is chiefly due to it. Evidences and estimation of nerve activity. — We say that the nerve is excitable ; we notice on the other hand that no alteration is visible in it, no apparent movement ; what proof then have we of its activity if the latter consists of purely molecular movements ? In order to demonstrate and study this question, there are two methods available, both indirect. (1) We notice the repercussion of its activity on that of elements 44 ELEMENTARY NERVOUS FUNCTIONS capable of changes of form (the muscles especially) to which it com- municates its excited condition. The molecular movement of the nerve arouses a movement, also molecular, in the muscle, which move- ment is transformed in the latter into a mechanical one, into work, so much the easier to calculate as it is considerably greater than that of the nerve, although being (as is supposed, at least) proportional to it. The 7nechanical ivork of the muscle demonstrates and measures the molecular work of the nerve. ^ (2) We render visible and measure, with the galvanometer or the electrometer, the electric motor force which its stated activity pro- duces in the nerve, and which is commonly known as its iiegative varia- tion or its electric current of activity. These two methods have each their advantages and their drawbacks. So long as purely motor nerves are the subject of investigation, the first is without doubt the most convenient and accurate ; but when the point of departure of the stimulus is separated from the muscles by a certain number of relays of grey matter, the transformations which these latter produce as regards the impulse which passes through them clearly hinder the muscles from indicating in an accurate manner, as regards extent and succession, the conditions of activity of the stimulated nerve. It is, then, these transformations themselves that the muscles reproduce by their contractions. It is thus only the nerves in direct connexion with the muscles which will be of use in studying the conditions of nervous activity, and the way in which it conducts itself with regard to external shocks. But we, in principle, admit that all neurons, in nearly the same degree, respond in the same manner to stimulation. In applying this principle, the general laws concerning the stimulation of nerves have been deduced more especially from investigations made on those nerves which are directly motor in function. The second method is applicable to every nerve, to every nerve segment, even when isolated and detached from the animal while it is still living. It requires, indeed, that the nerve be divided at one extremity, in order to receive the derivation of the electric current which is to be estimated. It is delicate in application, and it is, for want of a better, in many cases by no means a despicable method of analysis. 3. Isolated Conduction. — It is allowed, and can be demonstrated, that the different fibres of the sayne nervous trunk do not communicate from ^ Tlie word worlc is here used in the sense of " expenditure of energy," according to the definition of M. Chauveau, which is accepted by many physiologists. THE NERVOUS ELEMENT 45 one to the other any of the impulses which separately pass through them. The only points by which the neurons transmit the impulse are situated at their initial or terminal extremities in the grey matter of the brain, of the spinal cord, or of the ganglia. The necessity of such an isolated conduction, in order that the functions may be properly carried out, is easily understood. If this were not so, nervous functions would be impossible. A difficulty arises in reconciling this law with that which has just been referred to, of the excitability of the fibres on their course, and of the facility with which the impulses may penetrate them. This difficulty is overcome when it is noticed that this faciUty is altogether relative. It is sufficiently marked for us to be able, hy pressure or electric currents, to arrive at the nerve trunks through the skin, and to arouse them to activity. But it is great enough not to permit mechanical or electrical actions, which are far Aveaker, and which result from the change of form in the muscles, or of their special current, to act efficiently on them. Further, it must be borne in mind, that these stimuli are artificial, and that we have no right to compare them (as regards efficiency) with the specific excitations of unknown nature (but probably neither mechanical nor electrical) which normally penetrate the nerve and are propagated through it. Nervous induction. — When two bodies or two systems of similar bodies are in presence of each other and when the plaj^ of forces in the one arouses in the other actions similar or analogous to those which exist in the first, it is said that induc- tion occurs. A very clear example of this is furnished to us by the studj" of electricity, by the so-called electric induction and that known as viagnetic. The word has in these two cases a precise significance, because the conditions of induction are rigorously defined in it. In the study of Ught, phosphorescence and fluorescence are sometimes assimilated to phenomena of the same order, although the conditions which give rise to them are far less accurately kno^vTi. When, by stimulation of a sensory nerve, we arouse the activity of one or several nerves (motor or other) which follow it, we may, for our part, see in this re-echo one after another of the same phenomenon through separate elements, an example of induction, but it must be distinctly tmderstood that the word " induction " has here (as all those expressions taken from jihysics — conduction, reflection, polarization, interference, etc.) merely a comparative value, and in no sense an explanatory one. There may therefore be a nervous induction, as there is a nervous conduction, without the electrical nature of the phenomenon in the two cases being in any way implied. Or, to speak more acciirately, we may prove that in the nerve a certain electric conductivity and certain electric or magnetic phenomena of induction exist, without prejudging the real nature of the mechanism of the stimulation of one nerve by another. We are, so to say, assured in advance that the conditions of the physiological phenomenon are not reducible to one of those elementary phenomena of energj' which physics demon- strates in a state of isolation in its schematic experiments. So far as concerns magnetic induction, it must be here once again remarked that special precautions have been taken in order that it may not be produced 46 ELEMENTARY NERVOUS FUNCTIONS by the action of one nerve fibre on another throughout the length of the axons of varying function, which are placed joarallel to one another in the same nerve trunk. The activity of one nerve fibre never involves that of its neighbour. It only involves it so far as it is placed in relation with it, in the interior of the nuclei of the grey matter, or of the ganglia. It is this fact, clearly demonstrated, which is expressed by the law of isolated conduction. Throughout the course of the fibres, not only is conduction (electrical or not) isolated, but induction (electrical or not) does not exist. And it was necessary that this should be so in order that the nervous system might discharge its functions of direction, of rearrangement of the impulse which are both variable and complicated, throvigh the tissues of the organism. The nerve fibre, the axon, is a closed systena analogous to that of our tele- phones. If a point exists by which this system may lose its definite lines of force, so that these may act on lines of force of similar systems, it can only be at its extremities, at its receptive and emissive poles. The structiu'e, still imperfectly known, of these delicate parts shows us nothing which can support the idea of the occurrence of a phenomenon of this kind. The experiments which would be adapted to demonstrate this phenomenon are absolutely impossible of per- formance. The properties of the bodies and of their surroundings, the nature, the origin and transformation of energy, are in the special case wholly unknown to us. The nerves transmit an impulse with the greatest facility in directions with which we are acquainted ; we are wholly ignorant of the manner in which they transmit it. 4. Propagation in the two directions. — In the normal performance of its functions, the neuron receives the impulse exclusively by one of its poles, which is invariably the same ; hence it follows that it trans- mits the impulse invariably in the same direction, that is to say, for example, from the spinal cord to the muscles in the case of the motor nerves ; from the skin to the spinal cord in that of the sensory nerves. When we stimulate the nerve artificially in its normal position, we have just seen that, starting from the stimulated point, the impulse proceeds towards its habitual destination. But is it possible that at the same time from thence it may be transmitted in the opposite direc- tion, that is to say, contrary to its normal course ? The question has no great practical interest, since it involves abnormal conditions ; but it has an importance as concerns the general properties of nerve substance. In order to settle the question, it would be necessary to turn the neuron round by inverting its two poles, the receptive end becoming the distributive end, the distributive the receptive, and then to see if the impulses (normal or artificial) are still transmitted in this new position. This total inversion being impossible, it would be necessary, after having removed a segment of the nerve trunk between two sec- tions, that we bring this inversion about, by welding together the two ends and then observing what happens in this new position. This experiment has been attempted by several authors, especially THE NERVOUS ELEMENT 47 by P. Bert, who employed different methods, chiefly on rats' tails, and in his hands it appears to have settled the question. Yet, on considering it more closely, it seems that the objection may be urged that the segment, in this way cut and reversed (even when the opera- tion has been performed at several sittings), degenerates and is replaced functionally by new fibres having the same orientation as the old ones, which obviously deprives the experiment of demonstrative value. The laws of degeneration being known, it would seem that the attempt to give to nerves connexions other than those which they acquire by their development must be given up. Yet a means exists by which we are able to observe the state of activity of a nerve in the extremity in which we believe that the impulse may be propagated. This method is that of the negative variation of the electric currents of nerves. If it were possible to deal with a nerve trunk either exclusively sensory or exclusively motor (not containing these fibres mixed together in any proportion), this method would settle the question. Unfortunately such nerves do not exist, at all events in the absolute manner required, so that the question must still remain undecided. Without doubt, the nervous system contains an arrangement by which the passage of impulses in a definite direction is assured, just as does the circulatory system. Observation proves this, the evidence being the same in the two svstems. In the second case the controlling: apparatus is known to us : it is the arrangement of valves by which the cavities of the heart are separated the one from the other. In the first case neither analysis nor our intelligence has been able to unravel the problem. But it may be that there is some remote analogy with the arrangement which holds in the vascular system ; it would be, according to this h^^pothesis, localized in the points of union of the neurons. Between these points the impulse would be able to travel in either direction, but these points once passed, it would be impossible for it to retrogress. 5. Integrity of structure. — If a nerve is cut in its course, it ceases to transmit the impulses beyond the point of section. Indeed, if its two ends are adjusted as exactly as possible, conduction is not effected. It depends on a special structure which has been destroyed in the axon at a certain point of its course. Nerve conduction is indeed a mole- cular process ; it is rendered impossible by a local derangement, even a very limited one, or an alteration in the arrangement in the molecules of the nerve structure. Welding of nerves.— Schiff and Herzen have maintained, and surgeons also, that the two ends of a freshly cut nerve, pared and sutured, can unite by first intention, degeneration being hindered, and in this way 48 ELEMENTARY NERVOUS FUNCTIONS the loss of function may be prevented. On the other hand, Ranvier, Vanlair, and with them the majority of physiologists, consider that degeneration is an inevitable consequence of section. 6. Local excitability of the different portions of the neuron. — In the normal condition the nerve transmits impulses from one of its poles to the other. Artificially, it can receive others in its course, which com- port themselves like the preceding. If it is cut across its length, the segment attached to its emissive or distributing pole ceases to receive the current from the receptive pole, and consequently to distribute it to the elements, nervous or other- wise, in which it terminates. This isolated segment nevertheless preserves its excitability until it degenerates, that is to say, during two, three, four days or longer, according to the animal. This tem]3orary conservation of excitability in the axon separated from its cell has an important signification. The axon (like all the other prolongations) depends on the cell for its nutrition, for the conservation of its internal organiza- tion. It does not depend upon it immediately for that which we know as its function. Isolated from it, it yet comports itself as an entire neuron. It is capable of receiving im])ulses and of transmitting them to its extremity. This is precisely the role of the nervous system in the organism, namely : to transmit an impulse from one point to another. The body of the cell of the neuron is an organ necessary for the organization and conservation of the latter, but it takes no necessary and direct part in its power of functional activity properly so called. This is a consequence of tlie general excitability of living matter, but the re- actional manifestation is here renaarkably clear. Along the whole length of the axon impulses may be made to penetrate the latter : contact, light pressure, extremely weak electric cvirrents equally sviffice to render its ordinary action manifest. Hence between the receptive jDole and the rest of the neuron there is no essential difference of properties. Excitability and conductivity. — Excitability is the property which the nerve possesses of reacting to the stimulus which it receives, not only from its recep- tive pole, but throughout its course. Conductivity is the property of transmit- ting throughout its length away to its terminal extremity the condition of excita- tion which it has received. An attempt has been made to ascertain experi- mentally if there is not here fundamentally one and the same property, the con- duction resulting from parts of the nerve being successively excited the one by the other ; as if the energy liberated at each point acted on the following point by causing the latter to give off its intrinsic energy, and so on. These experi- ments aimed at subjecting the nerve to different influences, in order to see if the two phenomena present parallel or divergent or even contrary variations. It may be said that, as a general rule, the influences which modify local excit- ability modify little or not at all the transmission of the impulse through the part of the nerve thus infivienced. Example : a small portion of a nerve being submitted to the action of CO 2, comparative stimulations are applied to the part affected and above the latter (with regard to the direction of the trans- mission) ; the local excitability is much diminished, while the stimuli applied above are as powerful as ever (Grunhagen). It is the same as regards variations of temperature, local or general (G. Weiss). When a nerve degenerates (after crushing) the voluntary impulses again become efficient, while the nerve is still locally incapable of stimulation by electricity (Erb). THE NERVOUS ELEMENT 49 In the reasoning which is at tlie foundation of these comparisons it is assmned that, as regards the hypothesis of conductivity and excitabihty being one and the same thing, the artificial excitant (electricity) and the natm-al excitant (physio- logical stimulation of one segment by another) possess the same efficacy. Yet, on tlie other hand, there are reasons for believing that the artificial excitant is less active, and that, if the excitability diminishes, the final advantage must remain with the natural excitant. The strongest argument in favour of the dissociation of the two properties lies in the fact that the temperature does not modify the conduction, and that the latter remains equal to itself in the whole course of the nerve ; it is only in these two points that there is still a variance between experimenters. 7. Rate of conduction. — The impulses which reach a nerve, at its origin, or in one of its points, traverse this nerve at a definite rate, which it has been possible to reckon. Helmholtz, who was the first to measure this rate in the nerves of the frog, has found it to be about 27 metres a second. Chauveau, who has studied it on warm-blooded animals, has found it to vary according to the nerves operated on, and according also to certain circumstances pecuUar to the subject or to the nerve itself. In the horse it is about 70 metres in the motor nerves of the larynx, and only about 8 metres in the motor nerves of the oesophagus. Method. — The rate (when it is uniform) is the distance traversed in a unit of time : t In the appHcation of this formula, e represents the length of the nerve from the point stimulated to the muscle ; it is calculated without difficulty in fractions of a metre, by direct measm-ement. t is the time which elapses between the stimulation of the nerve at a given point and the contraction of the muscle ; its determination is more difficult and requires special arrangements. The method most generally employed consists in inscribing, on a surface moving uniformly (a rotating cylinder) and at a given rate, the stimulation and the muscular movement on two parallel lines by the aid of styles which are exactly superposed, the one moved by tlie exciting electric current (electric signal), the other by the miTscle which contracts. The time which elapses between the two records is measured by the distance which separates them in the direction of the movement of the surface. This calculation is rendered very easy if, on a third line parallel to the two others, the movements of a tuning fork giving 100 or 200 oscillations a second are recorded. Length of propagation and latent period. — The calculation thus made may still be incorrect, because experiment has shown that, in its passage from nerve to muscle, and independently of its transmission through the nerve, the impulse imdergoes a retardation (or latent period), which it is necessary to deduct from the length of time inscribed on the registering cylinder. In order to overcome this difficulty two successive determinations of excitation are made on the nerve, by choosing two points of the latter sufficiently wide apart. If t be the diiration of the transmission of the impulse from the farthest part of the muscle, and n the dm-ation for the nearest ])oint ; t — f^ is the time occupied by the impulse in traversing the distance from the first of the points to the second ; as to the space traversed, it is the distance which separates the two points, and which is measured directly on the prepared and exposed nerve. P. E 50 ELEMENTARY NERVOUS FUNCTIONS In the first experiments of Helmholtz the values of t and of f^ were determined by making use of the method of Pouillet. Let a galvanometer be intercalated in a conducting circuit. If in this circuit a current of a given duration (fairly short) be made to pass, the deviation of the needle is pro2)ortional to the duration of the passage of the current. The exj^eri- ment is arranged in svich a way that the current which should pass through the galvanometer is made (that is to say, com- mences) at the exact moment of the stimulation of the nerve, and is broken (that is to say, ceases to joass) at the exact moment of contraction of the muscle. In other words, matters are so arranged that the stimulation makes this current, and the contraction which ensues breaks the con- tact which permits it to pass. Two ob- servations are made, by exciting the nerve at two points unequally distant from the muscle. In this way the two unequal values t and t^ are obtained, whose differ- ence represents the time sought, T. Chauveau, experimenting on the nerves Fig. 23- — Estimation of the rate of conduction in a nerve. Two contractions obtained by stimulat- ing two points of the nerve separated by a definite interval. The stimulation is made automatically by the rotating cylin- der at tlie same phase of its rotation. The delay of one of the contractions measures the time required by the impulse to pass over the space between the two points of the nerve successively. The duration of of the larynx of large mammals, which this delay is determined by a length which may be estimated by comparing it with other lengths representing hundredths of a second (waved line traced by the vibrations of a tuning fork). have, as is well known, great length (on account of their recurrent course), has combined graphic inscription of the mus- cular contraction with the indications furnished by the signal of Marcel Depretz put in action directly by the muscle, while another signal marked the moment of the excitation, and the vibrating style of a chronograph recorded the ^5-i^ of a second. Further, this author, instead of two observations, made three, by stimulating the nerve in three different i)oints. The comparison of the times with the space traversed leads him to conclude that the rate of propagation of the impulse is not uniform, but continually changes during its transmission through the nerve. C. STIMULATION OF NERVES Stimulation is a communication of an external movement to our own tissues. It must be added that this communication belongs to the order of shocks or of rupture of equilibrium ; the movement, once induced, continuing by the expenditure of a pre-existing potential. In this phenomenon, which is known as excitation, there is then a movement-cause (coming from the exterior) and a movement-effect (internal to the stimulated tissue). Usually this last greatly exceeds the first in magnitude ; this being so because it is divided into two effects, of which one is strictly the work of rupture or of shock (equal- ling the exciting work) ; while the other, secondarily evolved, is a measure of the liberated potential energy. Gradation of the effects. — In order that the effects of the stimulation may have free play, the exciting cause must attain a certain intensity, below which it is either inefficacious or insufficient. This degree of THE NERVOUS ELEMENT 51 energy corresponds to what is known as the 7ninimum excitation, or the threshold of excitation. If, starting from this degree of intensity, the force of the stimulation varies, two things may happen. In certain cases, as in the muscle of the heart, the effect is not increased. In many other cases there is a certain relationship between the intensity of the stimulation, or that of the shock, and that of the effect produced. The response of the stimulated tissue progressively increases, and attains a maximum which cannot be exceeded : this is the maximum excitation. At the precise moment at which it attains this maximum the excitation is optimum,. If the excitant still increases in intensity a contrary phenomenon, diminution, results : there is a decrease of the effect produced, which may proceed to its complete annihilation : this is the excitation known as pessimum. All tissues are excitable ; certain of them, like the nerve tissue, are so to a high degree, and are organized in such a way as to be capable of transmitting and communicating their stimulation to other tissues. A muscle supplied with its nerve (in practice a frog's foot whose nerve trunk has been isolated : galvanoscopic foot) is the most convenient subject for the study of stimulation and of its laws. Various stimuli. — Stimuli may be of very different natm-e. Some are natural : light for the retina, sound for the internal ear, the energy whose nature is un- known by which the motor nerve acts on the muscle . . . are of this order ; each of these, in the circmnstances peculiar to itself, reacts on an apparatus which, in the course of phylogenetic and ontogenetic development, becomes adapted to this stimulus rather than to any other. Others are artificial : mechanical shocks, electricity, chemical agents ... by attacking each one of the preced- ing organs they will elicit its special reaction, although, in the exercise of the functions of any one of the organs, these agents take no part whatever. The same facts may be expressed in other words if we say that, of these excitants, the first are specific, that is to say, display an adaptation of evolution, while the second are general, that is to say, manifest a common property, or a property coimnon to all protoplasm, that, namely, of reacting to external stimulation. Electrical stimulation. — Amongst the artificial excitants, electricity is that which is most convenient for the general study of the laws of excitation. Pres- sure or mechanical shock, even when very slight, may stimulate the motor or sensory nerves, and this means was much employed when experiments for the determination of nervous functions were first iindertaken. Certain weak chemi- cal substances, such as glycerine, chloride of sodixmi . . . may also act as stimulants ; but the rapidly destructive effects of these stimuli and the difficvdties which attend their graduated employment much limit their use. PRELIMINARY IDEAS It becomes more and more necessary for the physiologist and for the physician to be acquainted with the language of electrology and to be familiar with the exact signification of the ordinary terms made use of in this science, which is more and more appealed to as a means for the study of living beings. 52 ELEMENTARY NERVOUS FUNCTIONS A. Electrical Energy — Its Units Potential energy. — Metapliorieally speaking, potential energy may be defined as a difference of level. Two mill courses, two lakes, of which the upper levels are at a different height above the level of the sea, present a difference of poten- tial ; if they are united by a conducting pipe at their inferior level, a motor force of running water comes into action, being so much the stronger as the difference of level is greater. Two metallic isolated spheres, equal in size, imequally charged with electricity and united by a conductor, will, in the same way, give rise, in this conductor, to an electric cvirrent. Electro -motive force. — Electro-motive force thus originates in a difference of electrical potential between two given points on the conductor. The absolute value of this potential is not taken into account, any more than the absolute level of the jars in communication, but only the difference between two given values. In the case of reservoirs full of water, the zero potential would be the sea : for the water of lakes and of reservoirs of all kinds runs away into it until it is exhausted, if they are fm"nished with deep conducting pipes and if the water is not renewed. As regards bodies charged with electricity, the zero potential is the earth : every body electrically connected to the earth loses its potential if its charge is not renewed. Its unit : the volt. — The difference of potential between two points of a channel of water is measured in metres or fractions of a metre (reckoned verti- cally) ; the difference of potential between two points of an electric conductor is estimated in volts. The volt is the unit of electromotive force ; it is equal to the difference of potential existing betw^een the two poles of a Daniell's element. Unit of resistance : the ohm. — A force which is conveyed in canals, from the mere fact of its canalization, experiences a resistance wdiich transforms it par- tially into heat, and which economizes its expenditure to a greater or less extent. / For a given conductor (electric or not), the resistance will be so much the stronger as its length is greater and its section narrower. The unit of electric resistance is the oJmi ; practically it is equal to that of a wire of annealed copper 50 metres long by 1 millimetre in diameter. The converse of the resistance is known as the conductivity ; it is so much the greater as the condvictor is shorter and Fig. 24. — Resistance coil or rheostat, its section is larger. The resistance increases in proportion as Unit of Intensity : The Ampere. — The the keys 1, 2, 3, 4 are removed. intensity of a current of water is its yield (the quantity which flows in a unit of time) ; this yield is proportional to the difference of potential (difference of pressm-e, motive force) ; it is inversely proportional to the resistance, or directly proportional to what may be called the conductivity of the conduit. If there were no other available means of more easily measuring it, it could be expressed by a relation between these quantities. Law of Ohm. — The electric yield, or intensity of the current, is necessarily expressed by this same relation. Let I be the intensity, E the electromotive force and R the resistance ; then this equation arises : R If E is made equal to 1, that is to say an electromotive force about equal to a THE NERVOUS ELEMENT 53 Daniell's cell, and Rz:=l, that is to say a resistance equal to 1 ohm, the unit of intensity is determined, that is to say the ampere _ 1 volt , 1 = = 1 anipere. 1 ohm Quantity of electricity : coulomb. — In defining the intensity through the ratio of the electromotive force to the resistance, the time is left undetermined. If the time is multiplied by the intensity, we arrive at the quantity : Q = I x t, and if we assimie that the second is an tinit of time, the quantity will be the amount of electricity spent when an ampere is yielded in one second : this is the coulomb or ampere-second. In commerce the ampere-hour has been adopted. B. Different Electric Currents — Discharges — Ccrrents properly so CALLED Let there be two surfaces of a different potential ; when they are united by a conductor they give rise to a flux of electricity which is known as a current. This current may assume different forms and be of extremely different durations. Instantaneous currents : discharges. — If the surfaces carry charges which are opposed or unequal and which equalize themselves by traversing a simple recti- linear conductor, the fiux assunres an extremely rapid form, is, indeed, practically instantaneous ; this is the case with the discharges of static electricity. These currents may be usefully employed for the stimulation of nerves in conditions which are correctly known, when condensers of a known capacity are made use of, to which charges of a known intensity are communicated, and which may be directed through the nerve either at their entrance into the condenser or at their exit from the latter, in either direction. Continuous currents. — If the surfaces are in contact with the soiu-ce of energy (chemical, thermic, etc.) which renews the difference of potential as the latter becomes weaker, the flow is then continuous, as in batteries and accumulators. The renewal of the potential in proportion to its decline may also be effected by mechanical energy, as in Gramme's machine. Constant current. — A current is called constant when its value remains nearly the same during its passage. Polarization. — In certain batteries, of the type of tliat of Volta, for example, there arises, from the niere passage of the current in the interior of the battery, a contrary electromotive force which is due to the formation of opposing poles by the chemical reactions and the transport of the ions. The hydrogen, set at liberty by the action of the sulphuric acid on the zinc, is borne to the positive electrode, whose apparent conductivity it diminishes ; and, further, this hydrogen in proportion as it increases in quantity, tends more and more to reduce the zinc from the sulphate of zinc formed, whence arises the development of an opposed current contrary to tlie first, not so strong as the latter, but whose intensity continues to augment through the functional activity of the cell. Non-polarizable cells. — This inconvenience has been overcome by means of certain contrivances which afford, in principle, to the products formed during the reaction, an iinmediate point of accumulation by which they are hindered from being deposited on the material which is being destroyed, so that in this way polarization is suppressed by arresting the action contrary to that which gives rise to the current. Thus, in Daniell's cell, the hydrogen formed by the action of sulphviric acid on the zinc is taken up to form water (instead of reducing zinc from the sulphate of zinc which has been formed). Accumulators. — ^An accumulator is an apparatus constructed for the pm"pose of utilizing only the current of polarization, or secondary inverse cm'rent, which arises in cells after the passage of their special cm-rent, as also in the voltameter 54 ELEMENTARY NERVOUS FUNCTIONS after the passage of an external cm-rent. Thus it has been endeavoiu-ed by this apparatus (contrary to what is aimed at in cells) to make the polarizing modifica- tion as marked as possible. Accumulators are charged by causing them to be traversed by an external current for a certain time in one direction. When polarization is once produced, an inverse current of discliarge is received from it, which is remarkably constant, and which yields up, whether in quantity or in energy, the greater part of the electricity which it has accumulated. Like the condenser, the accmiiulator is a reservoir of energy. In the first this energy is electric, in the second it is chemical. In the first it undergoes no essen- tial transformation ; in the second this energy passes, dm-ing the charge, from the electric to the chemical state, and conversely during the discharge. Oscillatory current. — The cvirrent, without ceasing to be continuous, may present periodic augmentations and diminutions of its intensity ; it is then said to be osciUatorij. Alternating current. — In the electro-magnetic machines of the class of Pixii and of Clarke (rotation of a magnet before a current, or reciprocally), the current obtained presents not only periods of augmentation and of diminution, but is completely reversed at each half turn, unless by special contrivance this is corrected. In both cases it is oscillatory, but when it is not corrected it is alternating. Sinusoidal current. — If, fm-ther, the succession of values of the current follows the law which regulates the oscillation of a pendulum, the cm-rent is called sinusoidal. Diphasic, continuous and triphasic current. — In the electro-magnetic machines of former days the current, which is reversed at each half rotation, presents two phases at each complete rotation : it is diphasic. In Gramme's machine a special arrangement as regards the rolling of the wire on the electro- magnet permits the production of a continuous current. Other combinations of rolling allow the production of triphasic currents. C. Induction Between two electrified bodies attractions and repulsions arise which, when they are satisfied, lead to displacements of these bodies. Lines of force ; field of force. — These displacements follow certain lines between these bodies, which for this reason are called lines of force. The space traversed by the lines is known as the field of force. These lines indicate, by their number in a given space, the value of the field in this space. The field of force is said to be so much the larger as the lines in it are more closely compacted and more mmierous in a given space. They furrow the spaces comprised between electrified conductors, and consequently pass through the isolating bodies, which, for this reason, are known as dielectric. Electric induction. — When a conductor (for example, a metallic sphere) which is isolated is charged positively or negatively, it induces, by its lines of force, in a neighbouring conductor a quantity of electricity of opposite nature, which is exactly equal to its own : this is static induction, or electric induction properly so called, by the lines of electric force. Magnetic induction. — When a circuit conductor is traversed by a current, new lines of force are developed in the space which surrounds it, each of these forms a closed circuit and each is perpendicular to the lines of electric force or lines of flow of the current, and consequently perpendicular to the direction of the con- ductor which it envelops like a ring, and creates in the interior of the circuit and around it a field of magnetic force. If, in this field, another closed circuit conductor is arranged, so that the intensity of the cm-rent commences to differ in the primary circuit, the number THE NERVOUS ELEMENT 55 of the lines of force varies in the magnetic field ; this variation (positive or nega- tive), at the moment of its production, induces a current (inverse in the first case, direct in the second) in the secondary circuit. In the conditions which have just been described, it is the variable condition of the pi'imary current which gives rise to induction in the neighbouring circuit : during the permanent condition equilibrium once more prevails. Induction may be produced, but in a different fashion, by increasing or diminishing the distance which separates the circuits. In the one case, as in the other, induction ensues when tlie lines of magnetic force of the primary current change their number in the secondary cui^rent. Tlie number of lines of force which, from the primary circuit, enter into the secondary circuit is so much the gi*eater : (1) as the distance of the two circuits is shorter (these lines individually complete around the conducting wire, diverge more and more as they are removed from the jDlane in which the circvdt is con- tained) ; (2) as the planes in which the two circuits are contained are more Fig. •25.^^Induction apparatus of du Bois-Reymond (after Waller). I, primary coil ; II, secondary coil, which inay be approximated to or withdrawn from the former in order to vary the intensity of the induced current. a, situation of the key or interrupter CL for producing isolated discharges ; b, situation of key in short circuit ; c, ordinary arrangement for producing a series of tetanizing discharges d, another arrangement for modifying the relative intensities of the opening and closing discharges nearly parallel (wlien they are perpendicular to each other, no line of force passes from the fu'st to the second, and there is no induction). Solenoids. — A battery circuit is an elementary solenoid, that is to say, reduced to a single spiral turn ; like the solenoid, it is assimilable to a magnet, but to one extremely flattened out and having a north and a south pole situated, very near to each other, on the axis of the circuit on each side of the plane which contains it. Traversed by a ciu'rent and free to move, such a system (closed battery floating on water) would orientate itself as a magnet, directing its axis in the magnetic meridian. A solenoid is formed by an isolated conducting wire, rolled on itself a certain number of times ; the spiral tvu-ns form 56 ELEMENTARY NERVOUS FUNCTIONS a series of circular and spiral currents. A coil, such as that of an induction apparatus or of a galvanometer, is a solenoid. Magnet. — A magnet may be compared to a solenoid formed by molecular cvu-rents orientated like that of the spirals of a solenoid in a determinate manner ; it is provided with lines of force which, starting from one pole (the north pole for example), proceed to diverge in the magnetic field (siu-rounding space), bend more and more, and finally concentrate themselves at the opposed pole (south pole), then, returning in the opposite direction (from the south to the north pole) in the interior of the magnet, complete the circuit. A solenoid may replace a magnet, and reciprocally, in many circumstances. (a) Permanent Magnet. — A magnet which maintains its magnetization (except for slow loss) is called jDermanent. (b) Temporary Magnet. — The superiority of the solenoid arises from the fact that it is possible to suppress or to return to it at will its essential properties, by breaking or making the electric current which svapplies it : this is a temporary magnet. (c) Electro-magnet. — If in a solenoid, a rod of soft iron (or a bundle of soft iron wire) is placed, the lines of force are absorbed in large nmnbers by the metal, which is magnetically more permeable to these lines of force than is the surround- ing space ; during the passage of the current a very powerful field of force is created as regards the poles by the orientation of the molecular currents of the soft iron ; when the current is broken, this arrangement comes to an end, and the lines of force disappear. An electro-magnet is a temporary magnet, but more jDowerful than the ordinary solenoid. Magnetization and induction are two very different phenomena. In magneti- zation, there is simply a special arrangement of currents, which circulate, without any resistance, in the molecules of the steel (permanent magnet) or of the soft iron (temporary magnet) ; these molecules play the part of perfect conductors. In induction, at the instant of augmentation or of diminution of the lines of force in the magnetic field, there is the same impulse and movement of electricity in the secondary circuit ; but this movement, arising in an imperfect conductor, is soon arrested by the resistance of the circuit and ceases when the primary current has assumed its permanent condition or is itself arrested. D. Stimulating Apparatus The electric waves which are made use of for stimulation are furnished by different apparatus. Condenser. — If a nerve is electrically connected, on the one hand with a charged condenser, on the other with the earth, the floiv of the discharge i^roduces a wave capable of stimulating it. It may also be stimulated by placing it in the floiv of discharge. The discharge and the charge form two waves which are nearly equal in intensity and duration. There are different means of causing these two conditions to vary, but the shape of the wave is not well knowTi. The charge and the discharge depend on the resistance R, on the capacity C and on the self-induction L of the line. For R :^ ^ / — - the discharge is contmuous. \/ C For R < . / „ ,, „ oscillating. With ordinary wires which are not rolled up self-induction is a negligible quantity. Induction apparatus.— The induced current which takes origin in the secondary coil of the apparatus of du Bois-Reymond is very usually employed for the THE NERVOUS ELEMENT 57 purpose of electrical stimulation. The two induced currents, the one inverse on closing the circuit, the other direct on opening it, are equal in quantity, but inversely unequal in diu-ation and intensity. Self-induction considerably prolongs the fii'st, especially if a rod of soft iron is placed in the coil. The in- tensity is graduated (but imperfectly) by causing the distance of the two coils to vary on a graduated scale. The intensity of the induced current decreases as the square of this distance. Batteries ; accumulators. — Practically there is scarcely any means which is at the same time eas\- and correct of eliminating, in a continuous ciu'rent, waves of the character of those which will be described farther on in connexion with the law of excitation. The wave of making tlie current is made use of, and also that of cessation (breaking of cm-rent). The action of the second is gi'eatly complicated by the effects (electrolytic amongst others) which are due to the passage of the current. Rheotomes. — In order to make or break the current, different apparatas may Fig. 26. — Rheotome of Bernstein (application of the method of Guilleniin). A rod wliich carries a contact 1 closes periodically (by a rotatory movement of the axis which supports it) — a battery circuit inducing a cui'i-ent which stimulates the nerve. — Fui'ther, another contact 2 (in the shape of a bridge) closes, in a galvanometer, another circuit, in which the muscle is included, and allows of the determination of the electromotor phenomena which arise in the latter from the fact of stimulation. The contacts 1 and 2 may be mutually displaced so that (for a given rate of rotation) the interval of time which separates them may be increased or diminished. In this way the direction and the successive intensities of the electromotor phenomenon wliich is developed in the muscle in the period succeeding the stimulation may be studied. be made use of, oscillating pendulmns (Chauveau), contacts effected by a rotatory movement (Bernstein), which bring about these closings and openings of the circuit at equal intervals and at the same rate. E. Apparatus for the Estimation and the Measurement of Electromotive Phenomena Galvanometers and electrometers are employed, chiefly the electrometer of Lippmann. 