IC-NRLF 
 
 ORY 
 
 HERING 
 
UNIVERSITY OF CALIFORNIA 
 
 MEDICAL CENTER LIBRARY 
 
 SAN FRANCISCO 
 
 Ex Libris 
 C. K. OGDEN 
 
MEMORY 
 
 LECTURES ON THE SPECIFIC ENERGIES 
 OF THE NERVOUS SYSTEM 
 
 BY 
 
 PROF. EWALD.HERING 
 I . -' 
 
 UNIVERSITY OF LEIPZIG 
 
 
 FOURTH EDITION, ENLARGED 
 
 1913 
 
 THE OPEN COURT PUBLISHING COMPANY 
 CHICAGO LONDON 
 
COPYRIGHT BY 
 
 THE OPEN COURT PUBLISHING COMPANY 
 1913 
 
 
 
PUBLISHERS' PREFACE 
 
 DR. EWALD HERING, professor of psychology at the Uni- 
 versity of Leipsic, is not a voluminous writer, but he has done 
 considerable work in investigating the nature of specific color- 
 sensations and the processes of life. The results of his 
 studies have largely been accepted by his colleagues and his 
 little essay "On Memory" which contains a popular exposi- 
 tion of this fundamental problem has become classical. Con- 
 cerning its significance for physiology, Prof Michael Foster 
 of the University of Cambridge says in the Encyclopedia 
 Britannica, Vol. XIX, page 22: 
 
 "If the ingenious speculations of Hering, that specific color- 
 sensations are due to the relation of assimilation (anabolism) 
 to dissimilation (katabolism) of protoplasmic visual sub- 
 stances in the retina or in the brain, should finally pass from 
 the condition of speculation to that of demonstrated truth, 
 we should be brought face to face with the fact that the mere 
 act of building up or the mere act of breaking down affects 
 the condition of protoplasm in other ways than the one which 
 we have hitherto considered, viz., that the building up pro- 
 vides energy to be set free and the breaking down lets the 
 energy forth. In Hering's conception the mere condition of 
 the protoplasm, whether it is largely built up or largely broken 
 down, produces effects which result in a particular state of 
 consciousness. Now, whatever views we may take of con- 
 sciousness, we must suppose that an affection of consciousness 
 is dependent on a change in some material. But in the case 
 of color-sensations that material cannot be the visual sub- 
 stance itself, but some other substance. That is to say, accord- 
 ing to Hering's views, the mere condition of the visual sub- 
 stance as distinct from a change in that condition determines 
 the changes in the other substance which is the basis of con- 
 sciousness. So that, if Hering's conception be a true one 
 
(and the arguments in favor of it, if not wholly conclusive are 
 at least serious), we are led to entertain the idea that, in 
 addition to the rough propagation of explosive decompositions, 
 there are continually passing from protoplasm to protoplasm 
 delicate touches compared with which the nervous impulses 
 which with such difficulty the galvanometer makes known to 
 us are gross and coarse shocks. And it is at least possible, 
 if not probable (indeed present investigations seem rapidly 
 tending in this direction) that an extension of Hering's view, 
 with such modifications as future inquiry may render neces- 
 sary, to other processes than visual sensations, more especially 
 to the inner working of the central nervous system, may not 
 only carry us a long way on towards understanding inhibition 
 and spontaneous activity but may lay the foundation of a 
 new molecular physiology. This however is speculative and 
 dangerous ground, but it seemed desirable to touch upon 
 it since it illustrates a possible or probable new departure. 
 What we have said of it and of the more manageable mole- 
 cular problems of physiology will perhaps show that, vast and 
 intricate as is the maze before the physiologist of to-day, he 
 has in his hand a clue which promises, at least, to lead him 
 far on through it." 
 
 Two other popular expositions of highly important problems 
 in physiology have been translated for the Open