58 ELEMENTARY NERVOUS FUNCTIONS Galvanometers. — Two cui'rents, two solenoids, two magnets, one solenoid and one magnet are capable of reacting the one upon the other. If one is fixed and the other movable, when the current passes through the solenoid displacements ensue which may be made use of to prove the existence of such a current, to determine its direction and to estimate its intensity. Electrometer of Lippmann. — This consists of a tube ending in a conical capillary point ; it is filled wdth mercury, and dipped in a solution of sulpliuric acid which itself rests on mercury ; when this ap- paratus is traversed by a current, the capillary constant is changed and the mercury is displaced in the conical point to a certain extent which varies according to the direction of the current. The amount of the displacement (within certain limits) indicates tlu- difference of the potential. Non-polarizable electrodes. — The contact of metallic wires with the tissues gives rise to polar- izations which set up contrary currents capable of seriously invalidating observations. These difficulties may be avoided by making use of non-metallic electrodes, which are almost incajDable of polarization. Fig. 27. — Diagram of an as- tatic galvanometer. The current brought to the two terminals t, t is conveyed by a wire which is rolled succes- sively and in contrary directions round two conjoined magnets sn and ns, forming a movable suspended arrangement. The upper magnet carries a mirror on which a luminous ray is re- flected, forming an image on a graduated scale — the displace- ment of the image measures the intensity of the current. N S, fixed magnet intended to give a position of repose, which can be varied at pleasure, to the two others. D. LAWS OF ELECTRIC STIMULATION 1. The problem to be resolved. — Elec- tricity acts as a stimulus on every excitable organ, especially on a nerve when it passes through it in the form of a current, whatever may be the nature of this latter. It only exerts its stimulating and impulsive effect so far as it is itself in movement. A current has of necessity a period of inauguration {a variation ivhich is more or less rapid in its intensity starting from zero), a phase of persistence in certain conditions (constancy of the cur- rent), a phase of cessation (of more or less rapid variation the converse of the first). It has at every instant a determinate intPMsity; it has a total duration; it represents a quantity of electricity supplied, and it represents an energy expended. Which of these factors takes part in the stimulation in an exclusive or preponderant manner ? Historical ; facts and opinions. — Tliese questions have offered themselves for solution from the very commencement of methodical studies concerning the effects of electricity on the nerves and other excitable tissues. The researches commenced by a work of du Bois-Reymond (1848). THE NERVOUS ELEMENT 59 Experiment. — A nerve is traversed by a cvu'rent ; at the making of the latter a contraction of the muscle ensues ; duing the passage, the organ remains in Fig. 28. — Reflecting Galvanometer with its scale for reading the deviations. A sliunt is interposed between the galvanometer and the soiu-ce of the current to be measured . repose ; at the breaking of the ciu'rent, there is once more a contraction. Another fact of the same kind : a nerve is included in the circuit of a battery (by the Fig. 29. — Capillary electrometer of Lippmann. On the left, the apparatus as a whole ; in the middle, diagram of the apparatus ; capillary- tube full of mercury phuiged into a solution of sulphuric acid resting on a layer of mercury. On the right, appearance of the capillary column at its extremitj^ seen in the microscopic field. 60 ELEMENTARY NERVOUS FUNCTIONS aid of a rlieocord) ; tlie intensity of the current is caused to vary slowly, so that its strength may be greatly increased : there is no stimulation ; but, if a sudden variation is caused in the intensity of the current, there is contraction. 2. Old formula. — From this du Bois-Reymond has concluded that it is not the absolute value of the intensity, but the variation of the intensity of the current ivhich causes the stimulation ; in other words : if two Fig. 30. — Xon-polarizable electrodes for receiving the currents of muscles or nerves. 1 and 2, those of du Bois-Reymond ; 3, those of Burdon-Sanderson ; 4, those of V. Fleischl ; 5, those of d'Arsonval. The metallic wire, contact of which with the living tissues must be avoided, is plunged into a tube filled with a solution of a salt of the same metal (zinc in sulphate of zinc, copper in sulphate of copper). The extremity of the tube is closed by a stopper of kaolin impregnated with a neutral solution of chloride of sodium and is of form suitable for application to the nerve or the muscle. identical nerves are traversed by two currents / and /i which differ considerably, and if these two different currents are made to increase or decrease by the same quantity, during the same time and in the same manner, the two nerves will be equally stimulated. The stimu- lation will be so much the greater as the variation dl in the unit of time is greater, that is to say, the variable period is more rapid, the wave shorter. Contradictory facts. — It was thought at first that the experimental fact on which this theory of stimulation is based was of general application. In reality this is not the case. In operating on other animals, it may be found wanting. Grutzner (1887) has observed that the motor nerve of the toad responds better to slow and prolonged variations of the current than to those which are rapid. Fick finds that, even in a frog, a muscular response is not elicited when the passage of the wave is of very short duration. Brucke finds that it is the same as regards THE NERVOUS ELEMENT 61 curarized nerves, so long as the poisoning lasts. Pflliger observes that nerves react, in certain conditions, even to a constant current. Hence it cannot be admitted that the variation of intensity is the sole and only factor of the stimu- lation. Cybulski and Zanietowski, as also Waller, consider that the important factor is the electrical energy. But Boudet (of Paris) had previously found that " the same ciuantity of electrical energy expended did not ahva\-s produce the same effect." Fick (1869) and Wertheim-Salomonson attribute a direct influence on the magnitude of the reaction to the quantity of electricitj^ but Avithout regarding it as the sole factor in stimulation. G. Weiss (1901) has re-investigated the subject by the aid of very exact methods, and he proves that the rnxantity of electricity made use of is the important factor in electrical stimulation. 3. Conditions which must be observed ; method. — A wave of a simple form must be capable of being projected into a nerve, and the duration, as also the intensity of this wave, must be exactly kno\^ n. Acquainted with the duration, as well as with the intensity, it will be possible, in the preliminary discussions, to determine the threshold of excitation, that is to say, the intensity which, for a given duration, is just sufficient to cause a contraction. Having thus determined the wave which gives the threshold of excitation, an abbreviation of this is effected in such a manner as to reduce somewhat its duration, thus forming two waves of a total duration which is less than that of a standard wave. Contraction no longer . results (or, the conditions being the same, in order to obtain it, the voltage must be increased). If the stimulation were due to the variation of the potential, conformably to the hypothesis of du Bois-Reymond, contraction should have taken place. If two waves of a total duration equal to that of the standard wave are projected into the nerve, contraction follows. Result. — Thus it is clearly seen that, in every case, it is the quantity of electricity which is here the important factor, and not the energy, for the latter may be equal for a lesser, or greater for an equal quantity, without any change in the results. Apparatus. — For the purpose of having at command an electric wave of a known intensitj^ and duration which has been accurately determined, the major- ity of the apparatus employed (condensers, induction coils, etc.) are worthless. G. Weiss has invented the following arrangement, which is altogether satisfac- tory. (a) Estimation of the Duration. — The nerve is interpolated in the circuit of a constant battery. This circuit contains a derivation of a relative resistance such that (as regards the wires going to the nerves), all being entirely closed, the nerve receives no appreciable quantity of electricity. If, then, the derivation is broken at a point, the ciu-rent passes into the nerve ; if the interruption occurs at a second point, this time in the circuit, the ctu'rent ceases to pass, the wave is finished. Its duration is measured by the time which elapses between the two 62 ELEMENTARY NERVOUS FUNCTIONS interruptions. These interruptions are effected by the ball of a carbine worked by liquid carbonic acid, whose rate is 130 metres a second (at the temperature of the laboratory). The interval between the two interruptions depends on the distance between the wires, the latter being placed parallel to one another in the plane of transit of the ball. For an interval of one centimetre, the duration of the wave will be O'<^^000077, etc. By making the arrangement more complicated, several successive waves may be obtained having definite durations, succeeding one another at definite intervals of time, so that tlie comparisons and controls indicated above may be effected. Such is the mode of estimating the duration of the wave. (b) Estimation of the Intensity. — To calculate the intensity, the voltameter is connected with the terminals of the battery or the distributer of the potential. This voltameter registers a figure proportional to the intensity of the current, since in the successive experiments the resistance of the circuit does not vary (E ^ IR ; E^ = I^ R ; EE^is proportional to //). If the value of current is not determined with absolute precision, yet it is so within very narrow limits. (c) Ratio of the Energy to the Quantity. — The energy of the current is expressed : Elt = EH X R. It is, as a constant factor, nearly W = E'-t. The quantity of electricity employed is Q = It = Et x R. It is, as a constant factor, nearly Q = Et. It is seen that the quantity of electricity varies as E, while the energy varies as its square, E ". For a given duration the quantity remains constant. — Thus the first stage of the argument is estabhshed. For a given yeriod of time, in order to obtain the threshold of excitation, the same quantity of electricity is required. If this necessary quantity is not supphed to the nerve, the minimum excitablHty is raised. This minimum is so much the more elevated as the frequency of the wave is greater, that is to say, shorter. When the frequency is sufficiently great, very high voltages may be employed without attaining the threshold of excitation. The threshold of excitation implies, for a given duration, a given ciuantity of electricity. Supposing that the duration varies, will this quantity remain fixed or will it vary, and if so, how ? Variation of the quantity as a function of the duration, — From the formtda Q = Et it is obvious that a quantity of electricity remains the same if its two factors, intensity and duration, undergo proportionally inverse modifications. Will the same threshold of excitation be maintained if t and E are modified in- versely, in such a manner as to preserve Q equal to itself ? Experience tells us that this is not so. In proportion as tlie duration t augments, the Cjuantity Q augments equally. This shows that the quantity Q represents the svim of two terms, the one fixed, which may be described as A, the other B, proportional to the duration of the excitation. Hence the quantity required to produce the excitation becomes : Q ^ A X Bt. In order to prove this, a series of initial stimuli are applied to the same nerve in identical conditions, the waves being of variable duration but not exceeding ^sec Q()23, while at the same time in each case the quantity of electricity is measured. The duration of the wave (time of stimulation) is represented by THE NERVOUS ELEMENT 63 abscissfB ; the quantity of electricity employed is expressed in ordinates. Thus is obtained a straight line which does not pass tlirough the origin. It represents the quantity of electricity which is required for the minimum excitation of the nerve expressed as a function of the duration of the stimulation. 4. Law of electrical stimulation. — The researches of G. Weiss ex- plain the paradoxical facts of Grutzner, of Brucke, and, further, thej^ lead to a general la\\' which is applicable at the same time to the more prompt reaction of the nerve of the frog. This is in accord with the results of Dubois (of Berne) and of Horweg, which were obtained in making use of the condenser. The law may be thus expressed : In order to obtain the initial response of a nerve or of a muscle, electrical stimulation must make use of a constant quantity of electricity ; and, further, this quantity must be jyroportional to the duration of the discharge. " Buttery Rheocord Commutator Bheotome Myograph Fig. 31.— Arrangement for the stimulation of nerves. Action of the constant current. According to this observer, " everything tends to prove that a con- stant cj^uantity of electricity is necessary in order to stimulate a nerve, but that it is also necessary, during the whole of the operation, to cease- lessly oppose a process of return to the first state by means of another quantity of electricity which is proportional to the duration of the action." 5. Methods of stimulation. — In order that the nerve may be stimu- lated by a current, matters are so arranged that the nerve forms part of the circuit traversed by this current, at least for a small portion of its length. G4 ELEMENTARY NERVOUS FUNCTIONS According to circumstances, two arrangements are made use of. In the one the isolated nerve is placed in contact with the two electrodes, which are situated at a certain distance the one from the other. The lines of passage of the current follow the direction of the nerve as in a regular cylinder ; the density of the current is the same at its entrance, at its exit and in the intervening space : this is the bipolar method. In the other, the nerve, usually superficial, but adherent to the mass of subjacent tissues, is in contact (often through the skin) with one of the two poles, the other pole being placed at some distance. The lines of transmission, if the positive pole is on the nerve, diverge Position of the myo'jrftph for ch'irge ~ and discharge Fig. 32. — Arrangement for the stimulation of nerves. Action of the charge and discharge of the condenser. from the point of contact in every direction and proceed to complete the current at the other pole ; or, if the negative pole is on the nerve, converge parallel to the nerve starting from the positive j3ole : this is the monopolar method. Polar influence. — This second arrangement, designed and recom- mended by Chauveau, shows in a very definite manner the different action of the two poles. Let it be assumed that the two electrodes are symmetrically placed on two similar nerves, as the two facial nerves in mammals or the sciatic nerve in the frog, and let the current be pro- jected alternately in the two directions. As the intensity of the current increases from zero, the first contraction will occur in connexion with the negative ])ole, whether it is on the right or on the left, while the muscle in contact with the positive pole will remain in repose. When the strength increases, starting from this point, a new threshold of excitation arises for the positive jjole ; then the contractio7is become equal ; later. THE NERVOUS ELEMENT 65 those at the positive pole become 'predominant, irhile those at the negative pole tend to diminish. Finally, according to Boudet (of Paris), if the intensity still increases, the contractions at the positive pole diminish in their turn, once again become equal to those at the negative pole, then diminish more rapidly than these last, and cease before them. The effects due to each of the two poles may be represented by two curves, the one more extended and more elliptical (negative pole), the other shorter and with a still higher summit (positive pole) which present two points of intersection. Active pole ; indifferent pole. — In electro-therapeutics, it is not 1 -> f. K ^^ ■'-^^ N X /^ P / 1 2 3 ' 4- 5 5 7 3 i 1 1 12 Fig. 34. — Inequality of the polar actions (after Boudet, of Paris). The effects of closing and opening are examined separately in two experiments. XI 'S-, negative pole ; Pi P2, positive pole. The electromotor force of the current increases from 1 to 24 volts. Fig. 33. — Inequality of the polar actions of the ciu-rent (after Chauveau). N N, curve representing the values • of the contractions of the negative pole on the making of the current. P P, curve of the contractions of the positive pole. Equality at the point of inter- section of the two cm-ves ; inequality of opposite nature before and after it. 1, 2 ... 12, suc- cessive intensities of the stimulating cm-rent. generally necessary to stimulate the two nerves symmetrically and alternately ; usually a single nerve is alone treated. Thus the pole whose action is rec(uired is placed on the nerve, and on another locality situated at some distance, and but little sensitive, is placed the other pole, which is known as indifferent, and which covers a large surface (moistened sponge). Cathode ; anode. — The negative pole is usually known as the cathode (in the case of a feeble current this is the most active), and anode describes the positive pole (in the case of a strong current this is the most active). Direction of the current. — In the bipolar method, the lines of trans- mission of the current are projected, according to the direction of the nerve, alternately in two opposite directions, that is to say, according to the direction of its conduction [descending QurverA) or in the opposite direction {ascending current). P. F C6 ELEMENTARY NERVOUS FUNCTIONS The effects differ in the two cases. Contrary to what might have been thought, the differences are not due to an influence attributable to the johysiological conduction of the nerve, but once again to a polar influence. It is seen, indeed, that the results are inverted, as above, according to the intensity of the current, and that they follow funda- mentally the same law as in monopolar excitation. It is sufficient, in order to recognize it, to remark that, in the case of bipolar stimulation with the descending current, the lines of flux have the same orientation as in monopolar excitation with the negative pole on the nerve ; and the same with the ascending current in the bipolar excitation, the same also with the positive pole in monopolar excitation. Fig. 35. — Diffusion of the electric current in a homogeneous conducting medium, diagram of "the points of application of the two poles. The current is completed by lines of flux of which one only is straight, the others are inflected to a greater or lesser extent, c, Ci, Cg, C3, and represent the derivations of the current, feeble in proportion as they are further removed from the direct course. These lines are cut by the equipotential lines (represented in strokes) +, +1, +2, +3; — — 1> — 2> — 3- These lines connect the points of the conducting medium having the same potential, the same positive tension to the right, negative to the left of a line of zero potential, which cuts perpen- dicularly the straight line uniting the two poles. Unipolar stimulation in an open circuit. — When the two poles of the stimulating circuit are placed on the same nerve, the stimulation is called bipolar. When a single pole is placed on the nerve, the other pole, covering a large surface, being placed on a remote region of the animal's body, then it is a case of monopolar or unipolar excitation, such as has been recommended by Chauveau, and is ordin- arily made use of in electro-therapeutics. Bvit a nerve may also be stimulated in a unipolar manner by a different method of procedure. For example, on the one hand the indifferent pole, on the other the body of the animal experi- THE NERVOUS ELEMENT 67 mented on, may be connected electrically with the earth. This method is closelj- related to the jDreceding ; it differs from it inasmuch as the earth is interposed in the circuit. Lastly, after having isolated both the animal and the distributer of energy, communication may be established through a single pole with a nerve of the animal, and in this waj^ again, motor effects may result. The efficacy of this kind of stimulation which, at the first glance, would appear to be nil, is, as a matter of fact, tolerably great. In this case, as in all the others, the stimulation is due to a movement of electricity in the tissue [the nerve) ivkich is connected tvith the active pole. Fig. 36. — Diagram showing the internal polarization of the tissues (after Waller). All along the lines of the flow of the current, going from one pole to the other, secondary polarities are developed across the heterogeneous portions, traversed by electrolytic conduction. When, indeed, an induction shock is produced, if the induced current is open a more or less large nmnber of electrical oscillations are produced, whose frequency dejiends on several conditions, and more especially on the capacity of the system (Schiller and Mouton). This frequency amounts to 10,000 a second. The cur- rents furnished by this miijDolar stimulation are then alternately positive and negative, or diphasic currents, persisting a longer or shorter time according to circumstances. A. ChariJentier has methodically studied the action of these special currents {Archives of Physiology, 1893-1896). Periodic excitations. — So far we have assmned a simple stimulation due to an isolated wave. But, in practice, when, instead of studying the laws of stimvila- tion, it is merely desired to make use of the latter in oi-der to demonstrate the fmictions of a given nerve, the excitation is prolonged and renewed in the nerve by the passage of a periodical series of similar waves which succeed one another inverse^. This effect is realized by means of the trembleur of du Bois-Rej^mond's apparatus. These cm-rents are called tetanizing, because the muscular contrac- tions which follow them in the muscle ai-e so close together that they vmite them- selves in prockicing a physiological tetanus by which the tension of the muscle is maintained. In man, the object of the stimulation is not the determination of function, but the forming of a diagnosis or the production of a therapevitic action on the nerve. In animals it is the reverse. In the first case, the monopolar method is the only one applicable, and is very convenient ; in the second the bipolar method is some- times preferable, becavise it permits of the stimulation being localized on a given nerve. Usually the nerve has been cut and is supported on the electrodes of the current by the aid of a white silk thread which is very dry ; in this way all danger is averted of the derivation of the cm-rent to excitable parts other than the nerve imder investigation. Remark. — Elsewhere we have assmned that the stimulus is applied to a nerve element or to a bundle of nerve elements of similar fmictions, which thus together form a simple object. In practice this is rarely the case. The nervous bimdle 68 ELEMENTARY NERVOUS FUNCTIONS which is stimvilated will frequently contain parallel elements which terminate in peripheral organs having distinct functions, which the excitation will to a certain extent dissociate by revealing their presence. 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II, p. 225. — Raxvier, Legons sur I'histologie du syst. ner- veux. — AVali.er, Svir la reproduction des nerfs et sur la structure et les fonctions des ganglions spinaux. Arch, de Midler, 1852, p. 597. Chromophile substance ; chromatolysis ; reaction at a distance. — Ballet et Faure, Atrophic gr. cell, pyram. zone mot. apres section des fibres de project, chez le chien, Semaine niedicale, 1899. — De.jerine, Chromatolyse dans infections av. hyperthermie, Biol., 1879. — Lugaro, Rivista di pathol. nervosa e mentale, vol. I, 1896. — Marinesco, Pathologie generale de la cell, nerv., Fresse medicalc, 1897 ; Recherch. sur hist. cell, nerv. consid. physiol., C R. Acad, sc, 12 avril 1898. — Nissl, Tagebl. der Naturforsch. zu Strassburg 1885 . . . zu Coin 1889 . . . zu Heidelberg 1890 ; Allgem. Zeitsch. f. Psychiat., 1893 ; Neurol. Centralbl., 1894, et suivants ; Centralhl. f. N ervenheilkunde, Jhg., 18. Ne. — Onufrowicz, Jovrn. of nerv. and ment. dis., 1895. — Pugnat, Modif. cell. nerv. fatigue, C. R. Acad, sc, 8 nov. 1897. — Ramon y Cajal, Revista a trimestrial microgra/ica, vol. I, fasc. I, 1896. — Ranvier, Traite technicjue d'histologie. — De Quer- vain, Arch, de Virchoiv, Bd. CXXXIII, 1895.— Schaffek, Neurol. CentroJbl., 1893. — Schwalbe, lenaische Zeitsch., Bd. X. — Van Geiiuchten, Bull. Acad. mid. de Belgique, 1897 ; Acad. roy. Belgique, 1898. Ascending degeneration. — Dejerine et Sotta.s, Biol., 1895. — Durante, Assoc, avanc. des sc, Bordeaux, 1895 ; Editions scient., Paris, 1895. — Klippel et Durante, Rev. de med., 1895. — G. Ballet, Congr. neurol., Nancy, 1896. — Lugaro, Rivista di path, nerv: et ment., vol. I, 1896. — Monakow, Congr. natur. all., Xurenberg, 1893. — Marinesco, Neurol. Centrcdbl., 1892 ; Presse medicale, dec. 1895 ; Biol., 1896.— Moubatow, Soc. neur. et psych., ^Moscou, 1893. Atrophy of the spinal cord after amputation. — Berg, These Paris, 1896. — Fried- lander et Kr.auze, Forscltr.d. Med., 1884, Xo 4. — Hayem et Gilbert, Arch, physiol. norm, et path., 1884, III, p. 430. — Marinesco, Neurol. Centralbl., 1892, p. 463. — Redlich Obs. svir cobayes, Centrcdbl. f. Nervenheilk., 1893. — Vanderveld et Hemptinne, Journ. med. Bruxelles, 1893.— H. Will, Arch. f. Psychiat., 1895, XXVII, p. 585. Nervous suture ; reappearance of the functions. — Bachowiecki, Arch, f. microsc. Anat., t. XIII, 1876, p. 420. — Calugareanu et Victor Henri, Regeneration fonction, nelle de la corde du tympan sutviree avec le bout central de I'hypoglosse, Biol., Janvier et decembre 1901. — Herzen, Doctrine de Schiff, Rev. scient., 1894. — Langley, Journ. of Physiol., 1899, vol. XXIV. — Laucjier, C. R. Acad, sc, 20 juin 1864. — Xelaton, Soc de chir., 22 juin 1864. — Schiff, Rec de mem. : Vanlair, Assoc, avanc. des sc, Toulouse, 1887 ; Arch. biol. de Van Beneden, 1893, t. XXIIl ; Rev. scient., 4 aout 1894 ; Ann. Soc. medico-chirurg. de Liege, 1895. Regeneration. — Brown-Sequard, Biol., 1882. — JVIarinesco, Biol., 1894, p. 389, et nov. 1896. — PiiiLiPEAUx, Retour des fonctions du vague en trente jours, Biol., 1876. Conduction in two directions. — Ajibrosoli, Schmidt's Jahrh. 1860, p. 289. — Arloing et Tripier, Arch. f. Physiol., 1876. — Babuchin, Arch. f. Physiol., Leipzig, 1877 ; Arch, f. Anat. unci Phys., 1877. — P. Bert, Greffe animale, Biol. ; Soc. philotn. ; These de mede- cine, Paris ; Journ. anat. et physiol. ; Ann. sc. nat. ; Soc. des sc. de Bordeaux, 1863 a 1865 ; Transmission dans les nerfs de sensibilite, C. R. Acad, sc, 1877. — Bidder, . . . lingual . . . hypoglosse . . ., Arch, de Reichert et du. Bois-Reymond, 1865, p. 246. — Floubens, Svst. nerveux. — Gotch et Horsley, Philos. Trans., London, 1891, vol. CLXXXIl, p!! 485.— Helmholtz, Monatsb. d. Berl. Acad., 1894, p. 328.— VV. Kuhne, Monatsb. der Kon. Akad. der Wissensch. zu Berlin, 1859, p. 400 : Zeitsch. f. Biol., Bd. XXII, p. 305 ; Arch. f. Anat. und Physiol., 1859. — Langley, Journ. of I'hysicl., 1899, vol. XXIV. — Philipeaux et Vulptan, Journ. de la physiol. de Vhomnte et des anim. et C. R. Acad, sc, 1863. — Rosenthal, Centralbl. f. d. med. Wiss., 1864, p. 449. — Thier- NESSE et Gluc;e, Bullet. Acad. roy. Belgiqtie, t. VII, Xo. 7. — Vulpi.\n, C. R. Acad, sc, 1873, t. LXXVf, p. 146 ; Arch, physiol., 1876, p. 597 ; Leyons sur la physiol. gen. et com- paree du syst. nerveux. Direction of the transmission : part played by the dendrites. — Mislawskv, Biol., 1895, p. 489. Rate of transmission of the impulses. — Bernstein, Centrabl. f. d. med. ]Viss. 1866; Arch., ftd. ges. Physiol., 1868; Unters. und d. Erregimgsvorgang in Xerven u. IMuskeln. 1891. — Du Bois-Reymond, Jahresb. d. phys. Ges. zu Berlin, II, 1845. — Chauveau, C. R. Acad, sc, t. LXXXVIl, 1878, pp. 95, 138, 238.— Grigorescu, Biol, 1891 ; Biol, THE NERVOUS ELEMENT 71 1892, p. 634 ; Arch, phijsiol., 1894 ; Helmholtz, Monatsb. d. Berl. Acad., 1850, 1854 ; Arch. /. Ancit. und Phys., 1850, 1852. — Helmholtz et Baxt (mesure chez rhomme), Monatsb. d. Berl. Acad., 1867, 1870.— Hermann, Handbuch, 2 vol., 1 partie.— Mabey, Gaz. Died., Paris, 1866 ; Du mouvem. dans les fonctions de la vie, Paris, 1868. — H. Munk,. Arch.f. Anat. und FhysioL, I860.— Place, Arch. v. Pfluger, 1870, t. Ill, p. 424.— Re- GNARD, Influence des hautes pressions, Biol., 1887, p. 406. — Valentin, Molesch. Uniers., X, 1898.— WuNDT, Arch. f. d. ges. Physiol., 1870; Unters. z. Median, d. Nerv. und Musk., II, Stuttgart, 1876.' Rate ; sensory nerves. — Block, Arch. d. physiol. sc. exp., 1875, p. 588. — HiRSCH, Molesch. Unters., IX, p. 183. — De Jaager, Arch. /. Anat. und Phys., 1868, p. 657. — KoHLRATJSCH, Zcitsch. f. rat. Med., XXVIII, 1866, et XXXI, 1868.— Schelske, Arch. f. Anat. und Phys., 1864, p. 151. — V. Wittich, Zcitsch. f. rat. Med., XXXI, 1868, p. 87. Electricity (Ouvrages generaux a consulter). — Bordieb, Precis d'electrotherapie, Paris, J.-B. Bailliere, et tils ; Precis de pliysique biologique (Collection Testut), Paris, O. Doin. — Broca, Art. " Electricite," I>ic, Arch. f. Anat. und Phys., 1887, p. 363; 1888, p. 395, et 1889, p. 3.>0. — Pistrowski, Arch. f. Anat. und Phys., 1893, p. 205. Excitation of nerves.— D'Arsonval, C. R. Acad. sc. 1881, t. XCIT, p. 1520 : 1891. t. CXII, p. 625 ; Arch, de physiol., 1889, p. 246, et 1893, p. 387.— Bernheim, Arch. v. Pfluger, 1874, t. VII. p. 60.— Bordier, Arch, de physiol, 1897, p. 543.— Boudet (de Paris), Trav. lab. Marey. — Ciiarbonnel-Salle, These Fac. sc. Paris, 1881. — Dubois, Arch, de physiol, 1897, p. 740. — Engelmann, Beweg. an Nervenfas. b. Reizung mib Induct, schl.. Arch. v. Pfluger, 1872, t. V, p. 31.— Hallsten, Arch. f. Anat. und Phys. 1881, p. 90.— HooRWEG, Arch, de physiol 1898, p. 269.— Kries, Arch. f.Anat und Phys. 1884, p. 337.— Lewandowsky, Arch. f. Anat. und Phys., 1899, p. 352.— Marchand, Excit. des centres. Arch. v. Pfluger, 1878, t. XVII, p. 511.— Oelh, Arch, ital de biol. 1891, 1893. 1898.— Sestchenow, Arch. v. Pfluger, 1872, t. V, p. 114.— Tiegel, Arch. v. Pfliigcr, 1876, t. XIII, p. 598.— Wedenskt, Arch, de physiol, 1891, p. 687.— G. Weiss, C. R. Acad, sc, 1897 ; Biol, et Congres de physiol. de Turin, 1901. Interferences. — Charpentier, C. R. Acad. sc. et Arch, de physiol. — Patrizzi, Arch. ital de biol, 1896, t. XXV, p. 1.— Valentin, Arch. v. Pfluger, t. XIII, 1876, p. 320.— Wedenskv, C. R. Acad, .sc, 1893, t. CXVII, p. 240. Chemical excitants.— E.-W. Groves, Jojirn. of Physiol, 1893, p. 221.— Stefani, Arch, ital de biol, 1805, t. XXII, p. 183. — Sven Akerlund, Arch. f. Anal und Phys., 1891, p. 279. — Wertiikimer, Arch, de physiol, 1890, p. 790. Polar action : unipolar excitation. — A. Charpentier, C. R. Acad, sc, 1893 et 1899 ; Arch, de physiol, 1893, a 1S99.— Chauveau, Effets physiol. electric. J. Anat. et Phys., de B.S., 1859 ; L'tilisation de la tension electroscopique, Soc avanc sc, Lyon, 1874: Excitation unipolaire, C. R. Acad, sc, 1875-1876. — Coxtrtade, Arch, physiol, 1890, p. 579. — Engesser, Arch. v. Pfluger, 1875, t. X. p. 147.— Leduc, C. R. Acad, sc, 1900, t. CXXX, p. 524 et 730.— Magini, Arch, ital de biol, 1883, t. IV, p. 278.-0. Nasse, Arch. V. Pfliigcr, 1870, t. Ill, p. 476.— H. Sewall, Journ. of Physiol, 1880, 2, vol. Ill, p. 175. Sinusoidal voltaisation. — D'Arsonval, Arch, de physiol, 1893, p. 387. Currents of high frequency. — D"Arsonval, Arch, de physiol, 1893, p. 401. — Bordier et Lecomte, C. R. Acad, sc, 1901. — Radzikowski, Trav. lab. Instil Solvay, t. Ill, fasc. I. Magnetic field : waves, electric rays. — Danilewski, Arch, de physiol, 1897. — Rad- zikowski. Trar. lab. Instit. Solvay, t. Ill, fasc. I. Latent stimulation.— Bernstein, Arch. f. Anat. und Phys., 1882, p. 329.— Boruttau, Arch. f. Anat. und Phys., 1892, p. 454. — Johan. Gad, Arch. f. Anal und Phys., 1886, p. 263. — Mendelsohn, Arch, de physiol. 1880, p. 193. Local excitability of the different portions of the nerve. — A. Beck, Arch. f. Anat. und Phys., 1897, p. 415, et 1898, p. 281. — I>ni. Munk et P. Schultz, Arch. f. Anat. und Phys., 1898, p. 297. 72 ELEMENTARY NERVOUS FUNCTIONS Repeated excitations ; physiological tetanus. — Ch. Bohr, Arch. f. Anat. und Phys., 1882, p. 233. — Cyon, Electrotherapie. — Duchenne, Eleotrisatioii localisee. — Grun- HAGEN, Arch. V. Pfliiger, 1872, t. VI, p. 157. — Kohnstamm, Arch. f. Anat. und Phys., 1893, p. 125. — Kkies und Sewall, Arch, und Phys., 1881, p. 66. — Kronecker und NicoLAiDES, Arch. f. Anat. und Phys., 1883, p. 27. — Kronecker und Stirling, Arch. f. Anat. und Phys., 1878. — Von Frey, Arch. f. Anat. und Phys., 1883, p. 43. — Fried. Martius, Arch, f. Anat. und Phys., 1883, p. 542. — Schoenlein, Arch. f. Anat. und Phys., 1882, p. 357 at p. 369. — Valentin, Arch. v. Pfliiger, 1875, t. XI, p. 481. CHAPTER II NERVE ENERGIES The study of the muscle serves as model for that of the nerve ; but this latter study is incomparably less advanced, which is due to special conditions affecting experimentation on the nerve, these being less favourable than in the case of the muscle. A. ENERGIES WHICH ARE DETECTABLE IN THE NERVE : ORIGIN AND SUCCESSION In the muscle, energy in its initial condition is of chemical nature ; in its final state, it appears as heat and as mechanical work ; its inter- mediate transformations elude our observation ; we know, however, that certain electro motor phenomena are connected with the produc- tion of this work. In the nerve, we must suppose that a cycle of the same nature exists ; but, far from being able to indicate precisely the different conditions of the transformation of energy, we can merely vaguely point them out. Initial condition. — In the nerve, tlie initial energy should also be chemical ; as a proof of this may be cited the faculty which the nerve possesses (as does every tissue) of breathing, that is to say, of burning up something in taking up oxygen from the blood and in returning to the latter carbonic acid ; — the final and the intermediate energies are far less accurately known ; however, to the nerve must be ascribed electromotor phenomena analogous to those of the muscle. Final condition. — Chauveau teaches that nervous energy should be capable of entire reappearance in the form of heat, almost as in the case of the muscle, which contracts in vacuo, when no mechanical work is produced. According to him, this heat, arising at the point of termination of the nerves within the muscle, is added to that of the muscle, and may even increase it to such a degree that it may be appreciable by the thermometer. Yet this final state of all nervous energy is not that which occupies our atten- tion when we desire to express the special impulse wliich the nerve brings to bear on the muscle (or anj^ organ external to the latter) in order to overcome its molecular equilibrium and to cause it to expend its own individual energy. It is scarcely admissible that this shock is the result of calorific vibrations. Prob- ably in this case, as in that of the majority of organs, heat merely follows or accompanies molecular work of a special nature which is accomplished by the motor nerve terminations in the muscle, in order to arouse in the latter an excitable condition. 73 74 ELEMENTARY NERVOUS FUNCTIONS It has been asked, without it being possible to give an answer, if this work is of an electrical nature. This idea has been suggested by the analogj' which has been remarked between the nerve and certain electrical apparatus, and also by the fact that, amongst the artificial stimvili of the muscle and of the tissues, elec- tricity is the best known. But these are rough analogies whose insufficiency is obvious. An opinion on this subject can only be based on direct and convincing experiment. Unfortunately, we have no means of isolating the nerve termina- tions in order to collect directly from them the energies which they give off, and to determine how these latter affect the apparatus ordinarily used in physics. Transmitted and localized energies. — The initial and final conditions of energy in the nerve may be regarded in two different ways. They may, fcu^ example, point out as regards the first the form assumed by the energy at its entrance into the receptive pole of the nevu"on, and as concerns the second, that at its exit from the ramified extremities of the distributing pole : thus the process of nerve conduction is carried out. They may also express — and this is their most general sense — the successive forms of energy in each section, or at each point, of the isolated nerve. Source of energy and excitation. — Whatever idea indeed may be formed of the process of conduction or of the circulation of energy in the direction of the length of the fibres, it is necessary to admit that, at each point of the nerve, a local cir- culation of energy exists from the interior to the exterior, for this is proved by the exchanges with the blood at the point of contact with the capillaries which accompany the nerve throughout its length. It may indeed be said that, just as in the case of the muscle, this is the source from which energy is supplied, and that the latter does not take its rise in the centres or in the organs of special sense. As in the case of the muscle also, the source, properly so called, of enei'gj' which lies in the vessels must be distinguished from the source of the stimulation which comes to it from other nerves or from the surrounding medium by the intermediation of differentiated peripheral organs. The difference between it and the muscle consists in the fact that, in the latter, the expenditure of energy being the end of function, this energy circulates in it in a relatively enormous quantity ; while in the nerve, conduction and excitation being the end of func- tion, the cvirrent in it is infinitely weaker, whence the difficulty of estimating it, or even of proving its presence. Reserve of energy. — Yet, further, as in the muscle, this energy is not abstracted from the blood by the nerve immediately, at each moment of its functional activity, but is stored up in its tissue, in the forni of an alimentary intracellular reserve, which is expended (probablj^ by combustion) in proportion to its activity. In this way it is possible to explain how it is that this activity may persist in the nerve for sometime after all its connexions in the vascular system have been destroyed, and how it is that this activity definitely ceases when this isolation has lasted for too long a time. Of the nature of the substance representing the stored-up energy nothing is known ; it may be, under its form of a mobile and immediately utilizable reserve, a hydrocarbon, as in the case of the muscle, and, in its dormant form, one of the fats which surround the axon. ExjDeriment teaches us further that the reserve of energy in the nerve, which is immediately available, forms but a small portion of its total reserve. Each single stimulation gives rise in the nerve (and by rebound in the muscle) to a very limited expenditure of its total potential energy. The reconstitution of the portion expended is effected so rapidly that it seems to be an opposite and necessary phase of this expenditure, so long as the provision is not exhausted. When the nerve is separated from its vessels, this exhaustion is fatal ; when the nerve maintains its vascular connexions this exhaustion is very difficult to obtain, whence arises the somewhat exaggerated but relatively true notion, that the nerve is incapable of fatigue. NERVE ENERGIES 75 This limitation in the expenditiu-e agrees with another experimental fact, namely, that the activity of the nerve, like that of the muscle, can only be main- tained by an incessant renewal of the excitation in it. Since Weber, every time that we see a muscle in a state of tonic, that is to say, continuous contraction, we regard it as receiving from its motor nerve successive closely approximated but discontinuous impulsions, while at the same time the nerve receives them in the same order from the apparatus which excites it. We know indeed that the only correct way of obtaining this prolonged tension of the muscle is to supply it with impulsions of this order, and the employment of the interrupter (trembleur) of du Bois-Reymond corresponds to this necessity. To return to the energies which can be detected in the nerve. There are two which have been particularly studied and established in it : heat and electricity. Heat developed by the nerve. — The amount of heat given off by the nerve tissue is extremely small. In isolated nerve trunks it eludes observation, as is shown by the experiments of Rolleston, Stewart and of Boeck, who have endeavoured to estimate it without result, using apparatus (bolometres) sensitive up to -,/,,,, of a degree. In the brain, Mosso, by producing asphyxia, has caused obvious heat produc- tion, w^hich may be fairly attributed to a local process, but without producing the complication of displacement of heat brought from another part by the circulation (see Animal Heat, pages 389-396). Chauveau considers, for his part, that a small fraction of the heat evolved by muscle, during its stimulation, may be due to the nerve terminations. To this is limited our knowledge concerning the heat given off by the nervous system. Electricity developed in the nerve has been the subject of a very large number of researches and observations, and hence merits a separate study. B. ELECTRIC ENERGY OF NERVE The nerve is the seat of electromotive forces which are fairly easy to investigate. But their study necessitates a mutilation of the organ examined, and thus much complicates the conditions of the experi- ment and the explanations which are given of it. 1. Current of repose. — As has been remarked above, no means exists by which the neurons can be isolated, in the sense of detaching them from their connexions at either extremity and of observ- ing what passes at each of these extremities. We are compelled to experiment on fragments taken from the continuity of the nerves. Experiment. — Let a segment of nerve isolated by two sections be taken ; this small nervous cylinder will be seen to present at its surface an altogether special distribution of the electrical potential. Starting from the middle zone, which is 76 ELEMENTARY NERVOUS FUNCTIONS known as its equator, this potential is observed to gradually fall, in proportion as the two extremities are approached. Between the equator and each of the extremi- ties, this difference is at its maximum ; between points which are more nearly approximated, it is less in proportion as the distance itself is less. Between the two extremities, as between all points symmetrically situated as regards the equator, it is nil. If this segment of nerve be cut in two, each one of the two halves presents individually the same distribution of potential. The case is obviously analogous to that of a magnet which is broken in pieces, and of which each fragment is furnished with two poles, just as the primitive magnet of which it forms a por- tion ; but with this difference, that the detached fragments of the nerve are not furnished with opposite poles at their extremities, but the two poles of the same nature are electrically oi:)posed to their equator. So far as it is possible to carry this analysis, this arrangement and distribution of the potential will be found to be present and symmetrically repeated as concerns the two lialves of all the fragments. A fragment of nerve fibre will be found to behave in the same manner. It is possible that the component particles of the fibre, inasmuch as these particles represent the elementary organization of the nervous protoplasm, would present the same phenomenon. The reasoning, indeed, is applicable both to the nerve and to the magnet ; analysis in both cases proves that the distribution of the polarities is connected with the molecular structure, or with the component particles, these being of extreme tenuity. Intensity. — When an attempt is made to estimate the intensity of the current of repose, that which is determined is the intensity of the derivation received in tlie galvanometer. The intensity of the derivation is about 0-02 Daniel (or about 0"02 volts). It varies little between different animals, equally little betiveen different nerves, whether motor, sensory, or mixed ; but it is stronger in the non- myelinated nerves than in those provided with myelin (Kuhne and Steiner). Frederic has ascertained it to be 0-048 (Daniel) in the lobster. Origin of the electric currents of nerve. — To what must the currents which are thus observed in nerves be attributed ? Are they the result of a iuerely local reaction of tlie metallic electrodes on the nervous tissue ? No, because they are equally well, or even better observed when inactive and non-polar izable elec- trodes are made use of to collect them. The differences of potential which give rise to them must then pre-exist in the separated nerve segment, and this apart from any contact of the conductors by which they are carried to circulate in the galvanometer. The nerve segment, or rather, the particles which compose it, seem at the first glance to resemble open cells, in which the cui'rent only circulates when tlieir polar wires are united. Derived currents. — This hypothesis may be brovight forward ; but it may also be asserted, and with more probability, tliat these particles (like those of the magnet) are the seats of currents completed in themselves (much more compli- cated, it is true, than those of the magnet). When the nerve segment is con- nected with the galvanometer by two points on its surface which are unsym- metrically situated, there arises merely a derivation of these particular currents, similar to that which occurs in a batterj- circuit when two of the points of its interpolar wire are united by a second closed wire wliicli passes tlirough a gal- vanometer. Do nerve currents pre-exist ? — We have just seen that they pre-exist as regards the ai^plication of electrodes, but do thej' exist anterior to the mutilation of the nerve which is affected in order that the segment to be examined may be removed? In other words, will an isolated neuron, not broken by the cutting instrument, present them ? This question is still under discussion ; nevertheless, many physiologists think, with Hermann, that the answer is in the negative. Du Bois- Beymond regarded the currents thus observed in the nerve, apart from any func- NERVE ENERGIES 77 tional activity, as being currents of repose ; Hermann describes them as currents of alteration. Current of alteration. — Section of tlie nerve trunk by the cutting instrmuent does not merely separate the component particles of the fibres of this nerve ; it necessarily destroys the structure of these fibres to a certain extent, mixes the separated substances, which hence react amongst themselves, without taking account of the fact that the air penetrates and probably takes part in these reac- tions. Hence the origin of currents of chemical source, but which arise under caitirely artificial conditions. They wovild not have much interest for us if they were not themselves the image of converse cirrrents, more feeble, indeed, but otherwise altogether similar, which are connected with the functional activity of the nerve at the instant of stimulation. But these last are not observed with- out the complication arising from the presence of the first. Transverse longitudinal current ; axial current. — In the case of a detached segment of nerve, the gi-eatest difference of i:)otential is observed between its equator and one or other of its cut extremities. This is obvious when the equator, as also one of the two extremities (by non-polarizable electrodes) is connected with the galvanometer. But between the two extremities the potential value is not equal ; if, indeed, the two ends of the nerve segment are connected with the galvanometer, the existence of a much feebler current, which Mendelssohn has observed, and called the axial current, becomes obvious in opposition to the pre- ceding cvu-rent, which is the transverse longitudinal ciu-rent ; its intensity is about ten times less than that of this latter. The interest of the axial current arises from the fact that it has a definite orientation with regard to the phj'siological conduction of the nerve under examination. As regards this latter conduction, it has an opposite direction ; it is ascending in the posterior roots (centripetal) and descending in the anterior roots (centrifugal). Is this orientation of the axial current connected with the mechanism of the phenomena of conduction, or rather is it allied to a phenomenon of a trophic nature, of alteration, which would be unecjual in the two ends, according to the respective locality they occupy in the intact nerve with regard to the cell ? This is not precisely known. Like the transver.se longitudinal ciu-rent, the axial cui'rent undergoes a modification of its intensity (negative variation) tlirough the fact of the activity of the nerve, when the latter is stimvdated. 2. Negative variation ; current of action. — Let a nerve segment be prepared in such a way that two unsymmetrical points are connected Fig. 37. — Diagram of an experiment for the determination and estimation of the negative variation. EE', DD', living nerve. In EE' excitation. In DD' derivation of the longitudino-transverse current, called that of repose or alteration, whose direction is indicated by the largest of the two arrows. At the moment of excitation, contrary current of less intensity wliich declares itself as a negative variation of the longitudino-transverse current. 78 ELEMENTARY NERVOUS FUNCTIONS with the termmals of the galvanometer. The needle of the latter deviates in a certain direction to a certain extent which is recorded. The other extremity of this nerve is stimulated ; the needle returns towards zero, but without attaining this point, thus indicating either a diminution of the intensity of the so-called current of repose, giving rise to what is known as the negative variation (du Bois-Reymond) ; or an inverse current of less intensity than the current of alteration, and which, on this hypothesis, is known as the current of action. Its importance. — Whatever explanation may be given of it, this phenomenon is a very important one, because it is obviously allied to the active condition of nerves ; this may be inferred from the follow- ing observations : — K iilljiiiiiii \ i lil Fig. 38. — Cm-rents of the optic nerve and of the retina. Above, diagram of the arrangement of the experiment. Below, graphic expression of the modifications of these currents during repose (darkness) and activity (ihumination). The darkness is marked by hatching. On the left, current of alteration of the optic nerve. The surface of section is negative with regard to the longitvadinal surface. During illumination, this negative condition is diminished. When the illumination ceases it also diminishes slightly ; then the cvirrent returns to its initial value (Kulme and Steiner). On the right, retinal current. The surface of the rods is negative with regard to that of the fibres. During illumination, this negative condition is at first increased, it then becomes less marked (sometimes diminished). On the interruption of the illumination, fresh augmentation ; then return to the initial condition (Kulmer and Steiner). (a) The magnitude of the negative variation is definitely related to that of the stimulation, just as is the magnitude of the muscular con- traction. A. Waller has verified this point as concerns segments of detached nerves experimented upon in the moist chamber. He has noticed, it is true, that, when the intensity of the stimulation exceeds that which, in the case of a nerve in situ, would give rise to maximum contractions, the magnitude of the negative variation is still capable NERVE ENERGIES -9 of increase. The negative variation being the special response of the nerve to stimulation, and contraction that of the muscle to stimulation transmitted by the nerve, there is no reason Avhy the limits of sensation should be the same in the two organs. But the parallel progression of the two reactions demonstrates their union in nervo-muscular function. (b) Its time of propagation (interval of time which separates it from the stimulation) is definitely connected ivith the length of the course which it runs, and it is the same as for the muscular contraction. We may thus substitute the negative variation for the muscular contraction as evidence of nervous activity. As regards the ^m. 39.-Actiono£ancestlietic.(ehluro- form) on the internal activity of the nerve (negative variation), after Waller. On the left, series of negative variations obtained by stimulating the nerve not sub- mitted to anaesthetic vapours. On the right, absence of the negative variations during anaesthesia. The current of repose at this moment presents slow variations in its intensitv. superficial or deep nerves, which have no direct connexion with the muscles, it may serve as a means of research and of control in the study of their special functions. (c) The negative variation is in- dependent of the nature of the exci- tation made use of. It may be elicited by stimuli other than electricity, and more especially by the physiological excitant, in other words, the normal excitant of the nerve itself. Holmgren, Kuhne and Steiner have demonstrated it in the optic nerve and in the retina, by sub- jecting the latter to its specific excitant — light. Beauregard and Dupuy have observed it in the acoustic nerve, by stimulating the internal ear by the sonorous waves. Du Bois-Reymond had already noticed that it may be elicited in the motor nerve, by stimulating the latter in a reflex manner by means of a sensory nerve. It may also be elicited by the excitation of the cerebral motor area. (d) The negative variation is no longer produced ivhen the nerve is sub- jected to the action of anaesthetics (Waller). In every way, then, it is closely connected with the activity of the nerve, and may be taken as ■evidence and measure of this activity. Wave of excitation or of propagation of excitation. — From wliat has just been said, it naay be inferred that the negative variation is connected, in each section of the nerve, with the excitation of this section during the passage of the latter. In other words, it is connected with what is commonly called the icave of excita- tion ; assiu-edly a very complex phenomenon, in which, as has been seen, elec- -.tricity takes a part, but which we have no right, nor even any plausible reason. 80 ELEMENTARY NERVOUS FUNCTIONS to call an electi-ic wave, that is to say, one similar to those which pass through the wires of the telegraph, or which are transmitted through dielectrics. If, following the example of the majority, we neglect the current of alteration, which is wanting in normal conditions, we shall rejoresent the negative variation as a ivave of negative potential (abbreviated into negative tvave), which passes through the nerve from one extremity to the other. As a potential of a given denomination cannot exist apart from a jDotential of the opjDosite denomination, it follows that, at its entry into the nerve, it is confined to a positive zone, which is in front of it ; at its exit to a positive zone which is behind it ; in its covirse it is comprised between two positive zones, the one in front, the other behind it. Diphasic current. — In passing from the commencement to the end of the nerve, the relationship of the negative and positive zone is inverted. What is known as the current of action acts in the same way. Let a nerve trunk be prepared whose two ends are connected with the galvanometer ; if an impulse passes through it from one end to the other, the galvanometer (if sufficiently mobile) will undergo two oscillations, the one opposed to the other, corresponding to the entrance and exit of the impulse. The diphasic variation of the currents is difficult to demonstrate in nerves, but is easily observed in the case of muscles, especially the heart, which presents a real and somewhat slow wave of proi3agation of the impulse through its fibres. The electric currents of nerve and of muscle are siifficiently similar to be ex- pressed under the same general formula. Form of the lines of flow of the current of action in the stimulated point.— Considered in a given section of the continuity of tlie nerve (in the middle of its length, for example), the negative variation, save that its differences of potential are weaker and its polarities inverted, recalls the distribution of the so-called current of repose or of alteration of the nerve ; it is capable of being depicted in a similar manner. In this case the equator is of a higher potential than the extremities. As to the cui-rent of repose, it may be asked if these potentials correspond to opposed non-satisfied tensions, or rather to true ciu-rents circulating in completed circuits. The latter supposition is probably correct. These circuits may be depicted as follows : on the stu'face of the stimulated nerve (or that of the nerve mole- c\iles), their lines of flux proceed to meet each other, but without actually doing so, in a direction parallel to the length of the nerve ; having arrived at the nega- tive zone, they return, follow the opposite direction in the depth of the nerve and complete the circuit. In this way these currents form a double series of rings arranged round a common axis. They may be compared to two tyres fixed on the same axis, in which the currents circulate in opposite directions. It will be noticed that in such a tyre the lines of force circulate only in the interior. Displacement of the phenomenon with the propagation of the impulse. — In projjortion as tlie wave progresses in the nerve, the potential is inverted in the sections which follow each other. Each section, taken by itself, is indeed posi- tive when the impulse approaches it, negative when the latter reaches it, once again positive when it leaves it. We know the rate of the progression of this wave, but we are ignorant of its length ; in acting in the manner just mentioned, we do not employ, indeed, any direct or indirect means which are really accurate in order to ajipreciate it. We shall demonstrate, a little farther on, a method different from the preceding, which has been contrived for the piu-pose of measuring it. If the nerve is invaded by a series of successive impulses very closely approxi- mated (such as induction shocks), it will evidently be traversed by a series of these waves which, arriving one by one at its extremity, will give rise in it to a series of variations of the current of the same rhythm as themselves. If these variable cvurrents are received in a telephone they will produce a sound whose NERVE ENERGIES 81 pitch indicates their number in a given time. If they are received by a galvano- meter, the needle, owing to its inertia, will not return to its initial condition during the very short intervals between these successive variations, and will maintain a median position so long as these impulses last. Intensity of the current of action, or of the negative variation. — As for the current of repose, tliis word intensity must be understood in a purely relative sense, because the absolute value of the phenomenon is unknown. And for this reason. The wires of the galvanometer being arranged on a section of nerve in an appropriate manner, at the instant when currents arise in this segment they are divided into two portions ; the one (principal current) circulates in the nerve, the other (derived current) circulates in the galvanometer. In this case there is no means of ascertaining the connexion, as regards magnitude, which exists between the principal and the derived current. It is only known that the direction of the derived current is the same as tJuit of the principal current, and that its intensity is proportional to it ; yet this is very valuable information. • It is possible that the derived current is but a very small fraction of the internal current of the nerve, and that it represents merely the losses of the latter, which are due to faulty isolation, losses wliich are, however, too minute to alter the func- tional activity of the nerve. It is remarkable that, in case of a nerve in situ and unnautilated, only a very weak derivation is manifested when it is treated in the manner which lias just been referred to ; to observe the negative variation a cut nerve must be operated on, just as if the section had the effect of opening up some of the closed cycles in which the currents circulate. (a) Intensity in relation to the current of repose or of alteration. — -Measured, as has just been observed, by the derived ciirrent which is received in the galvano- meter, the intensity of the negative variation is much feebler than that of the so-called current of repose or of alteration. — It represents in the frog about the tenth part of it, and still less in mammals. Its denomination of negative variation takes its origin from the fact that it always presents itself as a diminution of tliis current of repose. Whether, indeed, the current of repose really pre-exists, or whether it arises through the mutilation of the nerve, which is necessary in order that the internal circuits of nerve elements may be made manifest by our galvano- meters, the electrical phenomenon which is connected with the activity of the nerve during its stimulation only appears to us as a converse (but unequal) modi- fication of this ciirrent which we cannot avoid vmder the ordinary experimental conditions. Other things being equal, the negative variation will be the greater in proportion as the current of repose is greater. When there is no ciirrent of repose, there is no negative variation. (b) Intensity in relation to the magnitude of the excitation. — As has been said above, the intensity of the negative variation is definitely related to the intensity of the excitation. It increases with the latter up to a maximum which is not exceeded. It behaves itself in this regard like muscular contraction ; it has nearly the same threshold as the latter ; and closely follows its variations. Remark. — The excitations which are brought to bear on a nerve are either simple (isolated closing or opening of the circuit, induction shock, static dis- charge), or composite (due to the repetition of a certain number of stimuli in a given interval of time). In order to observe negative variation, composite ex- citations are habitually employed (called tetanizing). The reason of this is that in order to overcoine the inertia of the needle of the galvanometer, or that of the mercvirial colvmm of a capillary electroineter, it is necessary that the impulses which arise from the electric current be repeated a certain number of times. These successive impulses fix the needle in the median position, or that of rela- tive immobility. The form of movement of a galvanometric needle does not in this case reproduce the elementary form of the electrical phenomena under obser- P G 82 ELEMENTARY NERVOUS FUNCTIONS vation, nor its periods ; it merely shows the general tenoiir of it. When the experiment is conducted, no longer on myelinated nerves, but upon non-myelin- ated nerves, in which the current of repose is stronger and the negative variation greater, the simple excitations j)roduced by induction shocks, or even the break- ing or making of a constant current, become competent to produce a correspond- ing isolated negative variation. In these conditions, and thanks to certain con- trivances, into the details of which we cannot enter here, it is possible to closely follow the general form of the electrical modification thus produced. Form of the negative variation. — This form closely resembles that of a muscu- lar contraction. Its period of augmentation is abrupt and immediately fol- lowed by a period of decline which is much longer. Positive variation. — When the negative variation is over, it is followed by an inverse variation of positive direction, which is more or less marked, and which may be absent. In the case of a nerve which has been mutilated in order to produce the derivation proceeding to the galvanometer, before any stimulation there may be observed : (1) a so-called current of repose ; (2) at the moment of stimulation a negative variation of this current of repose ; (3) a positive variation of this current of repose. Some authors think, with Hering, that the positive variation is connected with the phenomena of nerve restoration after stimulation, just as the negative variation is connected with the waste of this nerve during its excitation. Unipolar methods for the study of electrical variations of the nerve. — In the preceding experiments the excitations supplied to the nerve are bipolar, and the electrical phenomenon which is the consequence of them is a modification of its current of action received in a derived circuit which passes through the galvano- meter. In other words, the nerve, in two localities separated the one from the other, is intercalated in two circuits, the one intended to provide the current which excites it, the other intended to receive the current arising in it as the result of excitation. But the nerve may be also stimulated in a unipolar fashion, in a single point of its progress ; and, on the other hand, if the nerve presents at a distance a variation of its potential, it is possible, by connecting it to the earth by a conductor which passes through a galvanometer, an electrometer, or a galvanoscopic paw, to act in a unipolar manner on these different rheoscopes, which will demonstrate, each one in its special way, the current which passes through them ; the two first will undergo a deviation which will be of a different tenour according to the direction of the current (negative or positive), the last will respond by a muscular contraction. Precautions shoixld be taken to prevent the stimulating current directly reaching the rheoscope, which is intended to demonstrate the ciirrent which takes origin in the stimulated nerve. By this means, therefore, it is possible to render evident the current of action of the nerve without mutilation of this latter, and without giving rise in advance to a current of alteration. This is maintained by Charpentier, who has contrived this method. Oscillatory variations of the electric potential of the nerve during its stimula- tion, — According to this author, the nerve thus stimulated is traversed, from its point of excitation, by a wave which has obviously the same rate of propagation as the negative variation. This wave is accompanied with a variation of the potential and, on its passage to the point at which the nerve is connected with the rheoscope, this variation is made perceptible by the latter. On account of its very slow rate of progress, this wave is not that indeed of tlie stimulating current, but a physiological wave, in which electrical phenomena, amongst many others, take part. It should be noted that the stimulation which gives rise to it is not necessarily tetanizing, but may consist of a simple excitation, such as that which arises in the nerve from making or breaking the current. So far the phenomenon does not differ from that with which we are already acquainted, NERVE ENERGIES 83 except as regards the means employed to render it evident. The new fact brought forward by Charpentier consists in the oscillatory nature of the variation of poten- tial which takes origin at the point stimulated, and is thence transmitted along the nerve. A simple stimulation (induction shock, making of current) produces, according to this author, not a half-oscillation like that which is known as nega- tive variation, or even an entire oscillation, but a series of oscillations, which decrease locally while they are transmitted to a distance. To demonstrate the oscillatory nature of the phenomena, the possibility of making the communica- tion of the conductor with the electrometer is limited to determinate and succes- sively variable periods after the excitation of the nerve. It is then seen that the deviation of the instrument ensues, according to the length of these periods, in one or other direction, a certain number of times. Of these different periods which elapse between the moment of stimulation and that in which the deviation coinmences to manifest itself, the shorter serves to determine the rate of propa- gation according to the formula V = A rate of propagation of 26'" 43 per second is found, that is to say, practically that which Bernstein has pointed out as being the rate of propagation of the negative variation in the nerves of the frog. In comparing these different periods one with another we see that a "(T7o to -gJo of a second = O""" 00134 = t. After the formiila V t= X (length of wave) 26-43 X yJ^g = 0'" 035 ; whence J X = IT"""""' 5. The length of a wave = 3"="'* J. The frequency or number of oscillations per second = 750 circa. Nature of the phenomenon. — It may be asked if this oscillatory phenomenon represents the negative variation (cm'rent of action), or an electrotonic modifica- tion of the nerve (polarization at a distance), both being thus proved to be less simple than has hitherto been supposed. The author of these researches main- tains that it is not an electrotonic phenomenon, but a negative variation. Nervous interferences. — Inasmuch as the nerve is, throughout its length, the seat of a series of waves which pass along it, and inasmuch as we are able to cause these waves to act on a rheoscope, it will be possible for us to make them act on it in such a manner that they interfere with one another in definite fashion. In order to accomplish this, two points of a nerve are chosen sejoarated from one another by a wave length or half a wave length, and these two points are con- nected to the same electrometer or to the same galvanoscopic paw. In the first case (a wave length) the phases will be concordant ; they will supplement one another in order to produce an electromotor effect, there will be a stronger devia- tion of the instrument or a stronger contraction of the frog's foot ; in the second case (half a wave length) the phases will be discordant and opposed, they will neutralize each other ; there will be immobility of the instrument and repose of the frog's foot. Electrical resistance of the nerve. — Compared to a rod of copper having the same form and dimensions, a nerve is a very inferior conductor. The numerical estimations which have been made of its resistance have no great value, because its electrically active substance may be extremely reduced with regard to the protective or nourishing materials which enter into the composition both of the nerve and of its constituent elements. It is as if a tube of copper were compared to a similar tube of paraffin containing extremely fine copper wires in its interior. The most interesting observation is the determination of the variations of this resistance as regards their connexion with the activity of the nerve. Charpen- tier has found that the electrical resistance of the nerve increases with its activity ; and that, on the contrary, it diminishes when its physiological properties disappear. 84 ELEMENTARY NERVOUS FUNCTIONS Cocaine diniinislies it, curare and strychnine at first diminish it for sometime, but afterwards increase it ; crushing of the nerve causes it to fall by a half. This increase in resistance of the nerve during activity is attributable, according to this author, to the development of an opposing electromotive force which repre- sents the physiological work (the expenditure of energy of the nerve) and serves to estimate it ; he has found in one experiment that this expenditure is equal ^^ a^oooVo 0(7 of a. kilogrammetre. No lateral influence. — The electric currents whose intensity is capable of changing so abruptly are, in consequence of this faculty, liable to induce currents in circuits in their neighbovirhood, if special precautions are not taken to avoid this influence at a distance. Can the currents of action of one fibre excite, by influence, currents in a neigh- bouring fibre ? Experience reiDlies in the negative, and the least reflection shows that this must be so, otherwise nerves would discharge various and independent functions and would never act except simultaneously. Part played by the cell in the transmission of excitation. — The impulse, in travelling from the receptive to the distributive pole of the neuron, passes through the body of the cell. Does it undergo some modification in it ? This has been maintained, and the majority of neurologists still maintain it as a matter about which no discussion is allowable. I consider that experiment and reasoning both support the opposite thesis. (a) Arguments draivn frotn analogy. — In the muscidar element, it is not the granular protoplasm surroianding the nucleus which represents the contractile fimction, but rather its external, differentiated, striated portion ; in the neuron, the differentiated function, strictly called nervous, is not that which surrounds the nucleus, but rather consists in the fibres to which this mass gives origin, and which pass through it in order to proceed to a greater or less distance. (b) Arguments drawn from experiment. — There are neurons, such as those of the posterior root, which may be excited at will, either above or below their cell body ; it is impossible to observe any real and constant difference in the effects of these stimulations ; the cell produces no change in them, neither in the in- tensity, nor the latent period, and neither the form nor the distribution of the impulses is altered by it. Changes of this order, on the other hand, become marked when the impulse passes from one neuron to another in the felt works of the grey medullary matter. The experiment has been made on the frog (Morat). Bethe has performed a still more convincing experiment. Taking advantage of the fact that, in certain animals, as the crab, these cells are attached to fibres by long pedicles arranged parallel to each other and perpendicularly to the fibres, he cuts these pedicles, and thus at the same time separates the cells, while preserv- ing the continuity of the fibres throughout their length. The transmission of the impulses and the refiex movements to w^hich they give rise remain possible for several days. Functional activity ceases at the end of this period, on account of the degeneration which the nerves, isolated fron^ their troiihic cells, undergo. This experiment proves both the independence of the fibres with regard to the cells so far as concerns the external or nervous function of these fibres, and their dependence so far as concerns the maintenance of their nutrition. Exner (previously to the preceding authors) had studied, by the galvano- metric method, the influence of the cells of the spinal ganglia on the transmission of impulses by the posterior roots. These roots, cut very close to the spinal NERVE ENERGIES 85 cord, are connected by their end, tlxus divided, to a galvanometer ; a stimulus is applied to the nerve below the ganglion ; the transmission of the impulse is effected through the ganglion just as through an ordinary nerve, there being no exaggeration of the latent period. Contradictory experiments. — On the other hand. Gad and Joseph, experiment- ing on the jugular ganglion of the vagus, regarded as a spinal ganglion, and taking the respiratory movements as evidence of the reaction, have found that there is a difference in the duration of the period of latency when the excitations are made below or above the ganglion. According to these authors, the delay in the first case is from 0"123 of a second, in the second from 0'087 ; the difference of 0'036 between the two indicates the delay iindergone by the impulses in their transit tlirough the ganglion. The structiu'e of these ganglia, in reality less simple than was at first believed, will perhaps serve to explain these divergencies. The following experiment has also been performed. Let an isolated segment of the spinal cord be prepared, and let all the sensory and motor nerves be cut, with the exception of one of these latter. This root is stimulated between the cord and the muscles which it supplies. The muscular contraction is registered. Then this root is cut close to the spinal cord ; the stimulus is again applied and once again the muscular contraction is registered. According to Cyon, the first of the two contractions is longer than the second ; this he attributes to a re- flexion of the impulse which, starting from the point of stimulation, reascends to- wards the cord, redescends the same and arrives at the muscle immediately after the direct wave, in such a manner as markedly to lengthen it. Now I have re- peated this experiment very carefully, but I have never found any appreciable difference between the contractions in the two conditions. Modifications of the body of the cell during repose and functional activity. — The functional activity and the nutrition of the organs of cells are two things which nvAy in principle be distinguislied. One is often observed to be exagger- ated, while the other is diminished, and conversely ; but their limits are not clearly defined and, fundanaentally, they are reciprocally connected. Activity of nerve elements is manifested, not merely by changes external to themselves (motion and sensation), but also by visible modifications of the protoplasm of the nerve cell. These modifications, consisting in change of volume of the cell, displacement of the nucleus, re-ai'rangement of the cliromatic substance, ha^'e been studied by a large number of authors on different objects, such as the grey matter of the bulb, the ganglia of the great sympathetic, the retina, etc. The conclusions are slightly different, and sometimes contradictory the one to the other. Lugaro, who has undertaken critical investigation of the question, maintains that the cell increases in size when subjected to moderate excitation (electrical), while it again diminishes if the excitation is excessive. Speakuig generally, an organ, when it becomes active, tends at the same time to increase its nutritive reserves by a compensatory exaggeration of assimilation over dis- assimilation ; but, should the work be excessive, the compensation is insufficient and the reserves tend to become exhausted. Chromatolysis. — Between the meshes of its network the cytoplasm of the nerve cell contains a substance which is stained by methylene blue (chromatine of Nissl). This substance is regarded by histologists as being a reserve for the neuron and is seen to disappear, starting from the nucleus, in the cells of the nerves which have been fatigued by stimulation (Vas, Mann, Lambert, Lugaro). To the phenomenon of the disappearance of this substance the name 86 ELEMENTARY NERVOUS FUNCTIONS chromatolysis has been given ; it proceeds jjari passu with the changes of volume of the cell and the mechanical displacements of the nucleus. C. EFFECTS CONSECUTIVE TO STIMULATION : FATIGUE Definition, — The word fatigue is used in two very different senses in ordinary language and in that of physiology respectively. In the first, it expresses a sensation which is united to the work of the bodily organs when this work tends to become excessive, and which warns us that repose is necessary ; in the second, it expresses not merely a sen- sation, but the objective phenomena of exhaustion and of the wear and tear of these organs, which checks their movement. Thus the excess of activity tends to be self-limited ; but this is effected in two ways : the one, the most perfect, in which the regula- tion is carried out by a complex sequence, the nervous system inter- vening, and in which we once more observe the mutual relationship of motion and sensation ; the other, more elementary, in which work ceases through the absence of materials and of the conditions by which it can be supported. In order to study the details of this objective phenomenon of ex- haustion, the physiologist excites it himself in the different organs, including the nerves, by putting them in a condition which involves prolonged work. But, the activity of the nerve being estimated by that of the organ to which it communicates the impulse, it is necessary to have recourse to certain special contrivances in order to estimate its individual fatigue in that of the total of which it forms a part. Resistance of nerves to fatigue. — When the stimulation of a nerve (a motor nerve for example) is prolonged beyond a certain hmit, which varies according to circumstances, the contractions decrease and finally disappear. This result is attributed to the fatigue arising from the wear and tear, to the exhaustion of the excitable substance. At first it was supposed that this fatigue was equal in the nerve and in the muscle. Bernstein has proved that fatigue is far more marked as regards the muscle than as concerns the nerve. How can this be explained ? All the methods employed may be reduced to the following : the transmission of the impulses between nerve and muscle is interrupted for a certain period ; the work of the first is increased vigorously and for a long time (in other words, forced fatigue is induced), while the second remains in repose ; then the con- nexion of the nerve with the muscle is re-established (or is allowed to re-establish itself), finally it is observed if the impulses of the first are transmitted to the second. If this is so, it is because it has resisted the fatigue which the long and strong stimulation to which it has been submitted has not failed to produce in the muscle. NERVE ENERGIES 87' ^« N Fig. 40. — Indefatigableness the nerves. of Temporary dissociation of the muscular and nervous tissues. — What means- are there for effecting this interruption, which should be only temporary ? There are two : the action of the continuous current, and that of certain special poisons. Bernstein, and after him Wedinski, have brought about the temporary interrup- tion by exciting a state of electrotonus in the nerve at its entrance into the muscle for a determinate time. Bowditch made use of ciu-are, which is deemed to act only on the terminations of motor nerves, and Lambert has employed atropine, which acts in the same w^ay on the nerves of the glands, in experiments carried out on secretory nerves. The removal of the obstacle takes place by the gradual and spontaneous elimination of the poison. If the stimulation of the nerve is maintained throughout the cku'ation of the poisoning by curare or atropine, it is sur- prisiiig to see, when this poisomng ceases, that the muscle contracts and the gland secretes, thus proving the transmission of the impulse, and therefore the absence of fatigue, in this nerve which has been kept so long in activity. But does poisoning by curare or atropine, as also the action of the constant current, respond to a simple interruption between the nerves and the organs which manifest changes in them, or » has the nervous element become the seat of inertia throughout its length ; in which case it would be no longer susceptible of stimulation and fatigue ? Herzan prefers this second ex- planation, which he supports by experiments made on animals convulsed by strychnine ; indeed, in these animals the nerve appears as inexcitable, and consequently as fatigued, as the muscle itself. Wedinski, in order to eliminate these errors, investigates the activity of the nerve in a direct manner, by its negative variation, estimated by the aid of the galvanometer or the telephone ; he finds that this variation exists so long as does stimiilation itself, proving thus once again the inde fatigability, which is at least relative, of the nerve. Another method. — In the preceding experi- ments it is assumed that the agent (curare, atropine, electrotonization, etc.) made use of in order to physiologically separate the nerve from the muscle, localizes its action upon a definite and restricted area of the first (for example, ciirare on the motor plate) exclusively, and that the rest of the nerve preserves its normal activity, although hindered from manifesting it on account of its separation from the muscle. But it is possible that this hypothesis is inaccurate. Claude Bernard maintained that curare acts electively on the whole motor nerve ; electrotonus is undoubtedly transmitted to a certain distance ; and we do not know if the interior activity of a nerve, separated from its muscle, is precisely the same as that of a nerve which has preserved its natural connexions with it. For all these reasons, Carvallo has had recourse to a method different to the preceding, by which fatigue of the muscle and of the nerve may be separately made evident. Two muscles of like nature and supplied with their nerves are kept exactly at the same fixed temperature. During this time one of the nerves Two muscles, Mj, Mo, furnished with their nerves N„ No, are simul- taneously stimulated at x, by an induced current. The nerve N2 is anelectrotonized at B by a constant current Z, so as to prevent the impulse reacliing the muscle M2, thus to prevent this muscle being fatigued. The mu-scle M 1 is quickly fatigued and ceases to contract. If then the cell current be broken, while the stimulation of the two nerves continues at x, the muscle M2 wiU be seen to contract ; there- fore its nerve has not felt the effects of fatigue. 88 ELEMENTARY NERVOUS FUNCTIONS is subjected to temperatures varying from 0° to 30°, while all that happens from 10° to 10° is carefully noticed : it is ascertained that there is a temperature most Javourable for the motor nerve of the frog, which is 20°. Above and below this temperature the susceptibility of the nerve decreases. The chilled nerve becomes fatigued more rapidly than the nerve at 20°. In a repeated series of excitations, it is noticed that the decrease of the contractions (the curve of fatigue) is brought about so much the more quickly as the temperature is lower. But the nerve at 0° and fatigued by stimulation recovers without delay its first excitability when the temperature once again becomes favourable to it ; that is to say, when stimu- lated, the temperature produces in the muscle work equal to that which this organ produced before. This result clearly shows that, although diiring the time that it was chilled, the nerve was fatigued independently of the muscle, and merely from the fact of the conditions of temperature in which it was placed ; the muscle, kept during this time at the same temperature, had no reason to undergo the effects of fatigue. Hence, in these conditions, and according to the conclusions of the author, fatigue would appertain to nerves, but not to muscles. These experiments have also shown that, when a nerve is chilled to 0° over a certain limited portion of its course, and is fatigued by successive excitations, the portion thus chilled and fatigued (locally inexcitable) nevertheless conducts the impvilse just as the non-chilled and non-fatigued portions. This fact supports the view of the non-identity of the two processes of excitation and conduction. Conclusion. — From the fact that heat has a similar influence on the excitability of nerve, it must be concluded that this phenomenon of excitability (if not that of conductivity) is fundamentally of a chemical and not of a purely physical nature, in spite of the small quantity of energy which is made use of in producing it. The indefatigability of the nerves is only a smaller degree of fatigability in comparison with the muscles. The mechanism of neuro-muscular fatigue. — Muscular or nervous fatigue is usually attributed to the exhaustion of cellular elements which have been func- tionally active for too long a time. Abelous remarks that this fatigue arises partly from the products of disintegration formed by this very activity, which act as paralysing or strictly speaking, " curarizing " substances on the nerve element. These waste products formed by muscle act on the motor nerve termi- nations by a kind of auto-cur arization. When the nerve is subjected to the action of a tetanizing current, a moment arrives in which the contractions cease to occur ; but if then the muscle be directly stimulated, the latter is observed to respond to the stimulus ; this is practically the same result as is obtained with ciirare, except that the paralysing substance is in this case elaborated locally as the result of the muscular " chimisme " during contraction. Fatigue rapidly occurs in the case of individuals or animals whose suprarenal capsules are diseased (Addison's disease) or removed. It would thus be a function of these organs to neutralize the curarizing or paralysing action of muscular waste products. (Abelous, Charrin and Langlois.) See also Albanese, Arch, of Biology, 1892. D. ELECTROTONUS. The term electrotonus is applied to two orders of phenomena, the one physical (polarization), the other physiological (modifications of excitability), which take origin in this nerve when it is traversed by a current. For some (du Bois-Reymond, Pfliiger), there would be a close connexion between the two orders of phenomena ; for others (Matteucci, Hermann), they would be entirely independent. What- ever the facts may be concerning this possible relation, the term elec- NERVE ENERGIES 89 trotonic condition should be reserved for the physical modification, and that of electrotonus for the physiological modification of the nerve ; nevertheless, the two expressions are made use of indifferently in order to indicate, either the physical modification, or the physiological modi- fication which arises from the passage of the current. 1. Electrotonic Condition.— Let a piece of nerve of a certain length be taken ; it is placed on the two non-polarizable electrodes of a constant ciuTent, in such a manner that it extends beyond tliem on both sides, away from the portion submitted to the action of tlie current. Three areas may be defined in this piece of nerve : one intrapolar and two extrapolar. If the portion of nerve arranged in this manner were an ordinary conductor (or if, tlirough crushing, it had lost its nervous structm-e), the cuiTcnt would circulate only in the intrapolar portion. If it were a nerve in which both its structure and its vitality are preserv^ed, in addition to the current which cir- culates in the intrapolar portion, electromotive forces will be developed in the extrapolar portions which will give rise in them to differences of potential be- tween different points of its length, in such a way that, if two of these points be connected with the galvanometer, the existence of a current will be demon- strated. Historical. — Longet and Guerard were the first who observed that, when a certain length of nerve is subjected to the action of the current of a battery, a cvurent is developed (which they call derived) in the extrapolar region of the nerve (Longet, Anat. and Physiol, of the Nervous System of Man, 1. 1, 1842, p. 143). Matteucci, Gruenhagen, Hermann have endeavoured to establish the theory of these derivations, which they essentially attribute to the existence of a difference of electrical conductivity between the superposed layers of the nerve or of the nervous element. Du Bois-Reymond has studied the phenomenon in a detailed manner. He endeavours to explain it by his molecular theory" (generally aban- doned at the present time). Pfliiger, Chauveau and many others after them have studied the modifications of excitability of the nerve which accompany electrotonic currents. The terminology. — The battery cvirrent (or any other applied to the nerve) is called the excitiny or polarizing current ; those which arise in the extrapolar regions, which are tlie localities influenced, polarized or electrotonized, and in one or other of which two points of derivation are chosen in order to connect them with the galvanometer, are called derived, electrotonic currents, or currents of polarization. The region which approximates the anode (positive pole of the battery current) is called anodic, and that in the neighbourhood of the cathode (negative pole) is known as cathodic. The derived current in each of the respec- tive localities is known as anelectrotonic or anelectrotonus , and catelectrotonic or catelectrotonus. 1. Electrotonic currents : direction, duration, intensity. — The direction of the electrotonic currents is the same as that of the polarizing current ; their intensity progressively decreases in proportion as they are removed from, the locality excited in the two extrapolar regions. These characters suffice to clearly distinguish electrotonic currents from the current of action or negative variation of the nerve, of which the direction is constant, and which is connected with the state of stimulation of the nerve, whatever may be the natiire of the stimulus and the direction of the exciting current, when electricity is made use of. The intensity of the electrotonic currents further varies with the intensity of the polarizing current and the length of the portion polarized. It is not equal for the two localities excited, but anelectrotonus is stronger than catelectrotonus. Tliis intensity is not miiformly maintained dm-ing the passage of the polarizing 90 ELEMENTARY NERVOUS FUNCTIONS current, but varies as regards the two extrapolar portions: catelectro tonus at once decreases, wliile anelectrotonus progressively increases, and then itself decreases. Interference with the current peculiar to the nerve. — When the middle segment of the nerve is chosen for the space acted upon, as in the exjDeriment which has just been referred to, and one of the extremities for the locality of derivation, this latter is already the seat of a current of definite direction (current of repose or of longitudinal-transverse alteration) whose action is rendered manifest by the galvanometer ; this current reinforces algebraically the electrotonic current, which inay be much stronger than itself, and which may be reinforced or weak- ened by it, accordingly as it has the same or an opposite direction. It is this- which was first known as the positive and the negative phase of electrotonus, an incorrect terminology, inasmuch as the electrotonic current is altogether inde- pendent of the current of repose of the nerve. In order to demonstrate it, it is- merely necessary to modify the arrangement of the experiment, and to replace the derived portion by the influenced portion, and reciprocally. Derivation, investigated on the median segment of the nerve at two points perfectly iso- electric, presents electrotonic currents when one of the extremities is stimulated,. and these cvirrents still follow the same direction as the polarizing current, with- ovit any complication of the cvarrent special to the nerve. Rate of propagation. — Electrotonus is transmitted from the portion influenced to the derived portion at a rate which ajjpears to be clearly that of the propaga- tion of the impulses (du Bois-Reymond and Bernstein) ; others, it is true, have maintained that its development is instantaneous, like that of the electric current itself. It is in any case very rapid, and hence it is obvious that induced ciu"rents- may give rise to it just as do the continuous current. Chauveau, Charbonnel- Salle have observed it to ensue after discharges of static electricity. 2. Paradoxical contraction. — If, instead of connecting the portion influenced with the wires of a galvanometer, the latter is brought into contact with the freshly cut nerve of a galvanoscopic paw, and electrotonus is produced by the current of a battery or by instantaneous discharges, at every passage of the current, the galvanoscopic paw is observed to contract. The physical rheoscope has thus been replaced by a physiological rheoscope which receives the impulses, and these impulses are none other than the electrotonic ciirrents arising in the primary nerve, which reach the cut extremity of the secondary nerve (nerve of the galvanoscopic paw). The paradox consists in the fact that the excitation of the first nerve seems to be transmitted to the second, in violation of the law of the integrity of structure and of tliat of isolated conduction. But this violation is only apparent, because, of these two laws the second applies merely to the nerves which have not been cut, and the first is verified by the fact that, if the two nerves are placed in contact, end to end, contraction does not ensue, the electric differences to which electrotonus gives rise being not produced. Experiment. — The experiment in its classical form is carried out in the following manner : two principal branches of the sciatic nerve in the frog, these branches being the peroneal and the nerve to the gastrocnemius are prepared, and the trunk of the sciatic is cut in the thigh. If one of these nerves be stimulated (derivations being avoided), a contraction follows not only in its own muscle, but also in the muscle of the other nerve. When the trunk of the sciatic is not cut, the paradoxical contraction would not be obtained, but reflex contractions more or less numerous, which wovild be recognized by their much longer latent period. Difference between the paradoxical contraction and the indirect or secondary contraction. — The paradoxical contraction, obtained by putting nerve in contact with nerve, is not comparable to the contraction known as induced or secondary. NERVE ENERGIES 91 Fig 4 1 . — Paradoxical contraction. which resiilts when nerve is brought into contact with muscle (the latter being in a state of contraction). In this last case, it is the negative variation of the muscle stimulated (by elec- tricity or otherwise) which is the excitant of the gal- vanoscopic paw ; in the first it is electrotonus only and not the negative variation of the electrified nerve which excites the physiological rheoscope. The negative variation of the nerve is much too feeble to give rise to this stimulation ; it is for this reason that mechanical or chemical excitation of the nerve is always without effect on the secondary nerve. Elec- trotonic currents which are susceptible of being much stronger than the negative variation have, for this reason, a definite exciting effect. Inequality of the phases. — With equal intensity of the polarizing current, we have seen that the electro- tonic ciu-rents are unequal ; polarization in the region of the anode exceeds that which arises in the region of the cathode, independently of the cvu-rent of repose, which is diminished or reinforced according to the direction of the polarizing current ; it thus follows that when alternating currents which are equal and not too strong succeed one another with a certain frequency, the resulting effect will manifest itself by a feeble anelectrotonic current. This current must not be confounded with the negative variation, such as may itself arise from the emj^loyment of the so- called tetanizing cxrrrents. Charbonnel-Salle has proved that the electrotonic state may be induced by short currents by the aid of ■ the electrometer of Lippmann, and has also proved that, in these con- ditions, the same regulative laws are in action as when it is produced by continuous currents. Difference between electrotonus and negative variation. — Electrotonus and negative variation have this in common, that they are transmitted, both of them, through the length of the nerve outside the portion influenced by the excit- ing current ; both depend on the integrity of the structure of the nerve, and they disappear when the latter has been tied or crushed between the portion influenced and that which is derived. On the other hand, they are distinguished from each other by the three following characters : (1) the electrotonic modification may be demonstrated by connecting two isoelectric points of the nerve to the galvanometer, while negative variation is only observed when two points of a different iDotential are thus connected ; (2) the electrotonic tnodifi cation progres- sively decreases with the distance between the portion influenced and the derived portion, while the intensity of the negative variation remains the same ivhatever length of nerve may be interposed between the point excited and the extremity on which the derivation is taken ; (3) electrotonic variation is definitely related to the direction of the influencing current, while negative variation has a du'ection independent of that of the cm-rent made use of to excite the nerve. 3. Theories of electrotonus. — Tlie electrotonic current originates in a polariz- ation which the exciting current produces in the extrapolar regions. This polarization is differently interpreted by different authors. Soine, as du Bois-Reymond, regard it (althougli connected with the special Let there be a cut nerve trunk AB, whence are given off two branches BC and BM, the latter going to the muscle M. If the branch BC which is not in physio- logical connexion with the muscle M, be stimulated at C, tliis latter muscle will be seen to contract. Several equivalent forms may be given to this ex- periment and two nerve tnmks, of which one is fur- nished with its muscle, may be coupled together. The only necessary condition is that there should be con- tact between them for a cer- tain length. 92 ELEMENTARY NERVOUS FUNCTIONS structvire of the nerve) as beiiig of purely physical nature, and of the same kind as that, though much more complicated, which is presented by the molecules of soft iron when a current is made to pass through the latter ; others, as Mat- teucci, Hermann, look upon it as the result of the chemical phenomenon of elec- trolysis due to the difference of electrical conductivity of the different parts of Nerve Ane^eetrotnnie (Uirrent Po^nrisinf Current Cate'ertrotonic Current Pig. 42. — Polarization of the nerve in its two extrapolar segments and production of electrotonie currents in these two segments. The middle region is traversed by a constant current (polarizing current). The extrapolar regions show currents of polarization of the same direction as the preceding, but unequal in intensity. The anelectro tonic current is more intense than the catelectrotonic current. the nerve, or rather of the nerve element, the axis cylinder being a much better conductor than the myelin. If a current is sent through the badly conducting investing sheath, at the point of contact with the two bodies a polarization arises (the opposed current increasing the resistance to the passage of the current from the sheath to the axis cylinder). On account of this resistance, the current spreads to right and left from its point of application to the ramifications, and distributes genuine derived currents having the same direction as the current of the battery, and which progressively diminish in intensity in joroportion as the distance from the points of application of the princijDal current becomes greater ; these are the currents which are received by the galvanometer con- nected with the nerve and which represent the electrotonie currents. Nerves of different structure. — In the non-myelinated nerves electrotonus is generally wanting (Biedermann) ; yet this absence, is not total (Borvittau) ; the y Fig. 43. — Internal polarization of a narve fibre, giving rise to an electrotonie current in the extrapolar region. Only one of the influencing po'.es is represented (after Waller). ,A, axis cylinder ; MM, myelin ; bb, surface of contact between A and M, where it is supposed that the polarization gives rise to a resistance which obliges the current to diffuse itself following this surface. The intensity of the polarization progressively diminishes, starting from the point of.'appUcation of the pole under consideration of the influencing current. difference is merely quantitative (Mendelssohn), qualitatively the phenomena are the same. The non-myelinated nerves, generally little experimented uj)on, are thvis parti- NERVE ENERGIES 93 cvilaiiy well adapted for the study of negative variation without the complication of electrotonus. Integrity of structure. Electrotonus disappears when the nerve is crushed between the portion influenced and that derived, as Longet has observed. Schematic reproductions. — By means of special apparatus, it is possible to reproduce the principal phenomena of electrotonus, as has been done by Mat- teucci, Gruenhagen, Hermann. And it may be maintained that the essential and very simple conditions of these schemes are reproduced in the nerve ; but others also arise in this latter by which electrotonus is assimilated to the mani- festations of organized tissues. It is thus that this phenomenon disappears in the case of the dead nerve, and when the latter has degenerated (Schiff , Valentin), that it diminishes or disappears for a longer or shorter time under the influence of anaesthetics (Waller, Bieder- mann). Consecutive effects. — Post-electrotonic current. — Before whollj^ disappearing at the breaking of the polarizing current, electrotonus is first followed by an inversion of the direction of the cmrent (Fick), w^iich is clearly demonstrable only in anelectrotonus (Hermann). 2. Electrotonus. Modifications of Excitability.- — The polar- izing current, whose effects are exerted either on the interpolar region or on the neighbouring locahties situated outside its poles, causes in it similar local modifications of excitability, whose general tenor has more than one relationship to the physical effects which are produced by it. In order to study these modifications, the nerve is left in connexion with its muscle ; by its other extremity it may be connected or not with its centre. The polarizing current passing through its middle or interpolar segment, the extrapolar segment in connexion with the muscle will be described as myopolar ; the other will be called ceiitro- polar. When the influencing current follows the same direction as that taken by the impulses which proceed to the muscle, it wall be called descending : the cathode (negative pole) is then on the side of r€ * I - n ui Fig. 44. — Post-electrotonic intrapolar currents produced after the cessation of the polarizing current (after Waller). I, polarizing current ; II, ordinary post-current of contrary direction ; III, post-ciurent whose direction is tlie same as that of the polarizing current. .^ the muscle and the myopolar segment is in a condition of catelec- trotonus ; while the anode (positive pole) is on the side of the centres and the centropolar segment is in a condition of anelectrotonus. On the other hand, it is called ascending when the cathode and the seg- ment in a state of catelectrotonus are on the side of the centre ; the anode and the segment in a state of anelectrotonus, on the side of the muscle. 94 ELEMENTARY NERVOUS FUNCTIONS Proof of Excitability. — The local excitability of the different points of the different segments will be investigated before, during and after the passage of the influencing current. By experimenting with short excitations, the increased or diminished effects of the latter will then be demonstrated by muscular con- tractions, which may be registered in order that they may be compared one with another. In the extrapolar regions, electric stimulation may be applied without any difficulty by means of induced ciu-rents ; in the intrapolar region, this excita- tion may be effected either by electricity, certain precautions being taken so that the polarizing current be not deranged, or mechanically or chemically. Sketched out and successively investigated by Ritter, Nobili, Matteucci, Valentin, and formulated as regards its essential points by Eckhard, the study of electrotonus has been completed and perfected by the extensive and very methodical work of Pfliiger. General formula. — If it be assumed that the middle portion of the nerve be traversed by a current whose intensity is described as medium, the excitabihty of the nerve is modified throughout its extent. It is increased in the neighbourhood of the negative pole (cathode) : this is catelectrotonus ; it is diminished in the neighbourhood of the positive pole (anode) : this is anelectrotonus. The nerve is thus, as it were, divided into two regions, the one of increased excitabihty, the other of diminished excitabihty ; these regions being separated by a point called neutral or indifferent, situated in the intrapolar region, which preserves its initial excitability. Positive and negative modifications. — The modification, whether positive or negative, attains its maximum at each pole (positive at the negative, and vice versa) and thence progressively decreases in the neighbouring extrapolar region, as also in the portion of the intra- polar region which is adjacent to the pole in question. A curve in the form of an S placed horizontally, surrounding the nerve as an axis, expresses, in a sufficiently graphic manner, these modifications, save that the extremities are strongly inflected on the outside, the decrease of the modification being effected by a much more gradual incline in the extrapolar regions than in the intrapolar region. fi T ^ / \ \ ~ ^ -^ \y Fig. 45. — Diagram representing the contrary variations of excitability in the regions adjacent to the negative and positive poles. ' NN, nerve to whicli are applied the two poles of a constant battery. The two inverse varia- tions'* are never exactly symmetrical. When the current is strong, there is a tendency to the invasion of the whole nerve by anelectrotonus. The neutral point becomes more and more dis- placed towards the negative pole. When the current is feeble, this displacement is made in the opposite direction. NERVE ENERGIES 95 -According as the current is Spinal Cord A. medium current. — In the current which is known as 7nedium, this curve is almost symmetrical ; and the current is called medium by definition, when the inverse modifications of the excitability assume this symmetrical form. As regards intensities which are superior or inferior to that which gives this result, the curve assumes shghtly different forms, which are characterized, not merely by the stronger or weaker inflections towards the poles, but also by the displacement of the neutral or indifferent point, in one or other direction. This displacement, which is the greater in proportion as the current is stronger or weaker, gives to the curve that unsymmetrical form which has been mentioned above. In the case of iveak currents, the neutral point approximates the anode, and hence relatively increases the excitability, in one word, causes the catelectrotonic condition to predominate ; in the case of strong currents the contrary happens : the neutral point is displaced towards the cathode and the anelectrotonic condition predominates, and tends to occupy the w^hole nerve. Ascending and descending current, ascending or descending, these modi- fications are, in each case, of opposite denomination. In the case of the ascending current, the area of anelec- trotonus affects the myopolar seg- ment (that on the side of the muscle) ; in the case of the descending current, it is the reverse. As the state of excitability in the nerve is rendered evident by the contractions of the muscle, it is obvious that these con- tractions will vary in magnitude according to the direction of the current and according to whether the latter is w^eak, medium, or strong, in conformity with the definition which has been given of these words in the particular case. It has already been pointed out that, in addition to the intensity and the direction of the current, the distance between the excited point and the polarized region is also a factor ; and regard must also be paid to the length of the interpolar region and to the duration of the passage of the current. ?n Y Ascending Current Descending Current Fig. 46. — Diagram of experiment for the study of electrotonus. N, battery connected with motor nerves by two non-polarizable electrodes. Be- tween the two poles is the interpolar seg- ment ; below, the myopolar segment with its muscle ; above, the centropolar segment. In the diagram on the left, the cm-rent is ascending ; in that on the right it is de- scending. 96 ELEMENTARY NERVOUS FUNCTIONS It is the minute study of all the circumstances in their multiple associa- tions that makes Pfliiger's work very valuable. Excitability and conductivity. — Excitability is the more or less marked aptitude of the nerve to receive locally the exciting impulse ; conduc- tivity is the greater or less aptitude of the nerve to transmit this im- pulse along its length to the muscle. The modification of both these aptitudes has one and the same consequence : that of increasing or diminishing the magnitude of the contractions. The analysis of special cases proves that it is now to one, now to another of these two modi- fications that the change observed is due. B. Strong current. — Let it be assumed that a strong or very strong current influences the middle region of the nerve, and that this current is descending, excitability will be considerably increased in the area approximating the negative pole, and consequently in the myopolar segment ; the least excitation of this region will provoke strong con- tractions ; on the other hand, excitability will be greatly reduced in the centropolar segment, and stimulation of this segment, especially in the immediate neighbourhood of the positive pole, will remain with- out effect. Here the local excitability is increased on the one side and diminished on the other. Let it be supposed, on the other hand, that this same current is ascending. The catelectrotonic and anelectrotonic areas are inverted. The myopolar segment is anelectrotonized, and its stimulation yields no contraction, because its local excitability is extremely diminished. The centrapolar segment is catelectrotonized ; but, in spite of the local excitability of this segment being augmented, it yet gives no evidence of this, because the transmission of the impulse is arrested in the anelectrotonized myopolar region. This uni vocal manner in which excitability and conductivity comport themselves with regard to electrotonus is a strong argument in favour of their identity. Anelectrotonus and inhibition. — The stoppage of the contractions due to anelec- trotonus lias often been compared to the result of inhibition, so much so, indeed, that these two phenonaena have been completely assimilated. If by inhibition is implied every arrest due to the intervention of some force, this view is correct ; but it is necessary to remember that the phenomena usually described under this name, the stoppage of the heart by stimulation of the vagus, and a number of other more or less analogous phenonaena, are caused, not by a special form of action of a stimulus on a nerve, but by the action of the nerve on other nerves, an action, further, which is provoked by a trivial excitation. If the phenomena of inhibition be generalized, it is imperatively necessary to define categories in the disparate whole formed by the facts which are comprised in them. In the mean- time, there is nothing to prove that the inhibition of one nerve by another is due to an anelectrotonic action of the terminal pole of the second on tlie initial pole of the first. NERVE ENERGIES 97 C. Weak Current. — In proportion as the intensity of the polarizing current decreases, the catelectrotonic action tends to increase rela- tively to the converse action, and it may predominate over it. Thus with more feeble currents may be obtained contractions either with the ascending or with the descending current. Electrotonus in man. — Since Helmholtz, a large number of authors liave at- tempted to reproduce electrotonus in man, but generally with variable, uncertain and paradoxical results. Waller and Watteville have demonstrated that excit- ability is increased at the anode and in the anodic region, while it is diminished at the cathode and in the cathodic region. In other words, electrotonus follows the same rules in man and in animals. Different Conditions mhich Influence Electrotonvs. Direction of the current in relation to the physiological conduction of the nerve. Influence of the intensity of the polarizing current. Initiation and consecutive effects. Influence of the length of intrapolar region. C. J. ^ in H S S % 5 S ■ ^ R c. '(Centropolar seg- ment. ) Extrapolar as- cending cai- electrotonus. The effect first increases then diminishes and is invert- ed. Rapid initiation ; leaving after it a positive modification, preceded by a short negative phase. The effect first increases then becomes nil, and is invert- ed. o o (Myopolar seg- ment.) Ascending extra- polar anelec- ^ trotonus. Effect increases progressively without change of •sign. Slow in'tiation ; maximLim ob- tained after several minutes. Effect increases without chang- ing sign. (Slower than for intensity. ) < c. J. ^ ft seg- ( Myopolar ment.) Descending extra- polar catelec- trotonus. (Centropolar seg- ment. ) Descending extra- polar anelec- V trotonus. Effect goes increasing. Rapid start, slow, increase; leav'es a nega- tive modifica- ' tion, then a ! pos'tive modi- fication, which j disappears. | Effect grows rapidly. Effect increases progressively without change of sign. Slow initiation. Effect increases without change of sign. c f Z c. a z w (J .T. H o 1 PS H 7, Z Z r 'O V The length of the intrapolar polarized region has no effect on its own excitability. The extension of the modification is limited by the situation of the poles. The modification is positive near one and negative near the otlier. A neutral point of unaltered excitability exists in the interval between the poles. With the augmentation of intensity of the current, this point is displaced from the anode to- wards the cathode (consequently in the direction of the current). According to the situation of this point the total excitabiUty of the intrapolar area is augmented or diminished. Whatever be the direc- tion of the current (but especially ascending) it will be seen that with weak currents this total excitability is first augmented, Httle by little attains a maximum, and is then diminished. H 98 ELEMENTARY NERVOUS FUNCTIONS 3. Law of contraction — In the scheme which has just been given, the modifying action is supphed by a current (continuous), while the exciting action is effected by another current (induced), by means of which the different portions of the nerve under examination are in- vestigated. But the battery current at the instant that it is directed through the nerve produces an excitation, and when it is broken another excitation is brought about, in accordance with the laws governing the origin of the excitation. When a graduated series of makings and breakings of the continuous current is carried out on a nerve, whether ascending or descending, a series of excitations and of modifications of excitability is all at once initiated whose effects (muscular contractions) are explained by the law%s formulated above. Inrifn/j/mt Excitab. diminished Fig. 47. — Determination of excitability of the myopolar segment diu'ing the passage of a current tlorough a certain length of the nerve. In the upper figure, the polarizing current is ascending, excitability is diminished in the myo- polar segment. In the lower figure the polarizing current is descending, the excitability of the myopolar segment is increased (after Waller). Diagrammatic table. — The results of these series may be systema- tized in the following table. Current. Descending. Make. Break. Ascending. Make. Break. Weak. Contraction. — Contraction. — Medium. Contraction. Contraction. Contraction. Contraction. Strong. Contraction. — — Contraction. Discussion, — As is thus seen, the weak current causes two contractions at its making, whether it be descending or ascending. The medium current causes four contractions. The strong current once again brings about two, the one at NERVE ENERGIES 99 the making of the descending current, the other at the breaking of the ascend- ing current. An explanation of the absence of the contractions wliich are missing must be sought for. (a) Weak current. — As regards weak currents, the two breaking contractions are wanting on account of the weakness of the cm-rent, tlie exciting action of the break being regarded as inferior to that of the make, possibly in con3ec|uence of the alteration which is produced by the passage of the current. (b) Strong current. — As regards the strong cm'rent, the absence of tlie breaking contraction of the ascending ciurent is due to the marked effect of anelectrotonus (diminution of excitability) in the portion of the nerve which is in proximity to the muscle and in contact with the positive pole. On the other hand, the very considerable magnitude of the breaking contraction of this same ascending CTorrent is due, after the cessation of anelectrotonus in this region, to a contrary modification, which acts like catelec trot onus by augmenting excitability. After the cessation of the passage of the current from the battery, an after current of opposite direction is indeed developed in the nerve (for many reasons, amongst others, elec- trolysis), as in every polarized circuit. Lastly, the absence of the breaking con- traction in the case of the descending current is explained by the establislmient of an after cvirrent of the same kind which, after the cessation of catelectrotonus developed by the passage of the polarizing cmrent, gives rise to the development of a contrary modification equivalent to a strong anelectrotonus. (c) Medium Current. — As regards the medimn current, the exciting effect of breaking is now sufficient to bring about an excitation, and on the other hand the anelectrotonic modifications, both direct and consecutive, are still SLifificiently feeble not to hinder the occurrence of the contraction ; hence foiu- contractions — two on making and two on breaking of the two currents. Electrotonic theory of excitation. — On the strength of these facts, Pfliiger gives the following formula of electric excitation : Excitation arises through the production of catelectrotonus or through the cessation of anelectrotonus. Tetanus produced by the continuous current. — When a considerable extent of the nerve is traversed by a continuovis current (especially a descending current), dm-ing its passage a continued contraction — a tetanic muscular contraction — may be produced. Breaking tetanus. — Conversely, when the ascending cm'rent is strong and is long continued, it may produce, on breaking, instead of a shock, a tetanic con- traction similar to the preceding. These apparently tetanic contractions a^^pear to be inordinately prolonged shocks, such as are obtained when the nerve or the mviscle is fatigued or subjected to the action of cold or certain poisons (vera- trine). Neither making nor breaking tetanus (Hering and Frederick) causes secondary tetanus in the galvanoscopic paw. Volta's alternatives. — For seen by Ritter, incompletely formulated by Volta, the law governing these phenomena has been expressed in the following manner by Rosenthal and Wimdt : The continued passage of a current of a given direction increases the breaking excitability of the current of the same direction and the making of the current of the opposite direction ; it weakens it as regards the making of the first and the breaking of the second. If the existence of an after ciurent of polari- zation is admitted, and if the so-called breaking contraction be attributed to the luaking of this inverse cvurent, the law becomes simpler and may be formu- lated thus : the passage through the nerve of a current of a given direction increases the excitability of this nerve for a current of the opposite direction. Succession of effects with weak currents. — According to the authors who have studied it, and according to the point of view which each of them has adopted, the law of contraction assiunes a particular form, as is shown by the tables which have been prepared to express it. Its mam outlines are sketched in Pfiiiger's 100 ELEMENTARY NERVOUS FUNCTIONS table. That of Heidenliain expresses in detail the increasing effects of weak currents, until they attain the intensity which is known as medium. Current. Descending. Ascending. Intensity. Make. Break. Make. Break. I. ... — II. ... — III. . . . Contraction. IV. ... Contraction. Contraction. Contraction. Contraction. Contraction. Contraction. Contraction. Contraction. Contraction. Disappearance of contractions as the nerve gradually perishes. — The following table, due to Nobili, expresses the action of another condition, that of the varia- tion of excitahility of the cut nerve which belongs to a separated limb of the animal. This assumes that the stimulations are no longer extemporaneous, but succeed one another at long intervals, leaving to time the task of performing its work. The excitability of the nerve at first progressively increases ; then it regularly decreases, and this decrease is rendered evident by the gi-adual disappearance of the contractions one after the other. In the case of a nerve having its maxi- mum excitability it would be as follows : Degree of Descending Current. Ascending Current. Excitability. Make. Break. Make. Break. I. ... II. ... III. . . . IV. . . . V. ... Contraction. Strong con- traction. Strong con- traction. Contraction. Contraction. Weak con- traction. Contraction. Contraction. Strong con- traction. Strong con- traction. Law of Ritter- Valli. — This table expresses the variations of nervous excit- ability regarded in their succession in time. It should be added that this loss of excitability is not total ; it does not involve the nerve in its entirety, but follows a progression in the direction of its length. In the motor nerve it first affects the extremity which is farthest removed from the muscle, and then progressively involves the portions of the nerve which are nearest to the former. This mode of disappearance of excitability is the same whatever may be the nature of the alteration which produces decay of the nerve (anaemia, curare, CI. Bernard ; local anaesthesia, loteyko and Stefanowska). In sensory nerves, whose conduc- tion is the converse of that of the motor nerves, the progress of the loss of excit- ability is inverted, and proceeds from the periphery to the centres. (The same authors.) Electrotonus in the monopolar applications of electricity. — A nerve may be excited, as Chauveau has shown, by placing a single electrode of the current on it, while the other electrode is placed on a remote region (more or less symmet- rical). It hence follows, without any possible error, and contrary to that which Hermann maintains, that electrotonus (in other words, the modification of excita- NERVE ENERGIES 101 ability which accompanies the electrotonic currents) may and should arise in these conditions (Morat and Toussaint). Clearly the results of this mode of a unipolar application of electricity must be connected with those of bipolar applica- tion, and this connexion is made manifest by arranging, as I have done, in a parallel table the results obtained by these two methods of operating. A know- ledge of this concordance is the more useful inasmuch as if bipolar application is almost exclusi\-ely employed in physiology, the unipolar method is the only possible and correct one clinically ; hence it is necessary to make sure of the equivalence of the two methods in order that clinical practice may benefit from experimental results. From the theoretical point of view, for the explanation of the effects of elec- tricity, this comparison is also not without interest. It proves to demonstra- ^ Fig. 48. — Progressive loss of excitability in tlie cut motor nerve. The iioints A, B, C, D become successively inexcitable after having undergone a slight phase of hyperexcitabilit\-. tion that the specific nature of the action for a long time attributed to the direction of the current, really belongs to the nature of the pole in contact with the stimulated nerve (in the unipolar method), with the myopolar portion of the nerve (in the bipolar method). This is markedly shown in the following table, in which the succession of effects due to an increasing intensity of the currents in the two modes of excitation is noted. Current. Making. Breakixc;. Pole. Negative Positive Making. Breaking. < Ascending Contraction Descending — — Contraction — w ^ Ascending Contraction .Descending Contraction — Negative Positive Contraction Contraction = 'Ascending Contraction Descending Contraction Contraction Negative Positi^•e Contraction Contraction Contraction Med Ascending Contraction Descending Contraction Contraction Contraction Negative Positive Contraction Contraction Contraction Contraction Iz; o 'Ascending Contraction Descending Contraction Contraction Negative Positive Contraction Contraction Contraction g ^ m Ascending — , Descending Contraction Contraction Negative Positive — Contraction Contraction — Electrotonus in nerves of different functions. — Electrotonus, in so far as it is a modification of the excitability, is an absolutely general property of the nerves, just as is excitability itself, or the conduction of the impulses, and which mani- fests itself, like the latter, independently of the special functions of these nerves. The functions of nerves depend on their connexion with nervous or non-nervous elements, which they govern, or from which they receive the impulse. 102 ELEMENTARY NERVOUS FUNCTIONS (a) Secretory nerves. — If, instead of a motor nerve, a secretory nei've be stimu- lated, in i^lace of a nauscular contraction, there will be a flow of liquid from the gland. The law of contraction may be verified in these new conditions, as Bieder- mann has shown. Tliis author, operating on the glosspharyngeal of the frog, and the nerves of the tongvie, has found it advantageous to estimate by the galvanometer the cvirrent of secretion, rather than the secretion itself. (b) Inhibitory nerves. — By stimulating the pneumogastric, wliich is the type of inhibitory nerves, by continuous currents of varied intensity and direction, Donders has been able to prepare a table of the law of contractions similar to that of Pfltiger, except that in it contraction is replaced by arrest of cardiac action, and repose or want of contraction by the continuation of the movement of the heart. (c) Elements of association. — The inhibitory nerves are not terminal neurons, like those of the anterior roots proceeding to the muscles of the skeleton ; by definition they are considered as associating elements whose function, deter- mined by their special connexion with the motor elements properly so called, terminates in this apparently strange phenomenon, the arrest of movement. The internal elements of the nervous system (intercentral elements) thus present the phenomenon of electrotonvis. This is equivalent to saying that the pheno- mena of electrotonus appertain to every nervous element. Sensory nerves. — The initial nerves, like those which are terminal and the intercentral nerves, should present electrotonic modifications. In such nerves, and for reasons which are readily understood, these phenomena are not easily to be detected in animals, on whom we can only measure sensibility under its reflex form, and by no means withoiat difficulty (Zurhelle). Nerves of special sense — In man this study has been undertaken on the nerves of special sense. (a) Taste. — The passage of a cvirrent through the tongue produces a sensation of taste, acid at its entry, alkaline (almost bitter) at its exit (Pfaf¥, Volta and Bitter). (b) Sight. — A current proceeding towards the ganglionic cells (descending cur- rent) causes the sensation of darkness ; a curi-ent proceeding in the opposite direction (ascending current) causes a sensation of light (Helmholtz). (c) Hearing. — It may be shown, but not without difficulty, that the law of contraction is applicable also to the sense of hearing (Brenner). Polar influence on non-differentiated protoplasm. — Kiihne was the first to demonstrate the action of electricity on the non-differentiated or bvit slightly differentiated protoplasm of the lowest animals. Verworn has made a method- ical study of this action. It is generally held that the protoplasm of the unicellular beings represents living matter under an elementary form and apart from all the structures which are superposed on it in tlie more highly organized beings. Hence it has been supposed that the response of svich a substance to excitants would be equivalent to that of the least differentiated cells of superior animals, and even to that of the least differentiated substance which exists in these cells ; that, in a word, it should be univocal and very simple in its expression. As a matter of fact, the response to electrical excitations of unicellular beings is some- what complicated. This result appears to indicate that the comparisons or assimilations made between beings of. different organization are not perhaps very accurate. The simplicity of monocellular beings (amcebfe, infusoria, etc.) is not so great as has been supposed and, above all, its mode of expression is otherwise than that which has been held to be the case. In the amoeba all the essential functions of life are already represented, bvit, in a sense, undivided, in its protoplasm. In tlie course of phylogenetic evolution, a distribution of atri- NERVE ENERGIES 103 butes is made between the different portions of this protoplasm, a division in virtue of which each one of them increases its aptitudes in a given direction, for the performance of a given function, while it loses its aptitudes in an opposite- direction, to the gain of other differentiated portions with regard to other func- tions. Hence evolution, differentiation, results at the same time in the attain- ment of a greater perfection of the functions of the whole, and in a specialization, that is to say, a reduction of the functions of the part, hence the inability of the latter to live by itself when it is detached from the whole to which it belongs. Galvanotropism. — An amoeba, placed in a drop of water through which a current is passed, reacts under the action of this current. The 'portion turned toioards the anode (positive pole) is retracted, ivhile that facing the cathode (negative pole) is protruded and forms a pseudopodium. If the current be reversed, the same phenomena occur, but in the converse manner. Hence, imder the action of the two opposite poles, a double inverse deformation occurs. Which of the two is the equivalent of a muscular contraction ? It is very difficult to decide. In a definite differentiated protoplasm, such as that of muscle, nothing is more clear than the contrary state of repose and of contraction. But in the diffuse protoplasm of the amoeba, the direction of the forces varies according to circxmi- stances. As a matter of fact, both changes of form practically arise at one and the same time from a contraction in one direction, and a compensatory protru- sion in the other. Retraction is always the active phenomenon. When the force is parallel to the pseudopodium, it causes it to retract ; when it is circular and perpendicular, it causes it to elongate. In both cases there is an expenditm-e of energy ; in both cases there is a contraction, and from a certain point of view the phenomenon is less simple in the amoeba than in the muscular fibre. That which is remarkable is, that the anode excites one of these movements and the cathode the other, in a predominant or special manner. Yet, a double converse deformation, such as that which has just been de- sci-ibed, implies a tendency to locomotion. Certain unicellular beings, ciliated or not, present, under the influence of the cm-rent, a change of place to which the name of galvanotropism is more particularly applied. Electrotonus, the reactions of excitable substances varying according to the nature of the pole, or the direction of the current, are merely varieties of galvanotropism. E. DIFFERENT USES AND EFFECTS OF ELECTRICITY Action of the magnetic field ; electric waves ; electric rays. — When a current is projected into a circuit, lines of force are developed around this latter which create around it a field of force, known as the magnetic field. If another circuit is placed in this field of force (in an appropriate position), induction occurs, electric movement in this second circuit. If, instead of this second circuit, an excitable tissue be placed in the magnetic field, as the galvanoscopic frog's foot, what will happen ? This qviestion has been studied by several authors, more especially by Danielewsky, and more recently by Radzikowsky. As a matter of fact, when a frog's nerve is placed in a field of force, it is excited by the variations of the intensity of this field. In this tissue, the strvicture of which is very complicated, it may be admitted that induced currents are developed which arouse to activity its very excitable substance. In certain positions of the nerve the stimvilation is at its maximum, for example in that in which it is placed in the same plane as the inducing circuit and i^erpendicular to an element of this circuit, the muscle being turned to the outside. The arrangement of the exiDeriment may be greatly varied, and the nerve may be jilaced in the field in the neighbourhood of one of the isolated poles (unipolar excitation). The nerve must be isolated ; if it is left in position surrounded by other tissues, or if, after 104 ELEMENTARY NERVOUS FUNCTIONS it has been laid bare, it is covered with a conducting envelope, it will no longer be subjected to the action of the field of force. The surrounding tissue acts as a shunt (Radzikowsky) or, to speak more precisely, as a screen (Danilewsky). Electric opacity and transparency. — If, indeed, some conducting body be placed in tlie magnetic field so tliat it crosses the line of progress of the electric waves (a plate of metal, the hand), the excitation by induction ceases ; the interposi- tion of a dielectric (a glass plate) allows the waves to pass. The conductor is opaque to the electric waves (as to luminous rays) ; the dielectric, which is often known as the isolating agent, is transparent as regards these rays. Electric immunity. — It will thus be understood that tissues which are but little excitable, but which conduct electricity, help to render the more excitable tissues irresponsive to tlie action of the magnetic field, which is created around them by the electric phenomena developed in the organism. In the same way, as regards an excitable cell or fibre, a conducting envelope may preserve it from the excitation which would arise from this cause. In order to protect living elements against stimuli which are not intended for them, there would thus be two methods available : the one consists in surrounding them with dielectrics which preserve them from excitation by conduction, the other consists in sur- rounding them with conductors which prevent their being excited by induction. The same element may make use of these two methods. This special organiza- tion allows the element to reject the excitation in certain of its parts, and to receive it in certain other points of election. High frequency currents. — The so-called high frequency currents are those which attain the number of 500,000-1,000,000 per second. Their action has been studied on Hving beings, partly by direct appli- cation, partly by placing the subject in a solenoid, which creates around it a field of force (d' Arson val). (a) Direct action. — The very high frequency of these currents confers a kind of immunity at the site of their passage on the organism which they traverse. At an equal voltage, they are infinitely better sup- ported than currents of an ordinary rhythm (say 100 to a second). However, by augmenting the intensity, they may be rendered injurious, and death may ensue in animals (Bordier and Lecomte). {h) Indirect action through a field oj force. — When an animal is placed in the interior of a solenoid in which such currents circulate, an aug- mentation of the respiratory exchanges is observed (d' Arson val). This influence on the exchanges is nevertheless not due to the special action of the induced currents acting on the nervous system, or the tissues, but is a secondary effect attributable to the heat developed by the current. It is known, indeed, that heat by itself increases the activity of the exchanges, when it becomes increased to such an extent that the animal scarcely resists it. Death by electricity. — Prevost and Battelli have made a special study of the mechanism of death by electricity and the conditions which give rise to it as regards the electric agency. They have experi- mented on dogs, rabbits, guinea pigs and rats, by sending a current NERVE ENERGIES 105 through the body from the head to the anus. The resistance of the animal varied from 400 to 900 ohms. The duration of the apphcation varied from some hundredths of a second to two or three seconds. The resuhs differ according to the animal. The authors have investi- gated the action both of continuous and of alternating currents. (a) Alternating currents. — These currents must be divided according to their tension into those of low tension (10 to 120 volts), those of medium tension (about 620 volts), and those of high tension (1,200 to 4,800 volts). With currents of low tension a stoppage of the heart u'ith fihrillary tremor is observed in the dog. Respiration continues for some minutes, but in its turn ceases by anaemia of the medulla oblongata. The animal dies. The rabbit and the rat offer a much greater resistance. Sometimes 10 volts will suffice to kill a dog. Currents of medium tension produce, in the dog, a stoppage of respiration as also of that of the heart. In other animals stoppage of respiration is usually alone observed ; further, there is generalized tetanus and anaesthesia. Currents of high tension cause, in all animals, stoppage of respiration, while the heart continues to beat. In this case animals may be saved by the use of artificial respiration. But when, on the other hand, the heart stops first, artificial respiration is altogether ineffective. (b) Continuous current. — These currents have been raised up to 540 volts. Although breaking is more dangerous, accidents are not ex- clusively due either to making or to breaking. Further, the phenomena differ but little from those which are observed when alternating currents are made use of. Other things being equal with regard to tension, when alternating currents are made use of, the number of interruptions to the second forms an important factor. This has been made to vary from 9 to 1,720 to the second. A rhythm of 150 the second is that in which death is least likely to occur. Below, but especially above this number, the voltage must be considerably increased for the same effects to be produced. A continuous current acts like an alternating current of the same voltage at 350 to the second. It is a curious fact that the heart which has been stopped by a low tension current may once again be set in action by a high tension current. The results to which these experiments lead are somewhat different from those which are generally accepted. Mechanism of death, — According to the nature of the animal, accord- ing to the voltage of the current, according to its rhythm if it is alter- 106 ELEMENTARY NERVOUS FUNCTIONS nating, the first stoppage may affect either respiration (high tension), or the heart (low tension). The stoppage of the heart is clearly the only cause of death, inasmuch as artificial respiration is possible, and the normal respiration may spontaneously be renewed, which does not occur in the case of the heart. Currents of medium and low tension should then be more dangerous than those of high tension. As a matter of fact, when death ensues it is always by stoppage of the heart. Artificial respiration should, nevertheless, be attempted and persevered in. The observations which have been made in cases of death from accidents arising in the industrial applications of high tension elec- tricity must be regarded as being contradictory ; the tension of the current to which the victim has been subjected, by derivation or other- wise, not usually being that of the direct current which circulates in the line, but usually much weaker, its exact strength being indeter- minate. F. NERVE POISONS CI. Bernard was the first to analyse the tissues and their func- tions by means of poisons, which for this reason he called the " reactives of the physiologist." In this order of investigation he has left examples and methods which his successors can only imitate and make use of without essentially perfecting them. He divides j)oisons into general and special, according as their action affects every cell, or only certain species of cells ; or, further, certain varieties of cells, as happens in the case of the nervous system. 1. General poisons — Anesthetics. — CI. Bernard regarded anaes- thetic substances as general poisons, capable of suspending (without destroying, when their action is not indefinitely prolonged) the activity of all cellular protoplasm. They are for him the reactives of life. The germination of cereals, the growth of plants, the movements of the sensitive plant, are arrested by the vapours of chloroform or of ether, as is sensation in animals. In these latter, the movement of the vibra- tile cilia, the contraction of the muscles, the excitability of the motor nerves, in a word, that of all the tissues, may be paralysed if the tension of the vapours is sufficient and their action sufficiently prolonged. But it must also be added that, between the different species of cells and between the different systematizations which may be effected by them, there are well defined susceptibilities and fairly large gradations, from which it results that in limiting precisely the doses (tension of the vapours) these systems and these elements may be attacked one NERVE ENERGIES 107 after the other ; hence arises another form of physiological analysis carried out by the aid of poisons. The anaesthesia of chloroform and ether. — The action of anaesthetics (chloroform and ether being especially considered) may be divided into three periods, up to the instant when insensibility is complete. The first period is characterized by a sort of drunkenness which, in more than one point, resembles alcoholic drunkenness : vertigo, loss of equilibrium, stimulation of the different senses and of cerebral activity, general excitement. This is the so-called period of excita- tion, which is never completely wanting, and which should not be con- founded with a purely local irritation of the anaesthetic vapours on the upper respiratory tract. The sensations, which are at first exalted, are afterwards dulled and, before consciousness is lost, a condition is manifested of fleeting duration and difficult to obtain and voluntarily maintain, which is yet not anaesthesia, but analgesia, and during which painful sensations are alone suppressed. The second period corresponds to a true condition of anaesthesia characterized by insensibility to ordinary painful impressions, as well as those which are not painful, but in which reflex excitahility is entirely preserved, if not increased. The anaesthetic action should be carried slightly farther in order to ensure the muscular flaccidity which is favourable to operation and to the manoeuvres of surgery. In the third period, that of complete anaesthesia, the reflexes of the life of relation disappear, especially the winking of the eyelids con- secutively to touching of the cornea, the patellar reflex, the labio- mental reflex (Dastre and Loge). The muscles of the limbs are relaxed. The respiratory reflexes, all those which maintain the acts of the life of nutrition and whose sphere lies in the internal organs, are preserved. Not that these functions do not receive the rebound of the toxic action which gradually attacks the nervous system, as is evident by the modi- fications of the circulation and of the temperature, but the more simple and more resisting sj^stems which govern them preserve their united action to a sufficient degree. Anaesthetic syncope. — If the intoxication is puslied farther, a serious plieno- nienon gradually supervenes, menacing life ; this is respiratory syncope, the stop- page of the movement of respiration which may be combated by the emploj'ment of passive respiration artificially maintained by the aid of movements of the thorax. In chloroform anaesthesia cardiac syncope may occur suddenly, and is almost hopeless ; further, this accident may occur at the very commencement of anaesthesia. In animals, of which some, like the dog, are liable to this com- plication, it may be prevented by the previous injection of a small dose (half milligramme) of sulphate of atrojiine, which diminishes the inhibitory action of the vagus on the heart (Dastre and Morat). This method may be combined with 108 ELEMENTARY NERVOUS FUNCTIONS that of CI. Bernard, who administers morphine to the animal previously to its being ansesthetized. During complete anaesthesia, the pupil remains contracted even when the eyelids are closed. At the instant when respiratory asphyxia occurs it abruptly dilates. The state of the pupil should be watched so long as anaesthesia is maintained. Cocaine. — Cocaine may be classed amongst the general poisons of the nervous system in almost the same category as the anaesthetics. Employed in watery solution, wherever it comes in contact with nerve protoplasm, it suspends or destroys its excitability. Sensory nerves, motor nerves, white and grey matter, all are subjected to its paralysing influence. It is employed locally (clinicalljO in order to extinguish for a time the sensibility of certain surfaces, such as those of the larynx or the cornea. Cocaine has been regarded in turn by some (Laborde, Arloing, I>affont) as a special poison, a sensor;/ curare ; by others (U. INIosso, Danilewsky, Charpentier) as a general poison, a genuine ancesthetic. The first of these two opinions is based on the local anaesthetic effect, easily obtained by painting mucous mem- branes or the skin with a solution of cocaine, while the excitation of sensory nerves which leave these localities, proves them to be very sensitive ; the ulti- mate ramifications would thus alone be affected and not the nerve trunks, or the cerebral centres. The conclusion is not accurate. It is known, through the example of atropine, that an action which is purely local as regards the pupil after installation into the eye does not exclude the general action of the sanae substance diffused in the blood, even in a mininium dose. The comparison of the relative weight of the substance employed on the one hand and of the re- acting tissue on the other hand, proves, on the contrary (for both poisons) that local action is only produced by large doses, while the general action is obtained by doses which are comparatively verj- small (say 2 milligrammes per kilogramme in the dog). The opinion that the substance injected into the blood would only affect the peripheral ramifications of sensory nerves cannot be maintained. A nerve trunk being isolated, it may be subjected locally to the action of cocaine ; it then loses at this point its excitability and its conductivity. The excitations made above the point operated on are no longer transmitted to the terminal sensory or motor organs. This effect at once vanishes with the elimination of the substance. In this manner a delicate means of replacing the section of the nerves is available, and it allows also the complete return of function (two to four drops of the one per cent, solution injected under the sheath of the vagus are sufficient to paralyse it). The action of cocaine thus ap23lies to all varieties of nerve protoplasm (sensory, motor, voluntary, involuntary nerves, etc.). It also applies to the protoplasm of the muscle. When the substance is brought in contact with the muscles, it modifies their power of contracting (Sighicelli, U. Mosso). The anaesthetic effect, in the general sense of the expression, is displayed in all the branches, whether they have or have not a nervous system (Danilewsky). It hinders the diapedesis of the white corpuscles, without however suspending the chemiotaxis, as does chloroform (Massart and Bordet). It stops fermentation and germina- tion (A. Charpentier). Thus, therefore, the action is a very general one as concerns all protoplasm, but it is an action wliich varies unequally according to the nature of the element of the tis.sue ; it must be added also, that it is an action which varies according to the doses employed (stimulating in a weak dose, paralysing in a strong dose). When diffused in the organism, cocaine gives rise to somewhat complicated effects, like all anaesthetics, but which, on account of their special nature, make NERVE ENERGIES 109 this substance inapplicable as an ordinary anesthetic, such as is employed surgi- cally. Hence it has been reserved for local application, especially for superficial application, so that there is no danger of marked absorption of this sulastance causing genex'al effects. It has been shown experimentally that cocaine acts on the grey masses of the nervous system in the same way as it acts on its terminations or its conductors. When applied directly to the cerebral cortex (motor area), it diminislies its excitabilitj- (Charpentier, Carvallo) ; when injected into a segment of the sj^inal cord, it diminishes its reflex power (U. Mosso). Gaglio has made use of it in order to study the functions of the semi-circular canals. When injected into the general circulation, it first attacks the higher functions of the nervous system, and more especially sensation, but it does not produce the dissociation, at the same time regular, prompt and gradual, which gives their therapeutic value to ether, chloroform and nitrous oxide. TheoreticaUy it is an anaesthetic, but not practically (Dastre). 2. Special Poisons. — Specificity must not probabty be taken in the absolute sense of the word. But it may be admitted when it is appHed to the action of agents which in infinitesimal doses paralyse certain elements, while in a large dose they leave the properties of others intact. This is the case with curare and with a certain number of substances whose effects are similar or analogous. Curare. — Curare is employed in an aqueous solution of y^rr strength injected usually under the skin or into the veins. According to its equality, its toxicity varies. Curare of good quality produces its ordinary effects in one centigramme doses (one cubic centimeter of the solution of 1 in 100) to the kilogramme of animal injected hypoder- mically ; when injected into a vein, the dose may be diminished by half. In an ordinary-sized frog, two to four drops suffice, injected hypodermic ally . Curare paralyses the motor nerves to the exclusion of the other elements of the nervous system and of those of the other tissues. An animal intoxicated by ciu-are becomes paralysed. If the excitability of the nerves and of the muscles is immediately investigated, it is seen that these last respond freely to the electric stimulus, while the stimulation of the motor nerves is without effect. Intoxication by curare is effected at the terminal extremity of the nerve ; in the new phraseology, by the distributing pole of the motor nexu'on. This may be proved by the following experiment : a frog is taken and a ligature is tightly tied round the loins ; thus all conxmunication is prevented by the vessels between the anterior and the posterior portion of the body, but the lumbar nerves are left outside the ligature, and are thus respected. When curare is injected into the anterior portion, the anterior members and the head are incapable of move- ment ; the posterior members on the contrary are not jaaralysed, and this in spite of the fact that the origins of their motor nerves and the spinal cord itself are bathed in the poison which is diffused through all the parts situated in front of the ligatm-e. If the frog is thrown into water, it swims with its posterior limbs until it reaches the edge of the vessel. In intoxication by curare the sensory nerves are respected. If in the frog thus 110 ELEMENTARY NERVOUS FUNCTIONS prepared one of the antei'ior feet, paralysed as regards movement, is pinched, the animal reacts by movements of its posterior limbs. In intoxication by curare a distinction is fnade between nerves of animal life and those of organic life. If intoxication is effected with a very small dose, the motor nerves of the skeleton are alone paralysed, the nerves of organic life remain active ; the pupil dilates and contracts, as also the vessels, the stomach, the intestine, etc. But if the dose be increased, these nerves in their turn will be paralysed. The intrinsic nerves of the heart resist the longest and, in cold- blooded animals, the heart preserves its movements even when large doses are given. Struck by the fact that the paralysis of the motor nerves is only produced in proportion as ciu-are is administered by its muscular ending, Vulpian concluded that this substance acts locally and in an elective fashion on the terminal plate of the motor nerve, the rest of the nerve preserving its properties, but without the possibility of manifesting them, on account of its temporary separation from the muscle. Against this view CI. Bernard cited the following fact, the knowledge of which is due to him, as that of all the preceding facts. If, during the time that the intoxication is in progress, the excitability of the different portions of the nerves be investigated, it is seen that the latter disappears, not totally and at once, but gradually and successively from the spinal cord to the muscle. There is a kind of paradox in the fact that the paralysis commences in the ex- tremity opposed to that by which the poison is supposed to enter into contact with the nerve in order to affect the latter. A. D. Waller remarks that the different varieties of curare are not identical. Having experimented with curarine chemically isolated from curare, he finds that this substance produces motor paralysis like curare, but permits the nega- tive variation of the motor nerve to persist. Tlius motor paralysis would be due to a functional dissociation of the nerve and of the muscle, according to the hypothesis of Vulpian. The anaesthetics or general poisons, on the contrary, attack the nerve protoplasm and suppress the negative variation, in other words, the special activity of this protoplasm. Strychnine. — At the first glance strychnine produces effects exactly the con- verse to those of curare. Poisoning by strychnine is rendered evident by con- vulsive attacks, to which the most trifling stimulation of the extremities of the sensory nerves gives rise. This fact is expressed by saying that this substance increases the reflex excitability of the spincd cord. When the convulsive attacks are violently repeated a certain munber of times, the motor nerves themselves become inexcitable ; this may be partially due to the fatigue of the system which results from over-stimulation. Martin-Magron and Vulpian have nevertheless observed that, when given in large doses, strychnine destroys the excitability of the motor nerves, even if the latter liad l^een previously separated from the spinal cord. This is a fact which must be reckoned with, but it is not of a nature to explain the convulsions at the commencement and which are produced by doses far from large, and which is the striking fact. Knowing, as will be ex- plained further on, that the spinal cord contains both motor and inhibitory elements as regards movement, it is easily understood that naeduUary hyper- excitability is the result of a paralysing action, if this paralysis affects unequally the two species of elements, the inhibitory tnore than the motor. On the other hand, it is possible to explain how moderate closes of strychnine paralyse the motor nerves of the skeleton without convulsion, when these nerves are separated from the cord, as they do not contain, like tlie latter, inhibitory elements. Strychnine would thus be indeed a cixrare, but one affecting preferably the inhibitory nerves of the animal nervous system, which are hidden away in the interior of the vertebral column, in the cord and the superior centres. NERVE ENERGIES 111 One milligramme of hydrochloride of strychnine suffices to kill an adult rabbit ; 2| niilligrammes to 3 milligi-ammes will kill a moderate-sized dog ; 1 centigramme hypodermically, 2 centigrammes taken by the mouth, will jjlace the life of man in danger (Vulpian). Upas-antiar has an action closely analogous to that of strychnine, at least as regards certain of its effects (Doyon). Belladonna, Atropine. — Atropine, like the substances which are still to be mentioned, has an elective action on the vegetative nervous system, and more particularly on certain of its nerves. — As regards the secretory nerves, especiallj^ the nerves of sudation and of salivation, it acts like curare, by destroying their excitability, and consequently by preventing the secretion of these glands. As regards the nerves of the heart, it selects their inhibitory elements ; it causes a sort of convulsion of the heart, by hastening its beats. In this case, as the inhibitory cardiac nerves (by the pneumogastric) extend away from the vertebral column and are distinct from its motor nerves (contained in the great sym- pathetic), it has been possible to prove directly that atropine naakes them inexcitable. A half milligi'amme of suljihate of atropine, injected hypodermically, suffices, in man, to demonstrate the effect of the poison through a diminution of the saliva and sweat secretion and a very slight acceleration of the heart. Substitu- tutes for atro]iine are hyosciamine, daturine, duhoisine. Jaborandi, Pilocarpine. — The effects of pilocarpine are apparently the exact reverse of those of the preceding substance. It paralyses the accelerators of the heart (consequently slowing its beats, which atropine hastens) ; it convulses the sudoriparous and salivary glands, whose inhibitory elements, it must be allowed, are paralysed. This antagonism is indeed reversible or, as is still said, bilateral, in this sense that, by the alternative and superposed actions of the two poisons, it is possible to invert the effects a certain number of times (provided that a certain dose is not exceeded). The antagonism is not between the two substances which chemically neutralize each other, but between the systems of nerves {motor and inhibitory) which oppose one another in their function, and which the substances attack eleetively, the one preferably to the other, according to circmiistances. Substitutes for pilocarpine are eserine and muscarine. Bibliography Fatigue of the Nerves. — Different Alterations, Indefatigability. — Abelous, Arch, de ■phys., 1893, p. 437. — Abelous, Charrin et Langlois, Ibid., 1892, p. 721. — Abelous et Laxglois, Ibid., 1892, p. -165. — Albanese, Arch. ital. de biol., 1892 et 1893. — Bow- ditch, Arch. f. Anal, und Phys., 1390, p. 505. — Carvallo, C. B. Acad, sc, 1900, t. 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" Electrophysiologie " dans Diet, encycloped. des sc. med. — Ch. Richet, Physiol, des muscles et desnerfs. — Schift, Lehrbuch d. Musk, iind Nerv., Phys. physioL, 1858. — Strong, Pliysical theory, Journ. of Phys., 1900. — Valentin, Arch. f. d. ges. Phys., I. — VoLTA, Electricite dite animale, Ann. de chimie, 1797-1799 ; CoUez. dell, opere, Firenze, 1816. — G. Weiss, Technique d'electrophysiologie, Paris, Masson. En- cyclopedie des aide-memoire. — Wedinsky, Arch. f. Anat. und Phys., 1883. — Wundt, Traite de phys. et mecanique des nerfs. Negative Variation. — Bernstein, Ub. reflector, neg. Sehwank. d. Nerv. strom. und die Reizleit. im Reflexwegen, Arch. Psychiatr. Nervenkr.. XXX, 651. — Biedermann, Nerfs sans niyeline . . ., Sitz. Wien. Acad., XCIII, Abt. III. — Borruttau, Seuil de la variation, Arrh. f. d. ges. Phys., LXV, 1. — Danilewsky, Cerveau . . . C. P. 1891. — Du Bois-Reymond, Untersuchungen. — Charpentier, Oscill. nerveuses, C. R. Acad, sc, 1899, t. 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Anat. und Phys., 1893. — GrCnhagen, Die electromot. Eigensch. lebend. Gewebe, Berlin, 1873, et Arch. f. d. ges. Phys., 1873. ■ — Grutzner, Arch. f. d. ges. Phys., XXVIII. — Hallsten, Nerfs sensibles. Arch. f. Anat. und Phys., 1880. — Helmholtz, Opt. phys. — Hermann, Arch. Pfliiger, 1874 et 1879. — Kuhne, Unters. lib. d. Protoplasma, 1864. — Martin-Magron et Fernet, C. R. Acad, sc, I860.— Matteucci, C. R. Acad, sc, 1838, 1863, 1867, 1868 ; Traite des phe- nom. electrophysiol. des animaaix, 1844. — Mendelssohn, Electrotonus des nerfs sans myeline, Biol., 1900 ; N. sensitif, C. R. Acad, sc, 1884 ; Art. "Electrotonus" dans Diet, de phys., index bibliographicjue. — IMorat et Toussaint, Electrotonus dans I'excitation unipolaire, C. R. Acad, sc, LXXXIV. — Nobili, Ann. de chimie et de physique, 1830. — Piotrowski, Arch. f. Anat. und Phys., 1893. — PrLf ger, Unters. iib. d. Physiol, d. Elec- trotonus, 1859. — Regnauld, Journ. de physiol., 1588. — Schiff, Lehrb. d. Muskel und Nervenphys., 1858. — Stewart, Journ. of Phys., 1888 et 1889. — Tigerstedt, Studien iib. median. Nervenreizung, Acta Soc. Fennicae, 1880 ; Arbeit, phys. Lab., Stockholm, III. — Teschiriew, Arch. f. Anat. und Phys., 1879-1883. — Valentin, Lehrb. Physiol, d. Mensch., 1848. — Verworne, Galvanotropisme ; Act. electr. sur protoplasma, Allgem. Physiol., 1895 ; Arch. f. d. ges. Phys., LXII. — Waller, Journ. of Physiol., 1898. — Waller et Watteville, Electrotonus chez I'homme, Philos. Trans., 1882. — W^atte- ville, Electrot. chez I'homme, These, 1883. — Wedensky, Arch, de phys. norm, et path., IgQl. — Werigo, Arch. f. d. ges. Phys., 1885. — Wundt, Unters. z. Median, d. Nerven und Nervencentra, 1871. — Zurhelle, Nerfs sensitifs, Unter. Lab., Bonn, 1865 ; These Berlin, 1864. Anaesthetics. — Arloing, These doctorat, Lyon, 1879 ; Identite des conditions pour obtenir Tanesthesie generale dans les animaux et les vegetaux, Soc. sc med., Lyon, 1882 ; Biol., 1882 ; Nouveau mode d'administr. ether, chlorof., chloral a la sensitive ; application a la determination de la vitesse des licjuides dans les organes de cette plante, C. R. Acad, sc, 1879 ; Influence des feuilles envisagees coninie cavise de la circulation des lic^uides nutritifs, Soc. agricult., Lyon, 1883. — Cl. Bernard, Legons sur les anes- thesiques et I'asphyxie, Paris, 1875. — Budin et Coyne, Etat de la pupille, Biol., 1875 ; Arch, de phys. — Dastre, Les anesthesiques, physiologie et applications chirurgicales, Paris, 1890 ; Revue sc med., art. " Chloral," 1881. — R. Dubois. Anesthesie physiologique et ses applications, Paris, 1894. — Flourens, C. R. Acad, sc.,1851. — Joteyko et Stefan- owska, Anesth. gen. et anesth. locale, C. R. Acad, sc, 1889 ; Biol., 1902. — Richet, Art. NERVE ENERGIES 113 "Anesthesie" dans Dictionnaire, index bibliographique. — Soulier, Traite de thera- peutique. — Vulpiak, C. R. Acad, sc, 1878. Cocaine. — Albertoni, Protoplasma, Arch. ital. de biol., 1891. — Von Anrep, Arch, de Pfliiger, 1880, p. 38. — Belmondo, Ecorce cerebrale, Lo Sperimentale, 1891. — Dastre, Bevue sc. med., 1892. — -Ferreira da Silva, Reaction . . .,Journ. de pharm. etde chim., XII. — Fran(.ois-Franck, Techn. exper.. Arch, de phys., 1892. — Gley, Biol., 1891. Mosso, Arch. ital. de fetoL,1891. Curare. — Cl. Bernard, Physiologie experimentale ; snbst. toxiq. et medic. ; science experimentale. — Bezold, Arch. Beich. et du Bois-Beymond, 1860. — Bochefontaine et Tiryakan, Conine . . ., C. B. Acad, sc, 1878. — Boehm, Chemisch. Stud., Beitrag. z. Phys. Carl Ludvng. Festschr., 1887 ; Arch, de Pfliiger, 1875 et 1894 ; Arch, de pharm., 1897. — Gouty, Act. convulsi\^, C. B. Acad, sc, 1882. — Ciu-are et strych.. Ibid., 1882. — CouTY et Lacerda, Excit. muscul. du debut . . ., Biol., 1880 ; Le curare, son origine son action, ses usages. Arch, de phys., 1880. — Dastre, Action du curare, Biol., 1884. — JoLYET et Pelissard, Biol., 1868. — Joteko, Curare et poisons curarisants. Diet, de phys., index bibliogr. — Hermann, Bevue sc. med., 1879, t. XIV. — Kolliker, Arch. f. Anat. und Phys., 1856. — W. Mitchell, Journ. de la phys., 1862. — A. Moreau, C. B. Acad, sc, 1860. — Pelikan, Bull. Acad, sc, Saint-Petersbourg, 1857, t. III. — Pollitzer, Journ. of Phys., 1886. — Schiff, Ges. Beitr. z. Phys. 1896. — Tarchanoff, Arch.de phys., 1875. — VoisiN et Liouville, Journ. d'anat. et de physioL, 1867. — Vulpian, Ai-ch. de jihys., 1876 ; Lemons sur les svibst. toxicjues. Strychnine. — Cl. Bernard, Subst. toxiq. et medicamenteuses. — Couty, C. B. Acad, sc, 1883. — Delaunoy, C. B. Acad, sc, 1881. — Delezenne, Act. vaso-dilat., Arch, de phys.. 1894. — M. Doyon, Upas antiar, Arch, de phys., 1882. — E. Heckel, Bevue scient., 1879. — Magendie, C. B. Acad, sc, memoire, 1809. — Martin-Magron et BuissoN, Strych. et cxirare, Journ. de Br.-Seq., t. II, III et IV. — Th. Mays, Brucine et strychnine, Journ. of Phys., 1887. — A. Moreau, Biol., 1865. — Ch. Richet, C. B. Acad, s'c, 1880, t. XCI, p. 131. — ScHiFF, Infl. pupille. Arch, de Pfliiger, 1871. — Stannius, Mailer's Arch., 1837, p. 223. — Verworn, Arch. f. Anat. und Phys., 1900. — Vulpian, Arch, de phys., 1870 ; C. B. Acad, sc, 1882, t. XCIV, p. 555 : Lemons svu* subst. toxiq. et medicament., Paris, O. Doin, 1882. Atropine. — Heidenhain, Arch, de Pfliiger, 1872. — Iveuchel, Tliese Dorpat, 1868. — A. Marcacci, Cinchonamine . . ., Arch. ital. de biol., 1888. — A. Meuriot, These Paris, 1868 (bibhographie). — Nuel, Art. (Atropine) Diet, de phys., index bibhogr. — Schmiede- BERG, Ber. d. Sach. Gesell. f. Wiss., 1870. — Schiff, Moleschotfs Unters., 1865. Jaborandi. — Pilocarpine. — Gysi, These Berne, 1879. — Hardy, Bevue sc. med., t. XI, 1878, p. 767. — Langley, Stud. fr. the Phys. Labor., Univers. Cambridge, 1877. — Ortille, Bull, therap., 1879. — Oulmont et Laurent, Hyosc. et datur.. Arch, phys., 1870. — PiLiciER, These Berne, 1875. — P1T013, These Paris, 1879, No. 162.— Rabuteau, U7iion medicale, 1874. — Robillard, These de Lille, 1881. — A. Robin, Etud. physiol. et therap. sur jabor., Paris, Masson. — Vulpian, Lemons sur les subst. tox. et medicam. Antagonism. — J.-N. Langley, Brit. med. Journ., 1875 : Journ. of Anat. and Phys., 1880-1882. — LucHSiNGER, Arch, de Pfliiger, 1877 et suiv. — Morat, Bevue scient., 1892 ; Diet, de phys., art. " Antagonisme." — Is. Ott, Muscar. et atrop., Journ. of Phys., 1878- 1879. — Prevost, Arch, de phys., 1877, et C. B. Acad, sc — Rossbach, Arch, de Pfluger, 1875. — Sydney Ringer and Mokshead, Atrop. piloc, Journ. of Phys., 1879-1880. — Schmiedeberg, Elem. de pharmacodyn., trad, franc, 1893. — Sticker, Centralol. f Jclin. Med., 1892. Anaemia. — Cl. Bernard, Rapport sur la physiologie, 1867. — Richet, Les nerfs et les muscles. — Stefani et Cavazzani, Arch. ital. de biol., 1888, t. X, p. 202. FART II Systematic Functions The functions known as systematic differ from cellular functions as the whole differs from the part ; they are the functions ichich originate in the associations and definite relationships which are established between the cellular functions, just as the systems which thej^ sustain arise them- selves from the associations and relationships effected between their component cells. These functions and these systems form, the first from the dynamic point of view, the second from the static, perfectly coherent aggregations ; the animal organism itself is nothing more than an aggregation of this description whose nervous system helps to consolidate all the separate portions. By analysis this organism may be resolved into aggregations of the same nature, genuine partial systems, of which the surfaces of separation are continued into the nervous system in different directions. The definition of their fre- quently changing hmits, their specific functions, their mutual relation- ships, is the end which it is the object of the study of the nervous system, regarded as a system, to attain ; an end, in the present state of knowledge, still very imperfectly realized. The conception of the system. — A system is a whole composed of parts liaving a determinate imion amongst theniselves whicli gives to the whole, that is to say to the system, its cohesion, its existence. A system is at the same time both a unity and an aggregation : it is one or the other, according to the tenoiu' of the ideal or experimental analysis to which it is submitted. In the universe nothing is isolated ; every system is united to other systems by bonds external to itself ; if these bonds with the exterior are broken, the system appears as a vmity ; if again, its internal ties be abolished, it is resolved into its component elements. All the developments which follow each other in the nervous system show that tlais view is correct : the system contains bonds of union by wliich it is comiected with the external world ; on the other hand, its constituent parts are united together in such a way as to form a unity of which we are ourselves coruscious. The conception of an element. — Taken in an absolute sense, the word element signifies the irreducible limit to which analysis in a given science leads vis. Re- garded in a relative sense, it signifies some component part which analysis has isolated from a systematized whole. In an organized whole, such as the living being, the component parts which analysis demonstrates are themselves organ- ized wlioles, are systems in the real sense of the word, and these systems may be 116 SYSTEMATIC FUNCTIONS themselves resolved into still more simple systems. Biologists frequently describe the elements as the component cells of the tissues. In spite of the fact that it is now understood that the cell is not an ultimate element, but that it is itself a system of microscopic dimensions, this term will be employed, because, in biological analysis, the cell expresses a limit far better characterized than any other ; this is dvxe to its boundaries being well defined and to its clearly being a \xnit. Thus the nervous system is composed of partial or sub-systems, m.ore or less resembling it, and analysis of these partial systems leads us, as in the case of other apj^aratus, to cellular elements. These elements have been studied in the first part of this work and now, being better known as regards their limits and consti- tution, are described as neurons. We have seen that, far from being homogeneous and incapable of subdivision, they present an internal organization and differen- tiated functions. The conception of function. — In the living being all is function, because in it the whole tends to a determinate end. The idea of function is closely connected with and is superposed to that of system. Function represents the dynamic aspect of the living being, whose system represents the static aspect ; it is nothing else than the bond of union which co-ordinates the several portions of a system and confers on it its unity. Two kinds of function. — The study of the organism, that of its nervous system which reproduces its chief features, that of the partial systems into which it can be decomposed, shows that there are in it two kinds of bonds of union : some entirely internal, unite the different portions of the system so that its unity results ; others, external, connect it with still larger aggregations, on which it depends and of which it forms part. Thus, from the very nature of things, there must always be two kinds of fvmctions ; the first, of a purely internal order, are called those of conservation, which in living beings may be generally described as functions of nutrition ; the second are, on the contrary, functions of external relation, or of relation properly so called, of the organism or of the system under consideration with other organs and other systems of the same value. External and internal relations. — It is perfectly obvious that the nervous system possesses these two orders of relationships and of functions. By it we are brought into relationship with our fellow-creatures, and it connects the com- Ijonent parts of our being by its internal bonds of union. But in proportion as we resolve it, by experiment, or merely by thought, into its component sub- systems of continually decreasing importance, the two orders of function become attributable to each of these units ; each of these has its functions of conserva- tion and of relation, and the same function is at once that of relation and that of conservation, according as to whether the organs which it tmites are regarded as separate systems or as a coherent whole. Plasticity of the nervous system. — And, as a matter of fact, these systems are susceptible of isolation and of fusion ; thanks to the extreme plasticity of the nervous system, they approximate or separate according to extremely varied modes. Without absolutely breaking, the bonds of union relax at certain places while they become stronger at others. On account of this mobility and this gradation, the study of the fiinctions of the systems is extremely difficult, but is none the less essential. In order to thoroughly understand it, it must be borne in mind that the phenomena of conscious sensation render necessary the associa- tion of a large number of co-ordinated elements ; in other words, they are only developed in systematized assemblages of neurons. If, indeed, movement is everywhere present (whether visible or invisible) in cellular function, it is not the same as regards consciousness : conscious functicms are essentially systematic functions. PRIMARY DATA 117 PRIMARY DATA Relations of sensation and of movement. — Whether functions express external or internal relations of the organism, that is to say, external or internal to a definite system, they are all based on fixed relations between those two pheno- mena which are knowTi as movement and sensation. On the more or less com- plicated bond of union which exists between these two phenomena depends the importance of the function, and that of the system which brings it into being. In proportion as the organization becomes more complicated, that is to say, as the system becomes more closely united and superposed, the sensory pheno- menon becomes more obvious and, in its turn, presides over more differentiated iind more varied movements. This will be made clearly evident by analytical and detailed study of each one of the functions of the system. Only, in order to make this study more intelligible in proportion to the extent to which it is carried, it is necessary to recall some definitions and to clear up certain pomts concerning the two phenomena which are essential to the performance of every function of the living being. The study of the fxmction of innervation brings us face to face with two cate- gories of facts which our present means of observation and of analysis are unable to further resolve : these are on the one hand motion, and on the other hand sensation. Not that this study does not enable us to recognize at every moment theii- relations and their dependences ; but, doubtless, tlirough the imperfection of the information which it supplies concerning both, it does not suffice to fill up the gap which separates them ; hence it is necessary to accept the phenomenal dviality which cUstinguishes them. Origin of the two ideas. — The idea of movement comes to us primarily from the exterior. Our o^^^l special movements are only known to us as such artifi- cially and by reasoning (reciprocal control of the different senses) ; as regards the inmost movements of our being, we can and shall attain a knowledge of them only by employment with ever-increasing assiduity of efforts of reasoning and of scientific analysis. On the other hand, sensation is a fact internal to ourselves. The sensations of others, similar to those which we j^erceive in om-selves, are only kno-wTi to us artificially and by reasoning (by comparison of the motor effects of these sensa- tions in ourselves and in them) ; as regards the most elementary sensations of beings other than ourselves, they are and will be known to us only by continued efforts of reasoning and of scientific analysis ; the method of study of the two l^henomena, however contrary these phenomena may be, cannot differ funda- mentally. In short, we clearly perceive that external movement is continued in our- selves, and that sensation exists apart from ourselves ; but we do not succeed in our efforts to superpose the two phenomena (neither in nor externally to our- selves) so precisely that the two may seem to us as being merely two points of view, two aspects, two varieties of one and the same thing, instead of being two distinct categories of phenomena incapable of resolution. Anatysis of the two phenomena. — In the present state of our knowledge, the study of movement is infinitely more advanced than is that of sensation. Hence movement serves not mereh^ as a control, but also as a model for the study of sensation. We know that complex movements are capable of resolution into simpler ones, and that this decomposition may be carried to a greater or lesser extent, without om' being able to arrive at, in an absolutelj^ certain manner, the first elements of movement. It is the same as regards sensation : there are complex sensations which surround elements capable of isolation, themselves more or less easily decomposable into still simpler elements. It is obvious at the first glance that 118 SYSTEMATIC FUNCTIONS analysis may be carried much farther in the case of movement than in that of sensation. For a sensation to be presented to ovir consciousness, it is necessarj^ that it arise in a nervous field developed in a very complex organization. On the contrary, if we endeavour to study synthetically the movements which corre- spond to it, especially in the nervous system, we shall be greatly hindered in our efforts.! Once more, the two orders of perception approximate one another, but the boundaries between them overlie one another, but are not sujjerposed. Process of association. — The most simple sensation of which we are conscious is thus, in itself, complex, and this complexity conceals a progressive series of _ ojDerations in some degree based on the unity of the result. To adopt the more concrete language of anatomy, sensation is not a cellular phenomenon, it is a sys- tematic function. — If sensation is real, it implies the association of elements which proceed from one of our senses to the cerebral cortex ; if it is a recalled sensation, a memory, it implies the co-operation of cerebral elements appertaining at least to the cortex, and perhaps to other portions of the brain, for in this case the limit is not easily determined. In any case it is a phenomenon of evolution, which implies a process of association as regards the cellular elements of the nervous system, or, more exactly, of the dynamic unities corresponding to their cellular unities. Threshold of consciousness. — Consciousness has its degrees just as its field has also its variable limits. Every act which takes place in ourselves and of which we are not conscious may become conscious by the mechanism of the internal phenomenon which is known as attention. A moment ago, this act was below the threshold of consciousness ; then it has attained the level in which it becomes clearly conscious. This threshold corresponds therefore to a degree of consciousness somewhat arbitrarily fixed, in order to eliminate from the consciousness everything which has only a doubtful value or is perceptible with difficulty. Practically, w^e relegate to the unconscious all bare consciousness of the being, and we retain the name conscious for the recognition, more or less analytical and detailed, of the being. Simple sensation. — If sensation presents degrees and shades in its intensity, it presents yet more of these in its complexity. We accept, as we have said, as elementary a fact which we know is fundamentally complex, but which resists that internal analysis to which we endeavour to subject it in ourselves. This fact is what is known as simple sensation. The prick of a needle, the sight of a luminous point, the hearing of a short sound, supply us with ordinary examples of it. Simple sensation is yet accompanied with perception ; the object is per- ceived as being such in its owti character ; that is to say, it is recognized. If perception is wanting, there is merely crude sensation. Specific modalities. — These different examples of sensation, touch, light, sound, etc., represent what is known as specific varieties (incapable of reduction the one into the other) of sensation ; further, each one of thein may include various gradations (colour, tonality, etc.). Complex sensations.- — Simple sensations of varying gradations combine among themselves to form complex sensations, in which the component elements are so fvised together as no longer to apjDear distinct from one another ; every sense furnishes examples of this nature. The sjiecific sensations of the different senses are combined, in their turn, to form a jDhenomenon which no longer bears the 1 If this should cause surprise, it must be remembered that our organism is constructed for a practical and not for a speciilative end. Sensations which should be localized in areas which sliould correspond to our component cells would be, by their excessive accuracy of localization, useless to us ; just as direct syntlietic vision of movements which corre- spond to an ordinary sensation would distract us from the consideration of much more simple external movements, which it is important for us, on the contrary, to be acquainted with. PRIMARY DATA 119 name of sensation and wliich marks definite progress in the evolution of this series of internal j^henomena. Idea. — By their association, these sensations, whose form, source and com- plexity are so different, give origin in their turn to a new psychical process still more complex than themselves ; this is ideation. The origin of sensation, start- ing from these elements, is unkno-mi to us as regards its mechanism, because they themselves are not accessible to consciousness : in other words, conscious- ness onh^ becomes clear in proportion as these elements are associated in a sensa- tion : when isolated, they elude it. The genesis of ideation, by the association of sensations, becomes on the other hand accessible to internal observation. Psychology has always extensively employed this method, in order to study the formation of ideas. At the present time progress in this direction has been made by causing external observation and experiment to be made use of in this study. In man and animals lesions of the nervous system either exist or are induced experimentally, by which these complex manifestations of consciousness are disconnected, the one being suppressed, the others being allowed to continue ; and thus we gain an insight, although a feeble, and too often uncertain one, concerning the conditions of their existence. Cognition ; Recognition. — The psychical elements which form sensations and ideas are not merely associated in an actual and contemporaneous fashion, but also tlie regular functional activity of the nervous system further provides them with a bond, an association, originating therefore a continuity in time. New ideas and sensations which arise in us recall the existence of sensations and of anterior ideas of the same order ; we recognize the objects, movements, pheno- mena, symbols which have already made an impression upon us, when this imjDression is renewed. This recall of sensations and of previous ideas, which seemed to be effaced, implies that they were in reality preserved in a latent dis- sinuilated condition, which is called, specifically, the unconsciotts state. New impressions bring them back to the tlu-eshold of consciousness. This is the rememljrance or recognition of phenomena with which we have already been in touch. Residue. — In other words, every impression, every sensation leaves a residue in VIS. New sensations of the same order are added to it, conseqviently are asso- ciated in it, by recalling it to actuality. This identification of the new pheno- menon with the old through time permits us to recognize it ; were tliis identifica- tion wanting, there would be for us no experience of the past, the action of the external world on our senses would be continually a new one, that is to say, one perpetually unknown. Remark. — It may seem that these data on psychical processes would be more appropriately placed at the end of the study of the nervous functions, as being its finishing point, and not at its commencement. We shall meet with them again in the analysis of a certain number of special cases, particularly the func- tion of language ; but from the very first steps wliich we shall take in this study, it is precisely these phenomena of sensation that we meet with, occurring as they do as the inevitable consequence of every investigation, of every analysis of the nervous system. In the study of moveinent we may, if we wish, proceed from the simple to the compound ; in the order of sensation, an already synthetized phenomenon is offered us, as the first discernible dattun. Hence we are com- pelled to describe it smmnarily at first, at the risk of being forced to justify ovir affirmations, in proportion as the facts resulting from observation and experi- ment shall be displayed before us. Less than any other science can psychology proceed by deduction ; in the living being everything is allied, everything is closely connected ; hence it results that, in the study of its fiinctions, we must joroceed from the comjDlex to the simple in many cases. FIRST SECTION NERVOUS ORGANIZATION At the foundation of nervous organization lies a bond of union, a reciprocal dependence between sensation and motion. Its perfection is obtained through multiple and graduated forms which affect both phenomena, as also from the numberless associations wliich they are capable of effecting. The centripetal paths convey impulses which are from their origin both multiple and diverse (sense organs) ; the centrifugal routes terminate in muscles (or equivalent organs), them- selves both numerous and diverse, for the performance of visible or concealed acts, which indicate the end of the evolution of the nervous process. From the commencement to the end of the latter sensory phenomena are interpolated ; its value increases with the complica- tion of the nervous paths, followed by the impulse, and also by the length of time which it occupies in these paths. This complication is based on a regular system ; it proceeds on simple lines and on a plan, the object of which can be recognized. First of aU it is necessary to describe this general plan, by which the organiza- tion of impulses is rendered intelligible. By adding to it the data furnished by experiment, we shall be able to observe the progress of these latter, their results now divergent, now parallel, and again diver- gent ; their numerous conflicts ; their reinforcements ; their divisions and their postponements : all the circumstances which do not explain to us these results of internal observation which we describe as sensation and ideation, yet which manifestly give rise to them and give us command of them in experimental practice, and, nowadays, in the treatment of nervous diseases. Its scheme. — Anatomically a distinction is made between a peri- pheral nervous system and a deep nervous system (the latter being generally known as central). (a) Inferior system. — This distinction is justified physiologically on the condi- tion of a division being made, not at the termination and at the apparent origin of the sensory and motor roots, but at their real termination and origin in the grey substance of the medulla oblongata and spinal cord. The conventional limit of the two systems is defined in this grey matter. The long cylinder which the latter forms is the direct rendezvous of periplieral 120 NERVOUS ORGANIZATION 121 impressions, and is at the same time tlie point of immediate departure of the motor reactions ; it is a spot in whicli the imjaulses are systematized and organ- ized ; it is the keystone of the peripheral or inferior system. (b) Superior system. — Another locality, in which grey matter, assuming the form of a folded sphere, is found, is the surface of the brain, and is united to the preceding by connexions in which the conduction is, on the one hand, ascending and on the other descending, by which impulses are conveyed to it and by which they are carried from it. It has no communication with the exterior except in an indirect and mediate manner ; it works on the niaterials prepared by the grey axis, to which it gives a new organization, and it reacts on the exterior by the intermediation of this same grey axis, whose motor associations, also ready prepared, it makes use of : it is the keystone of the deep or superior system. T)ie conductors which carry the impulse (in two different directions) between the periphery and the grey axis are the so-called fibres of projection of the first order. Those which convey it between the two areas, previously pointed out, of the grey matter are fibres of projection of the second order. Extension of the grey axis beyond the spinal column. — Ganglia of the great sympathetic. — This very simple scheme requires some corrections and additions in order to accT.U'ately represent the real condition. Externally to the spinal canal, the gi'ey axis is prolonged in the form of small gi'ey disseminated masses, the ganglia of the great sympathetic, which jaossess the sensori-motor functions of the spinal cord. Superior prolongations of the grey axis. — High up, above the medvilla oblon- gata, the grey axis, after it has gathered together all the conductors of general sensation, is surmounted by discontinuous masses (the internal and external geniculate bodies, the corpora qv/xdrigemina, anterior and posterior, the mam- millary bodies), which appertain to the special senses (hearing, vision, smell) and form, from a fmictional point of view, differentiated prolongations of this same axis. Still higher occm"s an important grey mass, the optic thalamus, which introduces a new complication into this edifice, hitherto apparently so simple. This mass, composed of distinct areas, of which each belongs to one of the modes of sensation which are diffused through the nervous system, is no longer placed at the union of the fibres of projection of the first and second order, but in the very coui-se of these latter. To express the matter more clearly, the ascending or sensory fibres, which proceed from the grey axis to the cerebral cortex, are inteiTupted in the optic thalamus, the larger proportion of them as regards the majority of observers, entirely for others. This very obvious interruption is not the only one which the paths of ascending imjaulses undergo ; similar inter- i-uptions are found from the spinal cord, so that these paths are divided into two species : the one long, proceeding from the grey axis to the cortex, the other short, whose length varies according to circumstances. Another organ, which must also be regarded as possessing the importance of a system, is the cerebellum, which, through the connexions uniting it to the grey axis and to the cortex, comi^licates yet more this assemblage, which is known, in its totality, as the superior or dee}? system. In every case, the covu-se of the impulses which traverse this suj^erior system is extremely varied and, from this point of view, contrasts with the relative simplicity of their progress in the inferior systein. Equivalent juxtaposed systems. — In addition to the divsions which have just been pointed out, the nervous systeni possesses yet others in a quite different direction. In addition to the transverse incisions marking its stages, it displays fissm-es on its length which also have the value of differentiated systems. At its origin in the organs of sense, the nervous system affects territories as well as func- tions which are distinctly separated ; at the surface of the brain, the cortex (a 122 SYSTEMATIC FUNCTIONS remarkable fact) reproduces these divisions. It reproduces them by repeating, not merely the areas devoted to each sense, but certain topographical divisions and subdivisions of these territories themselves. The brain is metamerised like the spinal cord, the word metamerised being used, it is true, in its most general and not in its strictly embryological meaning. The optic thalamus offers a similar metamerisation, repeating that of the sensory, peripheral, medullary and, lastly, cortical sensory territories. The cerebellum, the least metamerised of these grey masses, has more special connexions with some senses than with others (equilibrium, sight, touch). Association of the systems. — Such is the plan, once again very simple, which divides the nervovis system into systems, no longer superposed, but juxtaposed, which I'eceive from the preceding ones their constituent elements. But once again, also, it must be repeated that these are merely the chief constructive outlines. They supjDort others, which ensure the multiple connexions between these equivalent bvit specifically differentiated systems, \vhich they render more obvious to us. From the spinal cord, the impulses coming from the periphery are co-ordinated by associations effected in the grey matter of the cord, and the impulses which, from the spinal cord are forwarded to the muscles, are in the same way organized by it. Of the original metamerisation of the sj^inal cord, but little remains in the case of the superior mammals, and above all in man, at the period of their complete development. The segments of the grey axis are fused into more and more niunerous functional associations, and special condi- tions must be present in order that the isolated function of these segments may be recognized. The optic thalamus presents connexions of the same kind, but still more marked. This mass of grey substance no longer collects merely the impressions arising from a single sovu-ce, like the spinal cord, the geniculate bodies and the corpora quadrigemina, but all those proceeding from all the senses which are rej^resented in it and which are in a certain measure organized therein. This great ganglion is qualified for the reflection of these impressions, when trans- formed, upon the organs of motion ; an essential role is attributed to it as regards- instinctive and emotional manifestations. In man the most i:)owerful organ of association is the brain. The sensorial systems, which terminate in its cortex, find in it and in the tangential fibres immediately subjacent to it those internal connexions which organize them. These same systems find in the commissures of varying lengths and direction which fiU"row the cerebral mass, the links which, in their turn, fuse them into a common functional activity. Functional localizations. — The nervous system is composed of unities, of elements, which, from the first to the last, have a differentiated function and one in a certain sense specific. But this differentiation is generally progressive and graduated, and resides principally in the connexion of these elements amongst themselves. For convenience of study, we sometimes voluntarily neglect these graduations, in order to collect together the objects and the phenomena imder a common type ; again, on the contrary, we give them undue imjDortance in order to mark, in the two cases, that distinction which is api^roj^riate between them ; for, otherwise, the infinite detail would lead to confusion. Hence have arisen the discussions between those who would localize and those who would not localize nervous functions, discussions arising miich more from the exaggera- tion of the individual point of view than from absolute error. Three points of view. — In order to render these discussions intelligible, it is necessary to clearly understand the natm-e of system and of function, such as- has been pointed out above. As regards the nervous system, three points of view which should be separated have often been confounded. These three points- NERVOUS ORGANIZATION 123 of view regard, tlie first the succession of phenomena, the second their equivaleyice, the third their gradation ; hence arise tliree orders of locahzation which have not ahvays been properly distinguished. (a) The several jaortions of the nervous system transmit acti\itj' in a definite direction. In this evolution sensation first appears, then the apparent move- ment by which functions are exercised ; hence we recognize in the nervous system a sensory portion and a motor portion successively arranged. (6) In the nervous system sensation and motion manifest different modalities which are developed in parallel and equivalent systems. The idea of localization corresponds above all to the distinction of these systems as ordinarily under- stood, that is to say, in so far as these systems become flush with the cerebral cortex at their superior terminations. (c) Lastly, whatever may be the order of sensation wliich governs the motor act (whatever may be the sensorial organ which furnishes the impressions) this sensation manifests degrees according to the more or less deep routes in which the impulses received in the organs of sense travel ; in other words, according to the height at which the impulses are reflected ; whence the division of acts into new categories of unequal value, of which the three principal are, from this jDoint of view, called automatic (reflection in the spinal cord), instinctive (reflec- tion in the oj^tic thalamus), vohmtary (reflection in the cerebral cortex). Con- sciousness has its degrees, which are determined by the more or less complex organization of impulses. Restriction. — None of these indications must be taken in an absolute sense ; not one of the phenomena to which they relate is an isolated one ; none of the sj^stems which support them is absolutely inde]iendent ; all mutually influence each other. Every time that some part of the nervous system is stimulated or suppressed there is at all events a temjiorary reaction of this modification on the whole system. This reaction is immediate or remote, and, in the latter case, is often so weak as to be inajDpreciable. The pertvu'bation thus induced varies according to the jDart of the nervous system which has been modified. Hence the inference may reasonably be drawn that the portions experimented upon possess different functions. But the argument is often carried still farther, the function being localized in the region experimentally modified, which has been the point of depar- ture of the observed disorder, and in doing this it is clear that the teachings of experiment are overstepped. The ner\-ous system is composed of parts which are at the same tune botli conjoint and functionally differentiated. CHAPTER I SENSATION AND MOTION— THEIR RELATIONSHIPS The ego in itself only perceives sensations, outside itself it only perceives movements. The movements of our own limbs would be for it merely those of foreign bodies were it not for the sensations which attach them to it and, indeed, they become such with regard to the ego when they are no longer sensitive. How does our ego recognize the existence of sensation in the case of beings other than ourselves ? By a process of reasoning founded on analogy, and not otherwise. Analogical proof. — If I wound an animal and it begins to struggle and cry, I say, without any doubt, that it feels ; seeing that it manifests the same reac- tions as those by wliich I express my own sensibility, I affirm that it also possesses the same, and I am able to estimate the quality and the amount of the sensation. Between it and inyself, as between myself and it, there is merely movement ; but in myself there is a definite link between sensation and movement, and this enables me to recognize sensation by means of movement, and by this indirect means to estimate it as a fact accessible both to observation and experiment. Law of continuity. — The more closely the motor reactions of the being are assimilated to mine, the greater I presume to be the resemblance of its power of sensation with my own. In projDortion as this resemblance givfes place to a more remote analogy, so does the an^iount of sensation which it represents become less and farther removed from that which is personal to myself. If, however, this degradation follows a regular progression, I shall then be in a position to connect the fact of this transformation and diminution with my power of sensa- tion, which is the only criterion available to me for estimating facts of this nature. It is the same with sensation as with life, wliich it characterizes : we see its field continually increasing, without oiu* being able to precisely define its limits ; and this increase is effected, at each new stage, by the adjunction of beings of a nature at the same time inferior and analogous to that of beings previously considered as being of the most elementary structm-e. Anthropomorphic reasoning. — Excej^t for the infinitesimal part which each one of us plays therein, a knowledge of the living world is based on antliropo- morphic reasoning, and it is impossible to base it on any other reasoning. Hence it is necessary to exert great prudence in employing it. A. THE ROOTS OF THE NERVOUS SYSTEM— THEIR FUNCTIONS The bond of union between sensation and motion is, in a living being, an obvious fact. We can destroy this bond ; we can cause movement and sensation SENSATION AND MOTION— THEIR RELATIONSHIPS 125 to reappear, isolated one from the other, bj^ making use of different portions of the nervous system. I. The Simple Facts ; General Laws 1. Nerve pairs. — The nervous system with its superior termination, the brain ; its fundamental portion, the spinal cord ; and its peripheral distribution, the nerves, being regarded as a whole, it will be noticed that these last are inserted along the whole length of the cord on each side by two roots, one dorsal (posterior in man, superior in animals), the other ventral (anterior in man, inferior in animals). These are indeed the roots of the nervous system, that is to say, the paths by which the relations of this system between it and the external world and with the exterior are established, and this in a double direction, and conversely. Thus symmetrically arranged, they form nerve pairs, one on each side, corresponding to each of the metameric divisions of the spinal cord. / P ost. root Fig. 49. — Two nerve pairss at their origin in the spinal cord. roots. Anterior and posterior As regards the upper pair, the figure shows the relations of the roots with the gvey axis and the fluted shape of the latter. In the lower pair is seen the emergence of the anterior roots^'at the surface of the spinal cord, and in the anterior collateral furrows. Distinct functions. — For a very long time it was supposed that these roots had different functions, and h\^otheses were elaborated with the object of pointing out the nature of these functions. To Ch. Bell (1811) belongs the merit of invoking the aid of experiment in order to solve this problem ; but, operating on animals at the very 126 SYSTEMATIC FUNCTIONS moment in which they were killed (rabbits killed by distension of the medulla oblongata), he was not able to determine the function of the posterior roots, which he considered to be devoid of sensation, and he merely detected the motor excitability of the anterior roots, which he considered, further, to be sensitive. In ordei' to understand tlie experiment of Ch. Bell and the signification which he attached to it, it is necessary to be cognisant of his point of view. Imbued witli the then current ideas of Willis concerning the nervous system, he endeav- oured to verify the latter. Willis regarded the brain as the centre of sensation and of movement, while the cerebellum presided over vital actions (circulation), nutrition, secretion, etc.). Guided by anatomical results, Charles Bell described the anterior roots as being conducting paths in direct continuation with -the brain through the intermediation of the crura cerebri, and the posterior roots as tracts directly connected with the cerebrum by the intermediation of the restiform bodies (it is now known that in reality the connexion is far less simple). The anterior roots would thus represent the functions of the brain (sensation and movement) ; the jDosterior roots those of the cerebellum (phenomena of nutrition). Experiment, as performed by Charles Bell, does not verify all these assump- tions : it only confirms one of them, the motor function of the anterior roots, but remains mute as regards all the others. On this account, indeed, it did not appear to liim contrary to the hypothesis which served as his point of dejoarture ; in any case, it seemed to liim sufficient to support the doctrine of Willis, which it was his object to verify. Ajiart from any hypothesis, however, the experiment of Charles Bell established a fact new for his epoch, namely, that the anterior root manifests functions which are not displayed by the posterior root ; in other words, that there is a functional difference between the one and the other root. As to the natvire of this difference, it wholly escaped him. It did not become clearly comprehensible (for him, as for every one else) until some years later, after the researches of Magendie on this subject. Nature of these functions. — To A. Magendie incontestably belongs the merit of having ascertained the truth on this point through his decisive experiments (1821). ^ Exjjeriment. — The experiment is easily made on the dog, and prefer- ably on a young animal (softness of the bones, length of the roots and the special arrangement of the dura mater, which directly invests the spinal cord). The animal is anaesthetized and is kept insensible during all the preliminary operations. It is only permitted to recover consciousness at the time when the 1 In France, as also abroad, the discovery of the separate functions of the nerve roots is generally attributed to Charles Bell. As a matter of fact, he misconstrued them, and they were exactly formulated for the first time by Magendie, by means of the very apt experiments which he employed to demonstrate them. Tliis has been recognized and maintained by all the authors wlio have studied this question of priority. (Consult CI. Bernard, De la physiologie generale, pp. 15 and 216. — Vulpian, Le(^ons sur la physiologic ginerale du systeme nerveux, p. 109 et suiw — Chauveau, Journal de Vanatomie et de la physiologie. — A. Waller, Elements de jjhysiologie humaine, p. 596.) SENSATION AND MOTION— THEIR RELATIONSHIPS 12: Co :^ roots are acted upon. A limited region being taken for investigation, as that of the posterior hmbs, the ^ roots of the nerves which correspond to them in the lumbo-sacral region of the spinal cord are laid bare. For example, all the so-called posterior roots (superior in the animal) are cut on the right, and on the left all the so-called anterior roots (inferior in the animal). (a) Effects of the section of the roots. — x\s a result of these sections, the limb on the right side continues to move, but ceases to be sensitive ; it may be pricked, pressed or burnt without provoking reactions on the part of the animal. The limb on the left side continues to be sensitive, hut ceases to move ; the animal is incapable of using it for walking or for any other purpose. Hence it follows that the posterior root is seyisory, that is to say is connected with the exercise of- sensa- tion ; the anterior root is motor, that is to say, connected with the per- formance of movement. (b) Effects of stimulation. — By cutting the posterior and anterior roots we have not destroyed or sup- pressed the organs of sensation and of movement, but we have inter- rupted the paths by which the impulse which arouses the first or excites the second is conveyed. In- deed, if the central end of a posterior root is irritated, the animal reacts, that is to say, feels ; if the peripheral end of an anterior root is irritated, the corresponding limb is moved. If, conversely, the peripheral end of a posterior root is irritated, or the central end of an anterior root, no reaction of such a nature as Fig. 50. — Medullary roots of the lum- bar and sacral regions in the dog. The diagram shows the point of emerg- ence of each of the roots from the spinal cord in relation to its exit from the inter- vertebral foramen. Obliquity and relative length of the sixth and seventh lumbars and the first sacral. Co, last rib. 1 to 7, transverse apophy- ses of the lumbar vertebrae, of wliich the posterior arch is removed. Is, illiac bone. 128 SYSTEMATIC FUNCTIONS those which have just been referred to, either sensory or motor, results. By this second test a new conception, and a very important one, is added to that of separation or distinction between sensation and move- ment ; that, namely, of direction, of orientation, or of polarization, as it is generally termed. Laws of Magendie.— 1. The posterior or dorsal root conducts impulses from the periphery to the central portions of the nervous system, to the special organs of sensation ; it is sensory. 2. The anterior or ventral root conducts impulses from the central portions to the periphery, to the special organs of movement ; it is motor. Dynamic polarity. — The dynamic polarity of the neurons has no other experi- mental foundation than this. We now say of the neurons that which it has been customary, since the time of Magendie, to say of sensory and motor fibres, nainely, that some conduct in one direction and others in the other, which presujjposes two poles for each. It is true that anatomy has enabled us to observe the exact situation of those of these poles which are located in the grey matter, and this is an incontestable advance ; but the idea of direction has been sujjplied by physiology, and can be furnished in no other way. 2. Current of entry, current of exit. — Hence, in the altogether similar fibres of these two parallel nerve trunks (posterior and anterior root) impulses circulate, of which some represent a current ivhich enters the nervous system and others a current which leaves it. The more special nervous phenomena (psychical phenomena or those of sensation) mani- fest themselves in the interval ; that is to say, in the complicated net- work which the elements of the spinal cord and brain form between them. Visible movement is external, in the muscles or analogous organs. Order of succession of the two phenomena, sensory and motor. — In the nervous process regarded as a whole, movement ensues after sensa- tion, of which it is the visible consequence. This general current of cyclic form, which commences in the organs of sense and finishes in the muscles, is never inverted. I7i the nervous system, as in the vascular system, circulation is effected in a definite direction. Temporary checks may occur, or different routes may be followed, but it never returns on itself. Abbreviated image of the whole cycle. — If it is desired to observe this cyclic process in its totality, it is only necessary to repeat the experiment in different conditions : between the posterior and the anterior root only the corresponding segment of spinal cord is left, by cutting the latter above the lumbar region, in such a manner as only to permit reflex action. When limited in this way, the complexity of the phenomenon disappears, and only the most obvious details are SENSATION AND MOTION— THEIR RELATIONSHIPS 129 observed ; but, on the other hand, it is possible to ascertain the direc- tion which it follows in the nervous system. Comparative Physiology. — Batrachia. — Fodera and J. INIviller, have extended the discovery of Magendie to cold-blooded animals. The frog, on account of the shortness of its spinal cord and the relative length of its lumbar roots, is one of the animals best adapted for every land of experiment on the nerve roots. Birds. — The fact has been verified as regards birds by Schiff, and more especi- ally by A. Moreau. Fishes. — A. Moreau has also proved it as regards fishes, making use of the ray, the torpedo, etc., species in which the roots, after their reunion in the ganglion, re- main easily separable for a certain length before forming a mixed nerve. Invertebrata. — It has been en- deavoured to ascertain if the in- vertebrata present an analogous dissociation of the nerves of sensa- tion and of movement at the spot where the peripheral nerves leave the ganglionic chain, which in them represents the spinal cord. Newjaort, Longet, Faivre, Vulpian have endeavoured to ascertain this, the first by dissections, the others by cutting experiments and excita- tion of these nerves laid bare in animals such as the lobster, crayfish, etc. It has not been possible to demonstrate that in these animals the sensory and motor elements are grouped in distinct bundles, as in the vertebrata. According to Vul- pian, the elements of the two orders must be mixed, inasmuch as all the nerves which may simulate roots give rise to manifestations both of sensation and of movement. No absolute distinction be- tween sensation and motion. — The experiment of Magendie therefore shows, in a very clear manner, the distinction between sensation and motion. But how must it be interpreted ? Is it to be assumed that there is an absolute locahzation within definite boundaries ; or rather, the two phenomena being everywhere closely associated, is it merely that there is an exaggeration of one or of the other in special organs ? This last is the true interpretation. P. K Fig. 51. — Diagram representing the medul- lary roots in the frog. S, sacrum represented in dots. Above it and also in dots are shown the limits of the vertebrae between which the nerve pairs find an egress by the intervertebral foramen, after a more or less oblique course. The spinal cord is much shorter than the vertebral canal which terminates at the base of the sacrum. I to X, spinal nerve pairs, whose posterior root and gangUon may be observed in the spinal canal, and the mixed trunk outside it. ]30 SYSTEMATIC FUNCTIONS Motion concealed under the sensory phenomenon. — And first of all, as concerns sensation, it is only in an abstract manner that we can regard it apart from movement. Not only, indeed, does it take origin from a movement (exciting impulse on the posterior roots), but, throughout its development in the deep masses of the nervous system, it is accom- panied b}^ a movement, invisible, molecular, but not the less real. The importance of this movement does not consist in its quantity, which is infinitesimal, but in its complexity, which is extreme ; it is the co-ordination and the synthesis of its component elements which give it its unity. And it is this which makes it absurd to seek for the mechanical equivalent of sensation. Thus, in stimulation of the posterior root, the phenomenon of sensation is so evident, so marked, that it arrests our whole attention ; but it must not prevent us from recognizing the phenomenon of motion, on which it is superposed, and which indeed presides over it. Sensation disguised under the motor phenomenon. — Conversely when movement is induced in the muscles through excitation of the anterior root, every trace of sensation seems to be absent in this organ and in the motor nerve ; yet both are composed of excitable elements, that is to say, of elements possessing the earliest germ of sensibility ; so that here also, under a more marked phenomenon, this time motion, we must suspect another, latent and concealed, which is in this case sensation. Both phenomena are present in every living portion of the body. But that which makes sensation evident is obviously or- ganization, the synthesis of living elements into a co-ordinated system. That which degrades sensation and reduces it to its simplest expression is the dislocation of the system, its reduction to its simple elements in which movement alone seems to be present, as in non-organized matter. This is the reason why the stimulation of the two roots of the nervous system (anterior and posterior root) has such different effects. Individually considered, they have the same mutual value as any other tract of the brain or spinal cord, but one is at the entrance to the system, and its stimulation extended over the whole of this system will elicit the most remarkable of its manifestations ; the other is at its exit and, as such, can only manifest the properties of the isolated element which it governs : the one takes part in a synthetic, the other in an analytic phenomenon. 3. Functional bonds of union. — The nerve roots display distinct functions, but these functions are not independent. The connexion which exists between sensation and motion is manifest, so far as it has been investigated, in all the experiments made on these roots. SENSATION AND MOTION— THEIR RELATIONSHIPS 131 Influence of the posterior roots on the excitability of the anterior roots. — Harless, Cyon, Dastre and Marcacci have observed that, after section of a posterior root, the excitability of the corresponding anterior root (by an induced current) is modified. The first of these authors has observed it to be diminished, the others, on the contrary, to be increased. Belmondo and Oddi. investigating the cause of these varia- tions, think that it Hes in the fact that the section of the posterior root acted for a certain time as a more or less persisting irritation, before suppressing the propagation of the impulses which are transmitted by it as they come from the periphery. In order to overcome the irritat- ing action of mechanical section, they cocainized the root in which they wished to suppress this phenomenon, and they then found that the excitabihty of the anterior root is always lowered after its physio- logical interruption. According to these facts, it would appear that, in addition to more or less lively accidental stimulations which are furnished it, the pos- terior root is the seat of a sort of shght but constant flow of impulses, which proceed to the spinal cord and thence to the motor roots. \Mien the diastaltic system is not interrupted in any point of its course and the anterior root is stimulated in its course, this impulse is added to those which are already circulating in it, and the effect of it is then greater, if, the posterior root being cut, this circulation should be from this fact interrupted. Hence the sensory nerve is in a constant state of tonic stimulation, a condition similar to that which is known to exist in the muscle, and of which it is the exciting cause. \Yhen a strong excitation arises, and visible movement is produced, this tension is exaggerated ; when every route available to the impulse is cut, this same tension disappears, and the tone is said to cease. The source of tone is, in fact, this permanent current of impulses, the latter being slight and imperceptible. Influence of the posterior roots on motor functions. — When only a posterior root is cut, its suppression does not act in a very obvious manner on the motor power of the corresponding anterior root. This is so because the grey matter of the spinal cord is a locality in wliiclj a large number of other impulses converge, both those coming from the uncut posterior roots, and those which have been stored up in the superior parts of the nervous system, and which easily supplement the deficit due to such a limited lesion. But if a certain number of sensory roots be cut, for example, all those which correspond to the posterior extremity, it is seen that the movements of this limb, without being abolished, are curiousl}^ disturbed. CI. Bernard, Chauveau, Tissot and Contejean, who have performed this experiment, point out the inco- 132 SYSTEMATIC FUNCTIONS ordination of the movements which is the consequence of it. In per- sons suffering from locomotor ataxy a lesion of this nature, affecting the inter-medullary prolongations of the posterior roots, gives rise to that inco-ordination of movement which is so obvious in their mode of progression. And it has been proved that there is an entirely sen- sory form of locomotor ataxy, which is the result of an alteration of the sensory nerves (polyneuritis), apart from any affection of the spinal cord. 2. Organic Complications ; Recurrent Sensibility. The very obvious distinction which exists between the nerves of sensation and of motion, in the posterior roots, presents, nevertheless, a paradox, which for a long time compromised the law so clearly enunci- ated by Magendie concerning the functions of these roots, and this obscurity existed until a rational explanation of this paradox was given. 1. The fact and its conditions. — The roots being laid bare, if, before cutting them the posterior root be pinched, it is found to be very sensi- tive ; but if the anterior root be pinched it will be found to be equally sensitive. The fact was in the first instance observed by Magendie, was successively accepted and denied by Longet (1840-1841), then re-discovered by CI. Bernard, who propounded the conditions which give rise to it. In order the better to observe it, it is necessary to wait until the animal shall have recovered from the operative shock by a suflficiently long repose. The logical solution of the paradox is due to Longet. The sensory elements contained in the anterior root do not arise from the origins of this root, but are nothing else but elements of the posterior root, which, instead of going to the skin, re-ascend into the anterior root by a re-current course, in order to confer sensation on the membranes which envelop the sjjinal cord. Analysis of the phenomenon. — //, indeed, the anterior root be cut and the two ends be investigated, the inferior will be found to be sensory (as also motor) and not the superior, irritation of which gives rise to no kind of result. // the corresponding posterior root be cut, all sensation disappears from the anterior root, whether cut or not. Sensation in the anterior root is, then, clearly a borrowed sensation ; the anterior root is only a place of passage for sensor^' fibres which go to the membranes either of the cord, or of these roots themselves. Hence the paradox is explained ; so-called recurrent sensibility is nothing more than a special case of general sensation. The apparent exception to Magendie's law is included in this same rule and fully confirms it. SENSATION AND MOTION— THEIR RELATIONSHIPS 133 Anatomical proof. — If, in reality, sensorj- fibres which have their trophic centres in the cells of the vertebral ganglion, enter by recui-rence into the anterior root, it is possible to make them evident by the method of Wallerian degeneration. By cutting the anterior root, these fibres are cut at the same time as those incomparably more nmnerous fibres (motor fibres) which have their trophic cells in the anterior cornua of the spinal cord. Both will degenerate, but in opposite dii-ections, some in one of the two ends of the cut root, the rest in the other, and they will in tliis way be recognizable, the degenerated fibres sur- rounded by healtliy ones, the healthy ones by degenerated. And this, as a matter of fact, is what happens : the medullar^' end of the anterior root which has been cut contains some degenerated fibres amongst its healthy ones, and the peripheral end some healthy fibres among the mass of degenerated motor fibres. The fibres which are present in small number, and which have an orien- tation contrary to the others, are clearly the sensory recm-rent fibres. This experiment was first made by Schiff on the roots (1850). Philippeaux and Vulpian have repeated it on the hyj50glossal and facial nerves. Arloing and Tripier also made use of this method of control when they investi- gated the question of recurrent sensibility under a new asjDect. 2. The reason. — It is surprising at the first glance that sensory ele- ments should be present in membranes such as those which cover the spinal cord or the coverings of the nerve trunks ; because, as a matter of fact, these membranes are generally found to be insensitive when experimented upon, and in theory, the only membranes which would seem to require sensibility are those which face the exterior, such as the skin or the mucous membrane. But sensation is not necessary for us only in our relations with the external world, it is indispensable as regards the relationship of all our organs to one another, of all our cells between themselves ; it is the great regulator of function. Only this sensation is not conscious in the personal sense of the word ; and yet it may become so as the result of changes due to traumatism, in the deep portions of the organism. This is one of the reasons which cause it to appear (by rendering it obscurely conscious or sub-conscious) in the dura mater and the roots a certain time after the opening of the spinal column after the animal has had time to recover the shock of the first operation. As regards the cranial dura mater, it has been found that filaments are given off by the trigeminal nerve, and this membrane was known to be sensitive by former observers. It possesses great numbers of nerve fibres, and these latter are furnished with receptive apparatus analo- gous to those of touch or of general sensation. Site of the recurrence. — The recurrence of the fibres which, from the posterior become involved in the anterior root, is not carried out at the union of the two roots, in the mixed trunk which is the result of this union ; it is effected much farther away, in the plexus which arises from the combination of these trunks. Section of the mixed 134 SYSTEMATIC FUNCTIONS trunk in the neighbourhood of the roots abohshes sensation in the anterior root, just as if the posterior root had itself been cut (CI. Bernard). CI. Bernard maintains that every anterior root acquires its recurrent sensi- bility from the corresponding posterior root, and iiot from another. According to him, this correspondence would be one of the characteristics of the physio- logical nerve pair ; he employs it more particularly in the study of the bulbar nerves in order to determine the sensory and motor elements which form the functionally active cranial pairs. It is possible that this character may have some importance, but it is not absolute. Recvirrence may be met with not only from sensory nerve to iTiotor nerve, but from sensory nerve to sensory nerve (Arloing and Tripier). 3. Generalization of the fact. — Through the labours of Arloing and Tripier, the question of recurrent sensibility has been at the same time extended and renewed. Recurrence of sensory fibres is not a fact peculiar to the motor roots, but is much more general. It is not merely a device to render sensitive the medullary or deep membranes, but it occurs in the distribution of sensory nerves in the skin itself and plays an important part therein. The more nearly the cutaneous investment is ajjproached, the greater is the imj^ortance and the extension of this recurrence. It here presents also a ncAV character : instead of being confined to the area of a nerve trunk, it mixes the extremities of the sensory fibres over a more or less extended surface. Experiment. — One of the most striking experiments of these authors is the following : in a dog the four collateral nerves (the two palmar and the two dorsal) of one of the digits of an extremity are uncovered. These nerves are successively cut, and after each section the sensation of the digit is investigated. After section of the first, of the second and of the third, sensation is found to be dulled, but the digit does not present any area which is completely anaesthetic. But if the fourth nerve be cut, the whole digit becomes completely insensible. The conclusion to be drawn from this experiment is that the four collateral nerves have not individually an area of distribution, but that they interchange their fibres by means of the terminal anastomoses which anatomy proves to exist between them. These anastomoses are the result of recurrence ; for, after section of one of the nerves, the method of degenerations invariably shows the presence of a small number of degenerated fibres surrounded by healthy fibres of the central end, and conversely. Applications. — These facts, well established both anatomically and physio- logically, supply a plaxisible explanation of persistence, or of rapid return of sensation, after section of more or less important nerve trunks (median nerve), consecutive to nervous suture, as well as in the absence of the latter. The SENSATION AND MOTION— THEIR RELATIONSHIPS 135 re-establislament of sensation is probably due to the fact that the apparent area of distribution of the cut nerve is invaded by fibres of neighbouring nerve trunks, thanks to the recurrent anastomoses which are so numerous at the peripliery. 4. Recurrent motricity. — The same reasons which cause certain fibres of the posterior roots to re-ascend in the anterior roots, in order to supply sensation to the corresponding meduhary area, should also cause certain fibres of the anterior roots to re-ascend in the posterior roots, in order to supply motor power to the organs of movement which are present in the interior and at the surface of the spinal cord. Just as sensation, so is movement everywhere, under a visible or invisible, mechanical or molecular form. Visible movement occurs in the spinal cord (as in the brain), since it contains (as does the latter) contractile vessels to which vaso-motor fibres are supphed. Independently of the invisible movements, there are intrinsic activities in the fixed cells of its membranes, exchanges with the surrounding blood which imply stimulation and a direction imposed by the motor portion of the ner- vous system. Remark. — Sensory and motor elements thus arise from the spinal cord, and, after a more or less lengthy course outside the latter, they return to it, having their point of termination (or being able to have it) quite close to their point of departure. The centre and the periphery are thus united in the same organ, and this organ is the spinal cord. Nothing more clearly renders evident the fact that these expressions, centre and periphery, have merely a relative and conventional value. It is important to remember, indeed, that the two words are used some- times in an anatomical and concrete sense, sometimes, on the other hand, in a metaphorical and abstract sense. In a differentiated system, such as is the animal organism, every organ, every constituent part is, in virtue of its differen- tiation into a given order of functions, a centre for other parts, which we then call the periphery, for want of a better word ; and reciprocally. The prolonged course of these fibres, which leave the spinal cord in order to return to it, seems contrary to the arrangement which governs the living organi- zation ; but this course is rendered necessary by a reason which is of embryo- logical natiu-e. The origin of the sensory fibres is in the spinal ganglia, that of the involuntary motor fibres is in the great sympathetic ganglia : it is necessary that they pass tlirough these ganglionic masses, wherever they come from or whither they go. Vulpian has investigated tMs recm-rent motricity by exciting the peripheral end of an anterior root while observing the state of the cii'culation at the surface of the spinal cord in its corresponding segment. He has not noticed any change. But it is possible that the vaso-motors of the sjainal cord, like those of the brain, have in the sympathetic chain a more or less considerably prolonged course, the result of which is that, leaving by an anterior root (or even a posterior root) of the dorsal region, for example, they re-enter the spinal cord by a posterior root (or even an anterior root) of another region situated above or below. Decisive experiment. — When the sympathetic is stimulated in the neck and when, as has been observed by several authors, changes are noticed in the vas- cular supply of the brain (dura mater or cerebral substance), it is a question, 136 SYSTEMATIC FUNCTIONS fundamentally, of a motor recurrent jjhenomenon. Having started from a nerve centre, which is here the spinal cord (which may receive it from the brain), the motor impulse has returned in the interior of a nerve centre (the brain) to the motor elements which the latter contains ; there can be no doubt that it returns in a similar way to the vessels of the spinal cord, by routes which remain to be exactly defined. 3. Conventional Definitions ; The Sensory and Motor Field The posterior root is called sensory, not that it exactly feels, but because it arouses sensation in a system which follows it ; the anterior root is called motor, not because it moves, but because it gives rise to a movement in the organs situated at its extremity. What are the exact hmits of the sensory system, and where does the motor system commence ? Formerly it was maintained that in the spinal cord, and afterwards in the brain, was situated a posterior area, sensory in function, and an anterior area, motor in function ; both, in a way, being a continuation of the roots, which were prolonged without any great alteration as far as to the two extremities. But the data furnished both by anatomy and by physiological experiments in no way con- firm such a supposition. 1. Comparison of the posterior and anterior portions of the spinal cord and of the brain. — So far as concerns the spinal cord, sensation may be aroused if certain tracts are irritated, and as the result of irrita- tion of certain other tracts movements may ensue ; yet these pheno- mena are far from reproducing quantitatively and qualitatively those which are due to stimulation of the roots. As concerns the cerebral cortex, the sensory and motor areas, instead of being disposed in different departments, seem to be superposed, which would appear to confirm the view of a cyclic process substituted for that of an isolated locahzation of sensation and motion. Indefinite limits of sensation and of motion. — It is consequently impossible to point out exactly where sensation ceases and motion commences, the passage from one to the other being gradual. All that can be said is that, from the mgans of the senses to the brain the develop- ment of the psychical phenomenon of sensation is progressive ; ivhile from the brain to the muscles it follotvs a retrograde course, ending by external movement. The former of these roots are ascending, and the custom of calling them sensory has become widespread : the others are descending ; they are generally called motor. At the present day it would be impossible to change these names for any better adapted for the purpose ; but it must be remembered that they have but a conventional value, and that the difference of function which they SENSATION AND MOTION— THEIR RELATIONSHIPS 137 indicate progressively diminishes in proportion as we approach more nearly to the cerebral cortex. With regard to the impulses which it receives, the nervous system first acts as a distributing agent, in its superior regions so singularly called centres, later as a concentrating one upon the organs of move- ment. The old criteria. — The cause is easily recognized which at first embarrassed authors in their efforts to define and classify the fimctions of the spinal cord and brain, and above all of the cerebral cortex. They were convinced that motion excluded sensation, and that, reciprocally, sensation excluded motion. As a matter of fact, these two functions are on the contrary confovmded and inex- tricably mixed. According then as one or other of these two phenomena chiefly attracted then* attention in some nervous assemblage, they elected either for sensation or motion. When, for example, with Schiff and rran9ois-Franck, the surface of the brain is compared to a sensitive siu-face, like the skin, the analogy is very true from the pi^irely experimental point of view, but from tliis point of view only, because it deals with a very special case, that, namely, in which the cerebral cortex being put out of action as an apjDaratus capable of transform- ing the impulse, this latter need be only reflected to the spinal cord in order to reach the motor organs properly so called. But if, the cortex being intact, the impulse arising from the skin passes through the latter, new characters are imposed on it by reason of this transit, and these markedly svu-pass those which are acquii'ed in the spinal cord. In the double jom-ney which it makes, the one ascending (from the spinal cord to the brain), the other descending (from the brain to the spinal cord), the un- pulse proceeds from reflection to reflection, or, better, from transformation to transformation ; but these transformations have very unequal values in the spinal cord and in the brain. When the impulse proceeds from the fu-st of these organs to the second, the sensory character predominates, as is proved by the absorption of the impulses wliich ensues in the brain without any motor effect resulting ; when the impulse descends from the second to the first, the motor character is that which predominates, yet it cannot be said that it is entirely dissociated from sensation, inasmvich as it recalls the latter in its inferior degrees. For the absolute meaning that these two words " sensation " and " motion " have up to now preserved, it is necessary to substitute a relative one, which alone corresponds to the real state of affairs ; for the opposition which they expressed a gradation must be substituted, a progression which connects them the one with the other ; but as, in the absence of expressions which indicate the different values of this gradation, the old terms are indispensable, we shall describe by the term sensory field all that part of the nervous system in which sensory char- acters predominate over those which are motor, and motor field all that region in which motion predominates over sensation. Arbitrary as it is, this definition is founded on fact. Sensory and voluntary excitation. — Our own powers of observation show us that sensation may exist in us independently of muscular movement, and they further tell us that movement may originate in us in an apparently spontaneous fashion under the influence of certain determining factors which we call volun- tary. In interpreting these two facts, it seems to have been often thought that sensation is localized at the termination of the ascending conductors, and the will at the origin of the descending conductors, just as if a plane of division existed between these two wliich could be clearly defined in the nervous system. In reality the arrangement is by no means so simple. 138 SYSTEMATIC FUNCTIONS (1) When an actual sensory impulse is only reflected on the muscles, we have no reason to believe that it is stopped at the ideal plane which has just been (" Ruban de Reil' Fillet Cran. Sensory N. Sens, decussation. N. of Goll and of Burdach. Column of GoU. Anterior column. Lateral column. Ant. commtss. Post. corn. Spinal If. Fig. 52. — Sensory field with its two chief orders of fibres of projectioru referred to ; on the contrary, we have cause to think that it often goes beyond this boundary. If, indeed, it does not produce immediate movement, it usually SENSATION AND MOTION— THEIR RELATIONSHIPS 139 produces a tendency to movement (sometimes slight tension, of the 'muscles), A.lc^J' Fig. 53. — Motor field, with its two principal orders of fibres of projection. arrested in its effects by antagonistic influences situated in the very interior of the nervous system. (2) When, on the other hand, the muscles are put in action by a voluntary 140 SYSTEMATIC FUNCTIONS excitation we have no reason to believe that this latter arises in this plane of separation, and we are justified in maintaining that it proceeds from the nervous conductors which precede it. It arises, in fact, from a memory which is equivalent to a resuscitation of anterior sensory excitations. In the first case, the imjDulse, after having traversed the ascending paths, is extinguished in the course of the descending paths, without reacliing the muscles : its effect is not lost, but it is postponed. In the second case the impulse, which traverses the descending paths in order to reach the muscles, still proceeds from the ascending roots, but not from their origin in the organs of sense. It pays the arrears of anterior excitations kept in reserve in the nervous system, esj^ecially in the brain. In the two cases the exciting cycle is formed in that jaart of the nervous arc which is called superior, that which gives to the phenomena of innervation their jDsychical value, and which is localized in the brain, although no precise limits can be assigned to it. Is not the process which maintains the impulse in the brain in a condition which may be described as potential itself of a cyclic nature, intended to store it up and causing it to reappear in an automatic and unconscious fashion ? We may certainly ask this question, considering the generality of the process and the slight expenditiu-e of energy which nerve actions require. But the verifica- tion of such a hypothesis is so far incajoable of being effected experimentally. Double difficulty. — The difificulty wliich attends the analysis of the functions of the spinal cord and brain is double. The first arises from the fact that the motor and sensory characters, which are so clearly recognizable at the two ex- tremities of the cycle, gradually change in proceeding from its origin to its termi- nation. The second is due to the fact that the nerve elements which represent these functions more or less modified, instead of remaining separate one from the other by forming distinct bundles as in the roots, are often intermixed, fibre by fibre. Further, this intermixture begins in the roots themselves. 2. Mixture of the sensory and motor elements from the medullary roots. — The distinction of the peripheral elements of the nervous system into two classes, of which one carry the impulse into the nervous system and the others convey it away, is an absolutely funda- mental principle of physiology, and one which has never been con- tradicted. Nevertheless, experiment does not demonstrate it in that clear and definite manner in which it appeared to the first observers, because these latter were ignorant of certain complications which have since their time been revealed in the nervous organs (roots) wliich they regarded as simple, and these complications alter, as concerns its anatomical enunciation, the primitive formula of the law of Magendie. In other words, the distinction of peripheral elements into centri- petal and centrifugal is not demonstrated at the present time by a single crude experiment, but is deduced by reasoning which is based on a varied assemblage of experimental facts. Nevertheless, amidst these secondary facts the experiment of Magendie remains so convincing, that it is customary to place these latter on one side, by giving to the anatomical terms (posterior and SENSATION AND MOTION— THEIR RELATIONSHIPS 141 anterior roots) a symbolical value equivalent to that of centripetal and centrifugal nerves. Structural complications. — Nerve trunks exclusively formed of identical elements do not exist in the organism. The spinal nerve roots represent those of these trunks which the most closely approxi- mate to this simplicity, but without, however, attaining it. Mixed functions of the posterior roots.- — The posterior roots, com- posed almost entirely of centripetal elements, contain a very minute proportion of ceyitrijugal elements. Anatomy proves this by its own special methods. The posterior roots contain neurons, which have the morphological character (polar orientation) of centripetal elements (Lenhosseck, Cajal). The method of degeneration confirms this. After section of the posterior roots some healthy fibres are found amidst those which are degenerated in the medullary end, and in the gangh- onic end some degenerated fibres in the midst of the large number of sensory fibres which are still healthy : this proves that the posterior Axis-cyl. Ant. fissure. Figs. 54 and 54a. — Motor fibres of the posterior roots. On the right, ch'awing from nature (Van Gehuchten) from the embryo of a fowl ; on the left diagram. j root contains elements whose trophic centre is in the spinal cord (Morat and Bonne). 3. Physiological proofs. — If, after having laid bare the lumbo-sacral roots (in the dog), one of them be selected (the sixth lumbar, for example), and be cut in its course, and if the peripheral end be irri- tated, it will be observed that the temperature of the corresponding hind limb rises considerably (Strieker, Gartener). If an animal be chosen whose skin is not pigmented, and the colour of the pulp of the toes be examined after careful washing, it will be seen that this colour progressively darkens while the excitation con- tinues, the former colour reappearing when the excitation ceases 142 SYSTEMATIC FUNCTIONS (Morat) ; this is a proof that the nerve trunks contain a certain pro- portion of vaso-dilator elements. Section of the posterior roots is followed, after a certain time, by trophic disturbances, such as cutaneous ulceration, falling of the nails and of the hair, thickening of the skin and of the skeleton, chiefly at the extremity of the limb, disturbances which are in reality explicable neither by vaso-motor changes nor by those of sensation (Morat). These phenomena are motor and as such under the domain of the centrifugal nerves, but their motricity is of a special kind unknown at the time of Magendie, when no other motion was known than that which was voluntary, and at the same time external and evident, of the muscles of the skeleton. Root elements of one system mixed with the intercentral elements of another. — \Mien applied to the nerve roots, the expressions posterior and sensitive, anterior and jnotor, are precisely equivalent, on one con- Ront Gangl. Post, root Affer. som. Fibres Root trunk Som. eff. F. Inter-vert, nerve Eff. Sp'a.-irh F. Sym. gangli- Aff. splanch. F. ^ Fig. 55. — Spinal nerve roots, great sympathetic, mixed nerve. Division between sensitory (blue fibres) and motor (red fibres). After their intermixture at the meeting point where the mixed nerve arises, there is a fresh division between consciousness (somatic fibres) and unconsciousness (splanchnic or sympathetic fibres). Structural complications arise from the presence of centrifugal elements in the posterior roots, from the presence of sensory recurrent fibres in the anterior roots, from the presence of recurrent sympathetic fibres involved in tlie inter-vertebral nerve, and finally from sympathetic fibres joining the somatic nerve (not indicated). dition : this is that the sensation is conscious and the movement voluntary. When it is a question of movement and sensation of SENSATION AND MOTION— THEIR RELATIONSHIPS 143 visceral organs, these expressions are no longer equivalent. As a matter of fact, as will be explained farther on, the medullary roots only merit the name of roots as regards a portion of the nervous system, that, namely, which is knowTi as the voluntary conscious side, or that of animal life ; as regards another portion, which is the in- voluntary, unconscious side, and which is represented by the great sympathetic, they are no longer roots, but interceniral fibres, extend- ing from the ganglia of this system to the spinal cord, which they bring into reciprocal relation bj^ exchange of impulses ; and, being such, they already present those intricacies which render the study of the central masses of the nervous system so extremely difficult. Spinal roots or those of the conscious voluntary system, and ganglionic roots, or those of the unconscious involuntary system. — The roots of the great sympathetic must not then be sought for in the spinal cord, but outside its gangha towards the periphery. It is proved (more especially by histological investigations) that neurons arranged inversely place these ganglia in connexion with sensory surfaces and motor organs. Indeed, these two effects may be physio- logically dissociated by stimulating comparatively the central and peripheral end of a sympathetic branch after the latter has been cut ; but, in the sympathetic, fasciculations which effect an anatomical dissociation of these two species of nerves, comparable to that of the medullary roots of the system of the life of relation, are nowhere to be found. B. SPINAL NERVES. METAMERISM The repetition of the roots of the nerves, which are arranged in graduated order throughout the length of the spinal cord, with identical characters and connexions, is a fact which of itself ^vould arouse attention. When connected w^th the form of the skeleton in the adult, and, above all, with the facts furnished by embryology and by comparative anatomy, it assumes a high degree of significance. Moquin-Tandon (1827) had termed zoonites those segments which are so easy to recognize in the external form of many invertebrate animals. Duges extended this conception to all the ramifications, all animals being, in the early stage of their development, formed of parts ranged in series, which, in spite of their reciprocal penetration, preserve the anatomical, and even functional traces of their primitive separation. The metameres of the vertebrate (Hoekel) are nothing more than the zoonites of the invertebrata. The segmentation which, in the adult, is rendered evident by the repetition of the medullary roots and of the ganglia of the great 144 SYSTEMATIC FUNCTIONS sympathetic, is, in the beginning, much deeper ; it divides up the muscles, the nerve axis, the skin itself, into distinct territories (myo- meres, neuromeres, dermatomeres) which correspond in each metamere. The osseous skeleton, whose development is slower, reproduces this arrangement in the spinal column, in which it becomes permanent. To return to the adult : if, starting from the medullary roots, their tracts be followed either internally in the spinal cord or ex- ternally towards the plexuses and nerve trunks which arise from them, the meta- meric disposition seems to have more or less disappeared, as the result of the in- tricacy of the peripheral nerve trunks and of the enormous expansion of the roots in the medullary tracts. For its recogni- tion it is necessary to make use of the analytical devices which pathology some- times furnishes in a very perfect fashion. 1. Root and spinal metamerism. — Brissaud, who has made a special and very exhaustive study of nervous metamerism, distinguishes between a radicular meta- merism (that which is rendered evident by the anatomical arrangement of the nerve roots), and a spinal or medullary metamerism (that which is based on the fact that the segments of the spinal cord correspond to the implantation of the roots or myomeres), but he is careful to point out that the one does not in any way imply the other ; and the reason of this difference is easy to comprehend. Radicular metamerism is genuine meta- merism : each nerve pair is a perfect reproduction of the nerve pair situated above and below it. When followed to the periphery, the nerve pair conducts us through territories (cutaneous or muscular) which are, if not entirely independent, at least clearly circumscribed and capable of being defined. On the other hand, spinal metamerism is a meta- merism which is reduced to a mere trace. The primitively independent medullary segments (as regards phylogenetic as much as ontogenetic evolution) are mutually interpenetrated by their exogenous elements, as well as by those which are endogenous and associating, in such a Fig. 56. — Spinal nerves in the frog, and its vertebral cokxmn. (front view). NO, optic nerve , A, atlas ; I to X, spinal nerve pairs. SENSATION AND MOTION— THEIR RELATIONSHIPS 145 manner as to consolidate them for the functions of the whole ; these functions being more extensive, more definite, and more perfect, are substituted for their uniform and rudimentary function ; and the higher we go in the supefposed structures of the nervous system, the more obvious will this be. The little independence which is left to the myelomeres is represented by functional connexions which unite, the one with the other, the posterior and the anterior root of the same nerve pair, for the exercise of the most simple reflexes. After isola- tion of the medullary segment (myelomere), each separate section, furnished with its sensory and motor nerves, can still, in a manner, perform the functions of a partially independent system. Apart from these elementary acts, it is intimately consolidated with the others. Number of Metameres. — In man there are seven cervical jiairs, twelve dorsal, five lumbar, five sacral, one coccygeal. In the dog, seven cervical, thirteen or fourteen dorsal, seven lumbar, five sacral, several coccygeal. The spinal cord does not occupy the whole length of the canal, whence arises the existence of a cauda equina, as in man. In the bird there are twelve cervical pairs, seven dorsal, thirteen lumbar, and seven caudal. The spinal cord occupies the whole length of the canal, conse- quently there is no cauda equina, no fihun terminale. In the frog there are ten spinal pairs. The cord is very short with regard to the spinal column, and ends in a long filum. In man and the majorit%- of animals the dm*a mater is separated from the spinal cord by a certain space. In the caniivora, especially the dog, the dura mater covers up the cord, with which It is in contact. These two characteristics, the lengtli of the cauda equina, and application of the dvu*a mater to the spinal cord, with an extra-dural space filled with fat, greatly facilitate section of the vertebra?, the laying bare of the spinal cord and operations (section, stmiulation) performed on the roots in order to determine their functions. In the herbivora, especially in the rabbit, these special facilities either do not exist, or are greatly reduced. 2. Radicular, cutaneous territories ; areas of anaesthesia. — If a posterior root is interrupted, an anaesthetic territory (or rather a hypo- aesthetic territory), whose situation and form is determinate, will be found in the skin. This territory is the corresponding derma- tomere. In the case of the roots which run a regular and non-plexiform course, such as the intercostal nerves, it will be easily understood that this territory itself will be regular, assuming the form of zones or of girdles ; but that it should be the same as regards the roots of the lumbar, sacral or brachial nerves, in spite of the plexuses which collect them together, would certainly be more unexpected ; as a matter of fact this is so. If it be conceived that an individual be placed in the position of a quadruped, or rather that the limbs are P. L 146 SYSTEMATIC FUNCTIONS separated and perpendicular to the trunk, the skin of this individual may be divided into as many superposed zones as there are nerve - d J ■ - ■■' d's- "■ d'f "■ ZT"-' ■ ' " d 3 ' d^^'^ --. ^ ^^ ..- Fio. 57. — Areas of radicular distribution of the spinal nerves (after Kocher). Front view. pairs (at the least the spinal). The dermatomeres assume the form of circular bands arranged in stages. As regards the upper limb, they SENSATION AND MOTION— THEIR RELATIONSHIPS 147 are prolonged along its length in parallel bands, more or less regularly arranged, but continuous.! As regards the lower limb, they are always Fig. 58. — Areas of radicular distribution of the spinal nerves (after Kocher). Back view. placed lengthwise, but are rendered discontinuous by the blurring of 148 SYSTEMATIC FUNCTIONS the territories of this region which has been more convulsed during development. Yet the plexuses are, in a sense, as if they did not exist. The nerve trunks which arise from them (radial, median, ulnar, crural, sciatic, etc.) form an artificial group of root fibres, in which no very definite functional arrangement can be detected. A knowledge of the areas subjected to these groupings is useful when their respective trunks are individually involved by paralysis ; or, again, if it is desired to investigate them experimentally. But the arrangement of these fibres, as regards their course, that is to say, far from their origin and their termination, does not correspond to any real systematization ; on the other hand, the relations of the roots with their dermatomeres is extremely simple, the form of the dermatomere being generally obvious, and their succession in the order of that of the roots them- selves. Mutual interpenetration of the territories. — Such is the very simple scheme of the cutaneous territories as regards their correspondence with the spinal roots. Yet these territories must not be regarded as having determinate boundaries. Every one of them is invaded, at its confines, by the sensory nerves of neighbouring territories (superior and inferior), which are superposed on its own territory, some in one half, others in the other half of its surface. It is obvious that these areas of overlapping increase the size of the area of distribution of each root, but they do not alter the general form of this area, as it is easy to understand. It merely follows that isolated section of a posterior root will in no case lead to the precise and definite an£esthesia of a dermatomeric cutaneous band ; to express the matter more clearly, experiment having shown that it is as described, it has hence been concluded that overlapping zones must exist. Thus the isolated section of a single posterior root will not point out to us its area of cutaneous distribution. Warned by this experimental fact, Sherrington has overcome the difficulty by allowing a single root to remain intact, amongst several others which have been cut, either in front or behind it (in animals). In this way a sensitive zone is defined (that of the root which has been spared) between two anaesthetic surfaces. In man, who is able to give an account of his sensations, as the result of multiple or isolated lesions of the roots (confirmed by autopsy), there may be observed, not merely total paralysis, but diminution of sensibihty, which is the consequence of it, and in this way it may be possible to deduce the topography of individual radicular innervations (Thornburn, Allen Star, Head). SENSATION AND MOTION— THEIR RELATIONSHIPS 149 The results thus supphed clinically agree fairly well with those furnished by physiological experiment. 3. Radicular muscular territories. — Isolated stimulation of a motor root causes the contraction of several muscles, sometimes of a fairly large number, according to its size and the number of its component elements. Hence it has its own special territory of motor action, just as the corresponding posterior root has its sensory territory on the surface of the skin. The first is far from having the regularity of the illiiiuiuiuirii\lil\\li\ Fig. 59. — Distribution of the sensory fibres of the thoracic nerves (diagram) (after • Sherrington). j ;., The areas of the III^' and the V dorsal join each other by each overlapping half of the inter- mediary ten-itory of the IV*". second. On the other hand, each muscle individually generally receives tnotor elements from several neighbouring anterior roots ; thus it follows that the territories of motor muscular innervation interpenetrate (Hke the sensory zones of the skin), and this interpenetration has no longer in the myomeres, the regularity which distinguishes it in the derma- tomeres. From the practical point of view (diagnosis of motor paralysis, surgical intervention in nerves), a series of tables maybe constructed, displaying, by a given number, the muscular area to which each anterior root is distributed, and the cutaneous territory presided over by each posterior root. So far, treatises on anatomy have preferably prepared tables which point out the field of muscular and cutaneous distribution of the nerve trunks (radial, ulnar, median, crural, sciatic, etc.), arising from the plexuses, that is to say, after the intermixture of the roots in these plexuses, and which are of use in the case of lesion of the trunks, or of surgical intervention in connexion with 150 SYSTEMATIC FUNCTIONS them. A knowledge of the second in no sense dispenses with that of the first ; because, as is thus seen, they are not superposed in any way. A radicular trunk is not a functional unity. — Have the roots which numerically follow one another at the origin of a nervous whole, such as the brachial plexus, individual special functions, such as extension, flexion, adduction, abduction, for the motor roots of the superior ex- tremity ? Ferrier and Yeo, P. Bert and Marcacci, who were the first to study this question experimentally, have answered in the affirmative. But their results have been contradicted by Lannegrace and Forgue, who have not been able to find between each motor root and its muscular territory anything beyond a purely anatomical or topographical cor- respondence, without any suggestion of function. The component elements of the same root resemble one another, in that they all possess a motor function ; but these elements enter into the com- plexus, which makes use of their function in a variety of ways. The natural movements, even the simplest, imply, indeed, an action which is at the same time gradated and successive of the muscles which execute them ; in other words, in order to effect the movements, a co-ordination both as regards time and space of the contraction of these muscles is necessary ; this is the result of associations, carried out in the grey matter, between neurons which are more or less approximated or separated, and hence which do not necessarily belong to the same group of original cells giving origin to a root. Dissociation of the root in its bundles. — In fact, the total artificial excitation of an isolated anterior root may certainly produce a defined movement in the corresponding limb (flexion, extension, adduction, etc.), but this is simply due to the fact that, the excitation acting upon a collection of fibres whose functions are different or antago- nistic, the resulting effect arises as regards the strongest muscle, or that which receives the strongest stimulation. If, as Russell has done, an anterior root be dissociated into its component bundles, and each one of these be separately stimulated, different and sometimes antagonistic movements will be aroused by these localized excitations. This proves that the same root is dis- tributed to several muscles whose function, further, is not univocal. Con- versely, the same inuscle receives the fibres of several roots. Practically, these results explain how it is that isolated paralysis of an anterior root only causes transitory disturbances of movement. These facts are in agreement with those of the same order observed clinically (Allen Star, Mills, Kaiser). 4. Mixed nerves. — The anterior and the posterior roots intermingle SENSATION AND MOTION— THEIR RELATIONSHIPS 151 their fibres just beyond the spinal ganghon, and thus form a mixed nerve. At the spot where the mixed trunk of the nerve pair emerges from the inter-vertebral foramen it receives from the ganglia of the great sympathetic sensori-motor elements of a new order, which still further complicate its composition. Thus it is mixed, not only by the addition of sensory and motor elements, but also by that of elements possessing different varieties of sensation and of motion. Thus constituted and completed, the mixed trunks to which the nerve pairs give origin extend to the periphery, where they are distributed to definite territories which are arranged in gradated order corresponding to that (or nearly so) of the roots from which they arise. Am. roof. Sinu-vert. V . . . / Symp. Gangli. Int. branch. Common Br. - La'.er br. Fig. 60. — General arrangement of a spinal nerve (diagram). An intercostal nerve is taken as a type. These territories are, some, cutaneous {dermatomeres) ; others mus- cular {myomeres), and yet others visceral (these may be called {splanchnomeres) . Definitions and Distinctions. — These divisions are not so absolute as would at first sight appear. The skin does not wholly represent the tactile sense, of which, it is true, it is the accredited organ ; the viscera penetrate it to a marked degree under the form of vessels, glands or other organs, whose function is iinconscious 152 SYSTEMATIC FUNCTIONS and involuntary. The muscles do not represent solely animal movement ; the vessels equally penetrate them ; and, further, a kind of sensation is acqmred by them which is intermediate between that of the skin and that of the viscera. As regards the viscera, properly so-called, if they send prolongations to the skin and the muscles, they do not in return receive anything from these organs, to which tl;ey serve a? a fundamental and indispensable base. The words derma- tomere, myomere, splanchnomere, will thus possess different senses, according as to whether the essential function, or the totality of the functions, which are represented in them are considered ; according also as to whether the essential part of an organ, or the prolongations which it furnishes to others, is taken into account. The constitution of the metamere, above all, that of its nervous system, explains these distinctions. The really fundamental portion of this latter consists of the ganglia of the great symjoathetic. From the vertebral ganglion branches arise which proceed to the viscera, and which, following the vessels wherever they penetrate, finally attain the whole field of the metamere ; this is the origin of the vegetative system. As regards the intestine (and the large viscera), these branches continue isolated. As regards the skin and the muscles, they are indeed duplicated by others, which skirt them, and which represent conscious sensibility and voluntary movement : this is the oi'igin of the animal system. Their origins are distinct from those of the preceding, and are in the spinal ganglia and the grey matter of the spinal cord. They accompany the sympathetic branches (as they become preponderant, it is generally said that they are accompanied by them) in the myomere and the dermatomere. They form with regard to the others a supplemental and perfected, differentiated system. They assume the function of external life, and from this fact have notliing in common with the viscera, which are organs of nutrition, while the converse is not so. When, in order to define the portions of the nervous system which (develop- ment being finished) have retained the metameric arrangement, we call the roots, medullary, we omit an important portion of the nervous system which has pre- served this disposition ; these are the branches (direct or intermingled with the mixed trunk) which arise from the ganglia of the sympathetic chain, and which have the same distribution as the corresponding roots. On the other hand, when, in order to furnish an example of true metamerism, we bring forward these same roots, the designation is only correct so far as we restrict it to the conscious voluntary elements of these roots ; these alone obey the usual laws of metamerism, while the unconscious or vegtative elements elude them. These latter indeed have not, like the first, a direct com-se from the segment of the cord to the peripheral nerve for which they are destined by the corresponding roots ; but, arising either above or below the ganglion which collects them, they pass for a certain distance vertically into the chain of the great sympathetic. The chain of the sympathetic destroys the metamerism of the vegetative system, just as the organization of tlie spinal cord destroys tJiat of the animal system. Trophic disturbances. — Among the facts which indicate metamerism of the nervous system, trophic disturbances of the skin have often been emphasized. Trophic troubles are motor phenomena of a special order. It is proved, both experimentally and clinically, that, in many cases, their appearance depends on the nervous system. The existence of nerves whose function is solely and speci- fically trophic is no longer admitted, but trophicity is regarded as a function of the nervous system. The disorders of nutrition may sometimes affect the same distribution as the areas of anaesthesia ;. some are accompanied with pain, as for instance, herpes zoster. These peculiarities have led to them being regarded as arising from an alteration of the sensory trunks, especially the ganglia of the posterior roots (Baerensprung). It has been thought that from this alteration SENSATION AND MOTION— THEIR RELATIONSHIPS 153 of the sensory nerves it would be justifiable to infer the origin of the nervous action ; in this case the action would be strongly reflsx ; and would be reflected on the skin (Brissaud). Centrifugal cutaneous nerves. — But the posterior roots contain centrifugal fibres ; and it is i^ossible that certain of these fibres proceed to the fixed elements of the skin, just as there are those which go to the glandular cells of this structure. The alteration of these fibres and the perversion of function in the cutaneous covering, which is the result thereof, leads, in the end, to a modification of its structvu-e (trophic disturbance). The areas of these neuro-trophic dermatoses may then be superposed on those of the anaesthetic zones, if, as is probable, the centrifugal fibres of the posterior root have, more or less accvirately, the distribu- tion of the centripetal fibres of the same root. Segmentary alterations of sensation and of nutrition. — Trophic disturbances and jDartial anesthesia affect an equally regular form in certain cases, but have an orientation altogether different to that of the preceding. Instead of being zonal, the cutaneous lesions are segmentary. This arrangement is particularly striking in the limbs (dermatoses assuming the form of a glove, that of a sleeve. Hand Fig. 61. — Diagram of the metameric and segmentary distribution of the nerves of the upper limb (after Brissaud). The cervical enlargement of the si^inal cord is divided bj- horizontal lines into three metameres to which the roots Ri, R2, Rs^ going to the thoracic limb, correspond ; by vertical lines into three segments Mi, M2, il-, of whicli the deepest gives off nerves of the arm, the middle one nerves of the forearm, and the most superficial nerves of the hand. etc.). In order that such forms may be the result of limited lesions of the grey substance of the spinal cord, they must be inscribed there in the arrangement of the origins of the nerves. According to Brissaud, the spinal cord is not only formed of superposed neurotomes (corresponding primitively to the roots super- posed in the same order), but its grey matter presents, in its lateral thickness, superadded layers, which reinforce it at the site of equally superadded portions 154 SYSTEMATIC FUNCTIONS (which are the superior and inferior extremities) by giving origin to its enlarge- ments (cervical and lumbar). In these enlargements the most superficial layers are those which sujoply the nerves of the hand or foot ; the deepest, those which furnish the nerves of the shoulder and of the hip. Hence it is easy to iinderstand how isolated alterations of these nerves may give rise to lesions of a form also singular in its regularity. Anaesthesia also is subservient to the same disposi- tion. Sensory visceral areas. — When the large viscera are the seat of a lesion (especi- ally inflammatory), their ordinarily unconscious sensibility becomes conscious, and we perceive the pathological excitation of their sensory nerves under the form of pain. But this excitation, on arriving at a medullary segment, is (in consequence of its abnormality and of the error to which it gives rise) often ex- teriorized in the cutaneous nerves which terminate in this segment. Head has made use of this circumstance in order to ascertain the medullary segments which correspond to each of the large viscera and to each of their principal portions. Clinical phenomena of this kind have been long known, for example, pairh in the left arm and the left little finger in angina pectoris ; pain in the right shoulder in hepatic colic, that in the testicle in renal colic. Mackenzie has also observed that, in intestinal obstruction, cutaneous pain is situated above the umbilicus, when the obstacle is situated in the small intestine, and, on the contrary, between this point and the symphysis pubis when the obstruction is in a portion of the large intestine. In order to determine the visceral sensoiy areas. Head takes into account not merely these painful radiations, but also the hypercesthesia which is obvious in certain cutaneous zones under the influence of slight stimulation (pressure). He has proved the existence of a large number of hyperagsthetic zones, which are very diversely situated, and whose relation to the roots of nerves, and hence, to the corresponding medullary segments, he has endeavoured to ascertain. The reasoning on which these researches are based is the following : these hypersesthetic zones (according to Head) can be superposed on the anaesthetic zones which are observed in the lesion of the roots and of the corresponding medullary segments, and whose areas have been determined by physiological and anatomical clinical methods. The hyperaesthetic zone indicates the affected root. Clinical experience, on the other hand, showing that such and such a zone is hyperaesthetic when such and such a viscus is diseased, we are hence justified in concluding that the sensory nerves of this viscus and of the hyperaesthetic zone (although topographically they may be far removed) terminate in the spinal cord in common nuclei. It is these common nuclei which, receiving the patho- logically exaggerated irritations of the inflamed viscus, exteriorize them in the skin. This external manifestation may thus become a diagnostic aid as regards the affected viscus. Such is the principle of this method. Practically it still presents too much uncertainty, and the results have proved too discordant for us to be able to give here a detailed table, this table still requiring very important revision. It may be added that the proofs are too indirect in nature, and lend themselves to too many objections, to be accepted without discussion. The relations pointed out between the hyperaesthetic zones of Head and vis- ceral lesions may, however, retain their symptomatological value, while still leaving untouched the problem of the systematization of the sensory visceral nerves. C. CRANIAL NERVES— FUNCTIONAL DETERMINATIONS 1. Morphological regularity of the spinal cord. — In the spinal cord, the origins of the sensory and motor nerves are systematically arranged SENSATION AND MOTION— THEIR RELATIONSHIP 155 in a simple and regular manner, so that they can be recognized at first glance, it being assumed that experiment has determined their func- tions. Every nerve pair has its component elements arranged in the same manner, and experiments carried out on one are applicable to all. 2. Irregularity of the medulla oblongata. — In the medulla oblongata this systematic arrangement has disappeared, or has become almost unrecognizable ; it is on this account that there is a question con- cerning the cranial nerves. The sensory and motor elements form in it, indeed, irregular groups which destroy the systematic arrangement by which they can be so easily recognized elsewhere, by merely regarding their relative position ; hence it is necessary to investigate these elements of diverse function by experimenting directly on each of the nerve trunks which arise from the medulla oblongata. New superadded formations. — The medulla oblongata is a place of transition ; the symmetrical and regular medullary cylinder ends in it ; the cerebral ex- pansion commences in it, or is more or less prepared in it by acces- sory formations. These new masses, of different function and type, at fu-st sight upset the primitive scheme of the medullar}" edifice. Nevertheless, modified traces of it may be re- cognized, but for this it is not too much to unite the data furnished by experiment \vith those of morphology. Vertebral Theory op THE Cranium. — The classi- fication of the cranial nerves is connected with the old theory of Goethe and Oken, a theory often fiirbished up, and which we will attempt to describe in a few words. So far as concerns the cranial nerves, tliere is a double problem : ( 1 ) to recog- nize the equivalents of the anterior and posterior roots ; (2) the nerve pairs being constituted, to ascertain their metamerism. Mei. Spinal. Fig. 62. — Topography of the nuclei of the cranial nerves situated in the floor of the 4th ventricle. The motor nuclei in red the sensory nuclei in Ijlue. 156 SYSTEMATIC FUNCTIONS Ventral and dorsal nerves. — In the spinal cord each pair is formed by a ventral nerve (the anterior root) and a dorsal nerve (the posterior root). The ventral nerve is exclusively connected with the muscles ; or, in other words, it is functionally motor. The dorsal nerve is connected with the skin ; in other words, it is sensory. It must be added that in the dorsal nerve some centrifugal elements exist, but in limited number, which thus make it (however small their number and their importance) a mixed nerve. The ventral nerves (motor nerves) arise from the cells of the anterior cornua, emerge from the ventral path of the nerve axis, and are distributed to the muscles taking origin in the myotomes. The dorsal nerves (sensory nerves) arise from the cells of the spinal ganglia, penetrate the spinal cord by the dorsal portion of the nerve axis, and are func- tionally related to the organs of touch situated in the skin. Their motor fibres . Spinal access. Hypogl. Fig. 63. — Apparent origin of the cranial nerves at the base of the brain (after Hirschfeld). are very few and have only been studied so far as regards their function in those varieties which make their classification difficult (vaso-motor elements). In the cranium, the series of ventral nerves would be represented by the motor nerves of the eye (oculo-motor, external oculo-motor or sixth nerve, pathetic or fourth nerve) and the hypoglossal. But the fourth nerve is still under dis- cussion on account of its posterior exit ; the series of dorsal nerves would be represented by the trigeminal, the facial, the glosso-pharyngeal, the pneumogastric and the spinal accessory. SENSATION AND MOTION— THEIR RELATIONSHIPS 157 The nerves of special sense are generally left unclassified, on account of their extremely pronounced differentiation. Comparative morphology. — In the case of man, the arrangement, dorsal and ventral, of these two series is not very obvious, but it is much more so in that of the inferior vertebrata. In any case the ventral series represents those nerves which are, in the cranium, the continuation of those which arise from the antero- external group of the anterior horn of the spinal cord, and which are distributed to the muscles originating in the myotomes. Difficulties and objections arise, on the other hand, when the dorsal cranial nerves are compared with the posterior spinal roots. The motor elements which enter the trigeminal, facial, pneumo- gastric and spinal accessory, are of such great importance that they can scarcely be compared to the few centrifugal fibres of the posterior roots. Fiirther, while the latter have a connexion only with the skin, the sensory cranial nerves are distributed to the digestive mucous membrane. Superadded nervous apparatus, branchial nerves. — This difference is explained, according to Kupj^fer, by the existence, in the region of the dorsal cranial nerves, of a superadded system, the system of branchial nerves, which is wanting in the spinal nerves. This branchial nerve distributes its motor ramifications, not to the muscles derived from the dorsal segmented portion of the mesoderm (somites, myomeres, myotomes), which, in the craniun:i, as in the head, are innervated bj' the ventral nerves (eye and tongue muscles ; oculo-motor and hypoglossal nerves), but to other muscles, which are wanting in the trunk and are present in the head, and which originate from the lateral mesodermic nerve, segmented by the branchial clefts. Buccal foss. Olfact. foss. Myotome Branchial arches {lateral plates) Branchial cleft Fig. fi4. — Diagrammatic representation of the cephalic extremity of an inferior verte- brate (after the description of Van Wijhe). The cejahalic mesoderm (myotomes and lateral plates) in red. The dorsal nerves, in the cranial region, are thus the result of the union of a postei'ior root with a branchial nerve. Metamerism. — In the trunk, primitive metamerisation of the individual is rendered evident, in the adult, by the vertebral bodies and the corresponding nerve pairs which emerge in the interval between them. In the other organs, including the nerve axis and the whole of the muscles, there is no trace of this arrangement. In the cranivim, the skeleton, as also the muscles, are a very untrustworthy and arbitrary evidence if examined in the adult. It is necessary to have recourse to the antecedent arrangements in order to recognize metameri- sation in this region, and Huxley, and after him Gegenbaiu", find the evidence of this in the disposition of the branchial apparatus, each of the visceral arches of wliich would correspond to a metamere provided with its nerve pair. Myomeres and branchiomeres. — But, with Van Wijhe, it is necessary to take into account the segmentation of the cephalic mesoderm, which gives rise, in the skvill as in the trunk, to a certain nvunber of somites or myomeres ; and this so nuich the more as, the strict and exclusive relationship of the somites with the 158 SYSTEMATIC FUNCTIONS ventral roots being allowed, we shall thus be supplied with a fixed base for the numeration of the metameres. The solution of the question would be clear and definite were it possible to be certain as to the exact number of the segments. But the rapid changes which ensue in the couj-se of evolution upset at every moment the actual condition, certain of the myomeres falling into a state of atrophy very soon after their appearance. Yet further, the parallelism between the somites and the visceral arches is itself of short duration and gives rise to micertainty, the myomerism and the branchiomerism not proceeding equally. It thus results that the dorsal and ventral nerves can only be classed separately, without endeavouring to collect them together into exactly corresponding pairs ; and hence the determination of the ventral nerves is difficult from the metameric point of view. (a) Physiological characters of the nerve pair. — Physiology, which is based, above all, on the study of function, allots to the constitution of the nerve pair other characters chiefly drawn from experiment. The nerve pair is in this respect essentially made up of elements which are functionally associated in the constitution of a simple reflex axis, as they are in the metamere. CI. Bernard emphasized another character, which expresses the signification of the preceding one : Tico nerves, one sensory and the other motor, form a ^physiological pair, tvhen the first gives to the second its recurrent sensibility. When it is remembered that, as CI. Bernard holds, the terminal apparatus of this recurrent sensibility is situated at the surface of the spinal cord and of its membranes regarded as receptive of excitation, in the very region of the roots under consideration, the functional associa- tion of the two nerves will be seen to be strengthened. Contingence of the associations in the course of functional activity. — But the physiologist also knows that these connexions are changeable, according to the progress and the necessities of the func- tions, and he is therefore the less surprised on learning of the diffi- culties which prevent a rigid classification of these associations. For him the conception of the nerve pair has a symbolical value, or one for convenience of description. Having made these reserves, it may be useful to point out those amongst the cranial nerves which approach the most closely to the primitive and ideal scheme which has been studied with regard to the spinal roots. Thus we find, in the trifacial or trigeminal those characters which are most clearly characteristic of a sensori-motor nerve pair, but of a nerve pair which has already lost its regularity and its symmetry, and which must be completed or dissociated if it is desired to re- estabhsh in it equilibrium between the sensory and motor elements. Nerve pair approximating the spinal type. — The trigeminal nerve takes origin in the medulla oblongata through the pons by two roots, the one sensory, bearing a ganglion {Gasserian ganglion), which is SENSATION AND MOTION— THEIR RELATIONSHIPS 159 obviously the equivalent of the ganglion of a posterior spinal root ; the other motor, which proceeds to form with the preceding a mixed nerve by interminghng of its fibres. This is not merely an induction from the resemblance of form ; if an experiment is made (with more difificulty, it is true) on the roots of the trigeminal similar to that on those of the spinal nerves, sensory and motor paralysis may be induced by the section of the nerve ; by the stimulation of each of the two roots sensation and motion may be elicited. But this nerve (as its name points out) has three branches, cor- responding to the three large subdivisions of the face, and of these three branches the third only (inferior maxillary nerve) receives the elements of the motor root. Hence the trifacial pair must be com- Op'it. n escend. Motor R. ; - Princ. Mnior N. Desc. Spiiuil R. Sup. Max. X. Inferior Max. y. Fig. 65. — Diagram of the real origins and the constitution of the trigeminal (after Van Gehuchten). pleted by the addition of motor elements. These elements are found in the purely motor trunks, which are distributed to the muscles of the orbit and which supplement the first branch of the trigeminal (ophthalmic branches), which confer sensation on the ocular region ; these are the oculo-rnotor nerve, the external oculo-motor nerve, and the pathetic. Functional association of the radicular elements. — The trifacial pair is obviously sensori-motor. It corresponds to the same sense as the posterior roots, namely, to the tactile sense or that of general sensa- tion. The other cranial pairs, more or less strictly so-called, which take origin from the medulla oblongata, have as a basis either purely sensory nerves, as the olfactory, the acoustic, or the optic ; or nerves 160 SYSTEMATIC FUNCTIONS in which sensorial elements are mixed with sensory elements, as the glosso-pharyngeal ; or, finally, trunks in which general sensation is mixed up with obtuse and subconscious sensation, as the pneumo- gastric and spinal accessory. Their complexity. — These associations between sensory nerves or those of special sense, and motor nerves, are from the point of view of function, both multiple and variable. Hence they are in no sense exclusive the one of the other. The motor nerves of the eye, which Trij. (G. Gasser). Wrisberg. Acoustic {Scarpa). 61. Ph. (Andersh). Pneumo. gust [G. PIe.c) L^<- Mot. tri;. (Mast.) Cervic N. Fig. 00. — Ganglia of origin sensory cranial nerves. of the Fig. Their roots, and the ascending and descend- ing branches of these roots. GOa. — Nuclei of origin of the cranial motor nerves (diagram). The nuclei are seen laterally through the cerebral trunk supposed to be transparent. are connected, by the ophthalmic branch of the trigeminal, Avith general sensation, are equally so, by the optic nerve, with the sense of vision, and may be connected with that of hearing, or of any other by the special conducting tracts of these senses. And it must not be forgotten that it is the same in the spinal cord ; the functional bond of union Avhich subsists between the posterior and the anterior root, and which is manifested by a reflex act, is interesting, because it illustrates the rough sketch of the nervous system ; but it is not strictly necessary, and the muscles of the limbs or of the trunk, as SENSATION AND MOTION— THEIR RELATIONSHIPS 161 well as those of the face, are functionally associated at every moment with the superior senses. Another example. — The jacial, with its two roots, of wliich one (small root) l^ears a small ganglion {geniculate ganglion), is also often compared to a spinal nerve pair. Tlie large root is obviously motor ; the small, known as the nerve of Wrisberg, is regarded as a nerve of special sense ; it shares with the glosso-pharjmgeal the faculty of conferring the sense of taste ; by the chorda-tympani it proceeds to the tip of the tongue, while the glosso-pharyngeal is distributed to the root of the tongue ; the tip and the root of the tongue comprise the area of the sens© of taste. Facia! A'. Intern. A', SolU. F. aetnc. Gangl Fig. G7. — Facial ner\e and intermediary nerve of Wrisberg. Facial pair. Multiple associations of the original elements. — The motor portion of the facial nerve does not go to the muscles of the tongue, but to the cutaneous muscles of the face, and it is consequently not cormected with the sense of taste except in a wholly contingent manner. The motor portion of the facial is, on the contrary, connected in a more direct manner, although a still very partial one, with the sense of hearing by the branches which it supplies to certain muscles of the ear (muscles of the external ear, muscle of the stapes). The acoustic nerve is closely attached, at its origins, to the two roots of the facial, and passes through the same orifice as the latter (internal auditory meatus) before separating from it m order to proceed to the internal ear. From the functional point of view, the facial is also associated with the sense of smell, in order to ensure the movements of the nostrils connected with the exercise of this sense ; with the sense of sight, in order to perform the movement of the upper lid ; and very generally with the tactile, or general sensibility, of the trigeminal. The hypoglossal nerve, its small inconstant root. — The motor nerve of the tongue, whose mvicous membrane is supplied with the organs of taste, is the hypoglossal. Usually it is formed of a single order of roots (motor roots) ; but P. M 162 SYSTEMATIC FUNCTIONS sometimes it is svipplied with a small root bearing a ganglion, and thus repro- duces the tj'pe of the spinal nerve pairs. Glosso-pharyngeal ; pneumo-spinal. — The glosso -pharyngeal, the pneiuno- spinal (formed by the union of the origins of the pneumogastric and of the spinal accessory) originally contained motor and sensory elements of different nature, which have caused them to be assimilated to the fxinctional pairs. Their centri- fugal and centripetal elements are mixed from their exit from the medulla ob- longata ; the ganglia which they pass through (ganglion of Elii-emitter and ganglion of Andersch in the first ; jugular ganglion and plexiform ganglion in the second) are partially comparable to the spinal ganglia of the i30sterior roots. Overlapping and reciprocal penetration of areas. — In short, all the motor, sensory, nerve trunks and those of special sense to which we have just referred are distributed to areas wliich overlap each other, fit together, penetrate each other, and are more or less superposed ; to such a degree, indeed, that only by experiment is it possible to determine their limits and to unravel the complicated skein formed by every kind of fibre which is woven in the tissues of the face and of the neck throvigh their multiple anastomoses. As they present themseh^es to our observation and to experiment in the adult animal, the cranial nerves are. some of them, isolated, such as the olfactory and the acoustic ; while others reproduce, more or less definitely, the arrangement of the sj^inal nerves (trigeminal, facial) ; others, again, have their sensori-motor elements mixed from their emergence. Method of study and description. — The method of study of these anatomical groujDings is based on a double analysis ; ( 1 ) it is necessary to separate in them the sensory from the motor elements ; (2) further, to separate in these groups, the different sensory and motor elements. From this second j^oint of view the elements are divided into two new general categories, that of the conscious and that of the unconscious, as will be explained further on. Anatomically, the imconscious is represented by the great sympathetic and its bulbar equivalents. On account of the intimate intermixture of these elements with the majority of these nerves whose anatomical arrangement is morjahologically irregular, it is necessary, in order to obtain perspicacity, to now describe the cranial sympa- thetic at the risk of repeating this description when we commence the study of the unconscious svstem as a whole. (&) Relations with the Great Sympathetic— In the spinal cord each nerve pair is attached to a ganghon of the great sympathetic and is, in a way, completed by its connexions with this special system. It is precisely the same in the medulla oblongata ; only the determi- nation of the elements which belong to it presents, in this region, greater difficulties. In fact, the great sympathetic, in its strictly vertebral portion, follows the same typical and regular arrangement as the nerve pairs with which it exchanges communicating branches. In the cranial portion, the shocks which have dissociated the primi- tive nerve pairs have, at the same time, changed the morphological characters by which they are recognizable, and compel us to identify them by the direct authentication of their functions. Normal Type of these Relations. — Yet, the normal type of the rela- tions of the great sympathetic with the other nerves has not entirely SENSATION AND MOTION— THEIR RELATIONSHIPS 163 disappeared, and it is again in the subordinate divisions of tlie tri- geminal that we recognize it. To the three branches of distribution of this nerve (ophthalmic branch, superior maxillary nerve, interior maxillary nerve) are attached three ganglia {ophthalmic ganglion, S'pheno-palatine ganglion, otic ganglion) ; further, a fourth ganglion {submaxillary ganglion) is attached to a large branch of the sub- maxillary nerve, the lingual nerve, all of these ganglia being clearly those of the great sympathetic. Fig. (58. — Diagram representing the principal cranial nerves and their chief anastomoses by means of tlieir ganglia, in the dog. Ill, oculo-motor ; V, trigeminal ; VII, facial ; IX, glosso-pharyngeal ; X, pneumogastric ; XII, hypoglossal. The great sympathetic forms •a^ common trunk (vago-sympathetic) with the pneumogastric in the neck, and becomes distinct before tlirowing itself into the superior cervical ganglion. In the skull, the sympathetic cham is represented by a branch proceeding from the superior cervical gangUon to the ganghon of Gasser, and from thence to the ophthalmic spheno-palatine and otic gangha of the trigeminal. The trifacial pair, which may be looked upon as condensed, when its roots and its Gasserien ganglion (the equivalent of a spinal gang- lion) are taken into consideration, is, on the contrary, found to be dissociated, when its sympathetic ganglia are regarded. Cranial sympathetic. — The three first of these gangha are attached to the chain of the great sympathetic by a double branch, which, 164 SYSTEMATIC FUNCTIONS leaving the superior cervical ganglion, proceeds to join them ; one of these branches forms the carotid, plexus and goes to seek them separately ; the other (and it is this which I consider to form the cranial prolongation of the cervical cord) terminates in the Gasserien ganglion, is distributed to the three branches of the trigeminal, and by the intermediation of these latter comes into relationship with the three gangHa (ophthalmic, spheno-palatine, and otic). The reality of these connexions is shown by experiment : excitations applied to the cervical cord pass through these ganglia in order to dilate the pupil, to cause certain glands to secrete, and to act on the circulation of certain areas of the face. Branches of distribution and branches of origin of the sympathetic in the skull. — Like the spinal pau's, the trigeminal receives elements from the great sympathetic, which are intermixed with its branches of distribution and proceed to the apparatus whose function it pre- sides over (vessels, glands, involuntary muscles) ; but, hke the spinal pairs, it supplies in its turn original branches to the ganglia which correspond to it, and this is proved by experiment : the stimulation of the origins of the trigeminal in the skull has the same effect as that of the cervical sympathetic ; it reacts on the pupil, on certain vessels, and on certain glands. Connexions with the oculo-motor nerves. — The motor nerves of the eye are also connected with the great sympathetic ; from it they receive slender branches which are destined for the vessels of the muscles which they supply ; further, by means of the oculo-motor, they give to it more than they receive from it, because it is through it that the thick and short branch is furnished which is one of the roots of the ophthalmic ganglion, and which represents the constrictor nerve of the pupil. The long and slender branch which is supphed by the ophthalmic division is its dilator nerve. Connexions with the hypoglossal. — At the other extremity of the meduUa oblongata the connexions of the hypoglossal are established with the superior cervical ganglion, which supplies it with a very obvious anastomosis and one whose function is weU defined (con- strictor nerve of the vessels of the tongue). On the other hand, the hypoglossal supplies few or no original elements to the great sympa- thetic (experiment does not reveal their presence). Ganglionic elements of the facial. — In the interval which separates the hypoglossal from the trifacial pair, the relations of the great sympathetic with the facial, the glosso-pharyngeal and the pneumo- spinal nerves, appear to be altogether interrupted. Yet these rela- tions exist, but in order to demonstrate them, it is once again necessary SENSATION AND MOTION— THEIR RELATIONSHIPS 165 to resort to experiment. The facial sends to three of the gangha of the trigeminal three important branches : the two superficial petrosal nerves (large and small) for the spheno-palatine and otic ganglia, and the chorda tympani for the submaxillary ganglion. It is obvious that these branches act on the vessels (dilatory nerves) and on the glands (secretory nerves) of the corresponding localities. They are then origins of the great sympathetic, which must be added to those referred to above. Geniculate ganglion : its nature. — It is usually considered that these three branches arise from the nerve of Wrisberg through the inter- mediation of the geniculate ganglion. The question to be determined is if this ganglion is purely sensory, or whether it is mixed in function. Even for the ganglia of the posterior roots, this question is not abso- lutely settled. According to Onodi, the ganglia of the great sym- pathetic and the spinal ganglia take origin in immediate proximity the one to the other, and are only separated afterwards ; is it possible that a portion of the first remains incorporated in the mass of the second ? This question may be propounded, but it has not been answered. Ganglion of Andersch : Jugular and Plexiform Ganglia. — The glosso- pharyngeal and the pneumogastric nerves pass through ganglia, to which may confidentl}^ be attributed the sensory nature of the spinal ganglia ; but, it being admitted that these two nerves have important vaso-motor and secretory functions, and that they possess these functions from their origin, it is probable that these ganglia are also mixed, that is to say, half sensory like the spinal ganglia, half motor of the vegetative life, like those of the great sj^mpathetic. 1. Nerves of Special Sense Those organs, and therefore those nerves, are called seyisoYial, which subserve the special senses other than the tactile sensibility, known as general. This convention might lead to the supposition that the tactile sense is only an element common to the other senses, which represent it merely in a differentiated condition. As a matter of fact, touch, properly so called, is itself a sensation differentiated in a special direction. That which earns it the name of general sensi- bility is not its more simple physiological modification, but its more extensive anatomical distribution. Just as the other senses, it also presents numerous gradations. Amongst the nerves of special sense, or sensorial nerves, there are some which in the medulla oblongata really reproduce the arrange- ment of a posterior root ; such are the glosso-pharyngeal and the 166 SYSTEMATIC FUNCTIONS auditory, which have nuclei of origin analogous to those of the tri- geminal, itself the bulbar nerve of touch ; but there are others, such as the olfactory and especially the optic, whose morphology has entirely broken away from this arrangement of nerve roots. These nerves are in reality tracts of the spinal cord or of the brain pro- longed to the neighbourhood of the organs of sense (neurons of pro- jection of the second degree), at the extremity of which grey matter is found (retina), and, arising from the latter, microscopical nerve elements (rods, cones), which represent neurons of the first or peripheral order. a. Olfactory Nerve This name must be limited to the collection of nerve filaments which creep in the pituitary mucous membrane, and are prolonged to the Saso-p'ilaiive Int. branch of N. the olf. N. Oil Hnlb F.thmoid F. Deep hranch of the ethinoid i.racl. Fig. 69. — Branches of the olfactory nerve. This figiire only shows the branches which are distributed to the internal sui'face of the nasal fossa (after Hirschfeld). olfactory bulb, by passing through the perforations of the cribriform plate of the ethmoid. These bundles have no ganglion in their course, but their fibres take origin from cells which have persisted in the olfactory mucous membrane, together with epithelial elements which cover the latter. This is an arrangement which is common to the neurons of special sense, and distinguishes them from the neurons of general sensation. This arrangement is, further, that usually found as regards the nerves of sensation in the invertebrata. SENSATION AND MOTION— THEIR RELATIONSHIPS 167 It is allowed without dispute that the olfactory nerve is the con- ductor of olfactory impressions, and that it is the only conductor thereof. Yet its function has been caUed in question by Magendie, who beheved that the sense of smell was preserved in animals after he had destroyed the olfactory bulb. The sense of smell would then have appertained to the fifth pair, whose filaments are distributed in the nasal mucous membrane, together with those of the olfactory nerve. But the substance which was made use of (annnonia) was an irritating vapour which acted on the terminations of the nerve of touch ; and this explains the evidences of disgust which the animals evinced. On the other hand, section of the trigeminal induced, after a certain period, trophic changes in the sensory apparatus to which branches of this nerve are distributed. Hence result secondary sen- sorial paralyses or pareses, which are the result of a mechanism entirely different from that which causes paralyses or pareses of the sensorial nerve itself, but which may be attributed to the latter if this circumstance is not taken into account. CI. Bernard, Le Bee, Testut liave ascertained the absence of the olfactory bulb and the olfactory tract (formerly known as the olfactory nerve) in persons in whom the sense of smell was present diiring life. Le Bee's case has been sub- mitted by M. Duval to histological investigation. This author has ascertained, on the one hand, the presence in the pituitary membrane of olfactory filaments ; and, on the other, in the brain, the existence of a real olfactory tract ; whence the conckision follows that the intermediate conductors must have been present, although following an abnormal covirse. b. Optic Nerve Just as the olfactory tract must not be described by the name of olfactory nerve, so the optic nerve should no longer be regarded as l^eing the extended link between the retina and the brain passing through the chiasma, but only the neurons which directly receive the luminous impression and communicate it to the ganghonic elements of the retina. In the limited thickness which separates the retinal surface from these ganghonic cells two superposed layers of neurons maybe defined : (1) the rods and the cones, (2) the bipolar cells ; and there is a discussion as to whether it is the first or the second which represents the sensory nerve of vision, and which is therefore equivalent to the neurons of the posterior roots in the exercise of the sense of touch. The function of these elements must not be sepa- rated from that of the retina, to which we shall several times have to return. c. Auditory Xerve The auditory nerve suggests, far more than the preceding forma- 1(;8 SYSTEMATIC FUNCTIONS tions, the typical arrangement of an ordinary sensory nerve. It is massed together into a trunk which, from the internal ear, is distri- buted to the lateral portions of the medulla oblongata, it runs side by side with the facial nerve and the intermediary nerve of Wrisberg in the interosseous portion of its journey through the internal auditory canal. Like the other nerves of special sense, it has nevertheless its cells of origin at the periphery, in the ganglia, included in the interior of the special apparatus of the sense of hearing. In reality this nerve is double, and corresponds to two distinct functions : that of audition or reception of sounds by a special apparatus contained in the cochlea and that of the reception of special impulses connected with the idea of movement or of fosition in space by another equally special apparatus contained in the semicircular canals. One is the cochlear nerve, the other the vestibular nerve ; the cells of origin of the first are situated in the ganglion of Corti or spiral ganglion ; those of the second in the ganglion of Scarpa. d. Nerves of Taste The elements of the gustatory special sense are contained, as regards their greater portion (base of the tongue and isthmus of the throat) in the glosso-pharyngeal^ and as regards a smaller part (tip of the tongue) in the chorda tympani, a branch of the facial mixed with elements whose functions are diverse. Their experimental study is included in that of the nerve trunks. 2. Sensory and Motor Nerves In a first natural group are included the motor nerves of the eye, namely : the oculo-motor (3rd pair), the pathetic (4th pair), the ex- ternal oculo-inotor (6th pair) ; then important and complicated nerves like the trigeminal (5th pair), the facial (7th pair), the glosso-pharyngeal (9th pair), the vagus -or pneumogastric (10th pair), the spinal accessory (11th pair), the hypoglossal (12th pair). As applied to the cranial nerves, the expression " nerve pair " is devoid of all physiological signification, in the sense of that which is given to it in the case of the spinal nerves (union of a sensory and motor root) ; it simply describes the two nerves which are symmetrically detached from the medulla oblongata at the same level. a. Oculo-motor The oculo-motor nerve supplies the levator palpebrae superioris, and in addition four (out of the six) of the muscles of the eyeball, namely, the superior rectus, the internal rectus, the inferior rectus, and the SENSATION AND MOTION— THEIR RELATIONSHIPS 109 inferior oblique. Paralysis of this nerve is rendered evident by ex- ternal strabismus (a non-compensated contraction of the external rectus), and a displacement of the eye both downwards and inwards, together with a slight rotation, due to the superior oblique. Down- ward and upward movements are impossible. The dipper eyelid falls on account of the predominant action of the orbicularis palpebrarum. Borrowed sensory elements. — The oculo-motor nerve receives an anastomotic fibre from the ophthalmic branch, which is of sensory function (muscular sense). Original ganglionic elements. — At its origin this nerve includes tlie highest root of the great sympathetic. It separates from it in the form of a thick and short filament, which proceeds to the ophthalmic ganglion and thence, by the ciliary nerves, to the muscular apparatus of the iris and to that of accommodation. In addition, therefore, to the movements of the muscles referred to abos^e, stimulation of the oculo-motor causes contraction of the iris and protrusion of the crystalline lens. The antagonistic action (dilatation of the iris and flattening of the crystalline) is effected by the slender branch of the nasal nerve, which originally arises either from the great sj'^mpathetic or from the trigeminal itself. The oculo-motor nerve receives gang- lionic elements not only through its proper roots, but also by its anastomoses with the chain of the great sympathetic properly so called. b. External Oculo-motor and Pathetic The external oculo-motor is distributed to the external rectus muscle ; its paralysis causes internal strabismus. The pathetic is distributed to the superior obUque ; its paralysis is followed by a deviation of the globe of the eye upwards and outwards. The oculo-motor nerve alone of the motor nerves of the globe of the eye appears to supply branches of origin to the cranial ganglia of the great sympathetic, but all receive from this nerve branches of distribution which, without any doubt, are destined for the vessels of the muscles of the eye. Sensory elements. — All the motor nerves of the eye are, on the other hand, in an anastomotic relation with the ophthalmic branch, as much by recurrence near to their extremities, as directly at the level of the cavernous sinus. These sensory elements are destined for the ocular muscles, whose degree of contraction they estimate and measure, so that it may be proportioned to the movement about to be undertaken. c. Facial The facial nerve arises from the medulla oblongata by two roots : 170 SYSTEMATIC FUNCTIONS the one larger, motor ; the other very small {pars intermedia of Wris- Oeiilo- Er oculO' nuitor N motor A Inj. max. Opiu. .bup. Spfieii. UplU. In/. CiUary bub. orb. N. N. max. Pal. G. obliq. Branches N. N. G. Branch Fig. 70. — rrincipal nerves of the eye, motor, sensory, sensorial and ganglionic. Their anatomical relations. Ophthalmic ganglion ; direct and indirect ciliary nerves. herg), terminating in the ganglion {geniculate ganglion), situated on one of the angles of the nerve in its transit through the intrapetrous ■ Svp. ocitlo-motor Br. Cav. plez. Br. of orijin oi the eir it. plexus. Sup. cervical O. Irido-constrict. Fibres. .Contract. Fibres of the ciliary muscle. „ Vaso-constrict. fibres of the eye. Irido-dilating fibres. , Relaxing fibres of the ciliary muscle. . Vaso-dilating fibres of the ant. segm. of (he e««. Fig. 71. — Constitution of the ophthalmic ganglion (diagram after Cuneo). portion of the temporal bone, and which would appear to be sensory. SENSATION AND MOTION— THEIR RELATIONSHIPS 171 The facial has, further, vegetative sensori-motor functions resembling those of the great sympathetic : this nerve was the S7nall sym'patJipMc of older observers. --. Right inf nbliqne. Ophthal. O. — ^ Oc. iiiolor. Frontal .V. ^ Lachrymal }J. ■ orbital branch of the superior maxillary ; q, lachrymal ; r, frontal : s, sub- trochlear ; t, sub-orbital. 1, styloid apophysis ; 2, digastric ; 3, pinna of the ear ; 4, masseter ; 5, large zygomatic : 6, scutellar ; 7, zygomatic arch 8, superior maxillary. 174 SYSTEMATIC FUNCTIONS palato-pharyngeus, the nasal tone of the voice to that of the levator palati ; the deviation of the uvula to that of the azygos uvulae. The fibres which proceed to the soft palate and to its pillars are conducted thither by the posterior palatine nerves, which proceed from the geniculate ganglion by the intermediation of the large super- ficial petrosal nerve, and of the spheno-palatine ganglion. It is a question whether these nerves are not closely united to these ganglia, or whether, as Longet holds, they actually form a component part of them. The exact origin of the motor nerves of the soft palate is not clearly known. 2. Sensory elements. — The soft palate receives for its pillars some sensory gustatory elements which come to it through the palatine branch of the nerve of Wrisberg (Vulpian). 3. Ganglionic elements : superficial petrosal nerves. — The facial contains a notable proportion of nerves, really ganglionic (vegetative in function), which it derives from its own origins (small root, nerve of Wrisberg). The same palatine nerve, which has just been con- sidered, like the great superficial petrosal 7ierve which gives rise to it, contains secretory elements destined for the glands of the mucous membrane of the soft palate ; it also contains vaso-dilator fibres for the same structure. The vaso-constrictor fibres are distributed to it by the filament of the great sympathetic, which is closely united to the large petrosal in order to form the Vidian nerve. The small super- ficial petrosal, which from the facial proceeds to the otic ganglion, is, as regards its function, but imperfectly known. Deep petrosal nerves. — The great and small superficial petrosal nerves each receives from t'he glosso-pharyngeal, by Jacobson's branch, an anastomotic branch, which is, as regards the first, the small internal deep petrosal and, as regards the second, the small external deep petrosal. The function of the first is but little known ; the second contains secretory and vaso-dilator elements of the parotid gland. Leaving the glosso-pharyngeal, they then follow a branch of the facial ; having passed through the otic ganglion, they enter the auriculo- temjsoral branch of the inferior maxillary and then pass into the parotid. Thus the glosso-pharyngeal and the facial both take part in the A'aso-motor and secre- tory innervation of the salivary glands and of the soft palate. This is proved by stimulating these two nerves at their cranial origin (Vulpian). Branch to the muscle of the stapes. — The facial supplies a small branch which proceeds in the middle ear to the muscle of the stapes. Its action is antagonistic to that of the filament to the internal muscle of the malleus, which takes origin in the trigeminal ; it relaxes the membrana tympani and lowers pressure in the labyrinth. Chorda tympani. — An anastomotic branch extends from the facial SENSATION AND MOTION— THEIR RELATIONSHIPS 175 to the lingual and skirts the internal surface of the membrana tympani ; for this reason it is known as the chorda tympani. Dis- tributed to the tongue, to the sub-maxillary and to the sub-lingual glands, this branch is vaso-dilator as regards these three organs ; it is secretory in function for the two glands. We shall see a little further on that this important branch also contains centripetal elements. With the two superficial petrosal nerves it is regarded as the principal continuation of the small root, or nerve of Wrisberg. 5 Jl* V V .; Great sup. petrosal N. Small iitp pelrosil N. ^~r~^^\ Spheno- pnl. ganjlion. Inf. dent. A'. Glni>v;',;..', Mylo-liyoid y Fig. 84. — Inferior maxillary nerve (internal aspect.) stridor of the pharynx, and of a portion of the muscles of the soft palate. Vaso-dilator elements. — Just as the gustatory fibres of the l-ngual and of the chorda tympani are accompanied by vaso-dilator elements, so also are those of the glosso-pharyngeal. By stimulating without or within the cranium the ninth pair in animals whose circulation is intact, Vulpian has observed a marked congestion at the base of the tongue in the area of distribution of this nerve. There is also dilatation of the parotid vessels. 188 SYSTEMATIC FUNCTIONS Secretory elements. — The same author has noticed that this stimula- tion (made within the skuU) causes the parotid glands to secrete. By combining this result with those previously obtained by CI. Bernard, Schiff, etc., the course of the secretory nerve of this gland is found to be the following : origin of the glosso-pharyngeal, the ganghon of Andersch, Jacobson's branch, small, deep external petrosal, otic ganglion, auriculo- temporal branch, of which certain ramifications are distributed to the parotid. Ganglia ; their functional nature. — The same remark may be made concerning the ganglia of the glosso-pharyngeal (ganglion of Ehren- ritter and ganglion of Andersch) as concerning those of the greater portion of the cranial nerves ; these ganglionic masses are without doubt the partial equivalent of the spinal ganglia of the posterior roots (sensory and sensorial fibres) ; but it is probable that they also represent the ganglia of the great sympathetic. Jacobson's nerve, which is given off by the ganglion of Andersch, is clearly a nerve of vegetative Ufe, as is shown by its connexions with one of the salivary glands. f. Pneumogastric The pneumogastric nerve (still called the median, sympathetic, vagus nerve, or nerve of the tenth pair), ramifies in the head, neck, thorax, and the abdomen, that is to say, in numerous and important organs which subserve very varied functions. It includes seiisory and motor elements, both of the life of relation and of the organic life, and this from its very origins in the lateral furrow of the medulla oblongata. A Typical arrangement. — In the constitution of the metamere, such as that which corresponds to a spinal nerve pair, or even to the trigeminal, the separa- tion of the sensory and motor functions on the one hand, conscious and uncon- scious on the other, affects a typical arrangement which much facilitates the analysis and the detailed description of these f mictions. In the pneumogastric nerve, this externally apparent systematization has almost disappeared, through the intermixture and intricacy of the different elements after leaving their place of origin. Nucleus of origin. — The nucleus of origin of the tenth pair is a mixed nucleus, enclosing elements whose functional value differs greatly. It receives a large number of centripetal fibres, among which the elements of unconscious sensation predominate, along with fibres of conscious sensation. It is the point of origin of a certain mmiber of terminal neurons which proceed to the voliontary muscles ; it gives off a much larger mmtiber of which the terminations are arranged in a graduated manner in the ganglionic masses belonging to the great sympathetic, thus showing in an obvious way the functional relations wliich this nerve main- tains with the system of vegetative life. Jugular and plexiform ganglia. — At its exit from the skull, the pneumogastric presents two ganglionic enlargements (jugular ganglion [ganglion of the root] and plexus gangliforniis [ganglion of the trunk]), which their structure designates as the SENSATION AND MOTION— THEIR RELATIONSHIPS 189 •equivalents of a spinal ganglion, at least as regards the larger portion of their •elements, but without, nevertheless, it being possible to affirm that they do not partially correspond to a great sympathetic ganglion. Field of distribution, — Taking origin in the medulla oblongata and soon anas- tomosing with important nerves, among which is the spinal accessory which yields to it its internal branch, the pneumogastric exhausts itself in ramifying suc- ■cessively in the neck, the thorax and the abdomen. Its inferior limits are not well determined because, falling into a complicated system made up of ganglionic relays which also contain fibres passing tlirough them, we have no anatomical ■or experimental method wliich is well adapted to determine the localities in which its fibres terminate ; these terminations being, further, arranged in a graduated series over a certain number of these relays. The pneumogastric is formed of fibres, some of which are myelinated, others non-myelinated. These Great sup. pet. xV. Spheno. pal. Q. Tub. br. Small deep petr. n Great deep. petr. n. Carol, tymp. Br. Br. fen. oi Br. fen. rot Jacobson N O. of A ndersch Spinal ac Int. jug. rein Jhjp. A Sup cervic. G Ext. carat. A 3rd cerv. N Pneumog Glosso-pharyngeal nerve (after Hirschfeld). latter are present in it in great number, and the former are in it much reduced in size. These characters approximate them to those of the great sympathetic. Protoneurons and intercentral neurons. — The elements of conscious sensation and of voluntary movement which the pneumogastric contains are confined to the neck and proceed chiefly to the larj-nx. Hence they are obedient to the law of metamerism, which demands that their terminations should be contained in the same segment of the body as that which includes their origins, or nearly so. They are initial or terminal neurons, or protoneurons. On the other hand, the elements of unconsciovis and involuntary function break altogether with meta- merism ; in doing this they reproduce the arrangement of the great sympathetic 190 SYSTEMATIC FUNCTIONS chain, wliich forms coinmixnications, no longer between the organs of a meta- mere, but between the different metaraeres themselves, for the functions as a whole. The gi-eater portion of the fibres of the pneumogastric are thus equiva- lent to intercentral fibres, which connect an important region of the grey axis (medulla oblongata) with the nuclei directly naotor in function, of organs which are essential to life (ganglion of the great sympathetic). Specific functions. — If, in the pneumogastric, the line of division between sensation and motion, and even between the conscious and unconscious, is not obvious at first sight, there is another which concerns the specific nature of the functions, and which is on the contrary very evident. This large assemblage of nerves exhausts its ramifications in three great apparatus : that of respira- tion, that of circulation, and tliat of digestion, whicli it helps to individually govern, and also to mutually harmonize amongst themselves, in union with other portions of the great sympathetic system. Yet the elements of specific functions, apart from the fact that they are each of them of different modalities (centripetal, centrifugal, motor, inhibitory, secretory, etc.), before they attain their ternninations, are mixed in the branches, which often contain them tmited Vdcia'. -■ T /■■--■-•' Facial. Great sup. pet.n. - - "'^j A."" '"', ^_-A_ '' ""-^'- /""• •'"/'• "»"/• Small deep pet. n. - "M»mT^>*'-^^~^;;5^,____^^^^^^^ ^^^T'>^o5=""~i-iCJ'" ' " ^«'"- '"^^- ^■^- '^'*''^- P'* Great deep pet.n. i?^-&'-^^\^ ■ (^VP^'^-^^~~^-'-~^-N.pet..pr.min.(Ar»,\ Small sup. pet. n. -- «- V ^^^^ Y ^V ^- V^t. sup. min. Y \ y yr^ G. olicum. k A ^~>J| \- - Chorda tympani. G. of A ndersch. ■ ■ — ^ \ a .A . . G. petrosum. Chorda tympani. Diagram A. Diagram B. Fig. 86. — Diagram of the terminal branches of the branch of Jacobson. A, French nomenclature ; B, German nomenclature. one with another. Thus we are forced to make an analytical and detailed review of them in connecting them only with their chief fvinctions. A. Respiration. — Respiration is represented in the vagus by the nerves, some centripetal, others centrifugal, which preside over very diverse kinds of acts. Superior laryngeal branch. — The upper portion of the larynx, the arytseno-epiglottidean folds, the epiglottis, the posterior and inferior portion of the tongue, derive from this nerve (chiefly by the superior laryngeal branch), the acute sensibility with which these regions are endowed, and which gives rise to the expulsive cough following the introduction of the least drop of liquid falling on the aperture of the glottis. The trunk of the vagus, when the nerve is cut in the region of the neck, is but slightly sensitive ; it contains nevertheless a large quantity of subconscious sensory elements which are distributed to the inferior portion of the larynx and the trachea {inferior laryngeal or recurrent nerve), the oesophagus, the pulmonary tissue, the stomach, and doubtless also the intestine and the liver. By the anastomosis of Galen, which connects the superior laryngeal SENSATION AND MOTION— THEIR RELATIONSHIPS 191 nerve to the recurrent, the lower portion of the larynx and the trachea receive sensory elements from the first of these nerves (Philippeaux and Vulpian ; Fr. Eranck), in addition to those which they receive from the second. Sensitiveness of the different portions of the larynx. — The great difference which exists between the sensitiveness of the superior and the inferior portions of the larynx can be shown experimentally : water injected from above downwards (upon the opening of the glottis) gives rise to a loud expulsive cough ; when injected from below upwards (by an opening made in the trachea), this defensive reflex does not arise. Dyspnoea. — No more than it suppresses hunger, does section of the two vagus nerves in the neck do away with the necessity of breathing. On the contrary, it may be said that it increases that necessity. Respiration becomes slower and deeper, assuming the characteristics of that which is known as dyspnoea. Eeffcts of the stimulation on the respiratory movements. — Stimula- tion of the superior end of the cut vagus hastens, on the contrary, the respiratory rhythm, and this so much the more as the stimulation itself is the more intense ; it may arrest the movements of the diaphragm and of the thorax, either in inspiration or in expiration, according to the branches excited, or according to the gaseous com- position of the blood. This arrest is due to a tetanic contraction of one of the two orders of muscles, combined with inhibition of those of the opposite order. Reflex sensibility of the viscera. — Doubtless the vagus nerve repre- sents the sensory element of a considerable number of reflex pheno- mena which are observed as regards the vegetative functions, and double section of the nerve should induce in the latter various per- turbations ; but these are not rendered obvious by disorders which are immediately visible like the preceding. Recurrent nerve. — ^\Vith the exception of the cricothyroid, which is innervated by the superior pharyngeal nerve, all the muscles of the larynx are supplied by the recurrent or inferior laryngeal nerve. Original and borrowed elements. — The pneumogastric in the region of the neck contains a large number of fibres which take part in very diverse motor functions ; these fibres come, some from the same origins as those of the tenth pair, and others from anastomoses with the neighbouring nerves, especially the spinal accessory, with regard to which we shaU refer to them again. The movements of dilatation which the glottis performs at every inspiration must equally appertain to the pneumogastric, for they persist after the extirpation of the 192 SYSTEMATIC FUNCTIONS Avcf. A. Occip Spi, ■Sup. Inierc. Vein (Esophag. br. ■Thoracic gang .Inierc. N. _i§] Sup. diaph. A RciM A Mreat splanehnie. Pneumn. (Right). Semi-lunar O. Fig 87 — Pneumogabtiic SENSATION AND MOTION— THEIR RELATIONSHIPS 193 Ophthal Sup. max. ' j lAngual. - / ;; [^1 Gl. pharyn Int. carot. - Hypoglossa' Thyro-hyoid. V Pharyngeal pie r -^ p I anal — f mtdiinjastnc. — hit tpin br. • Ext <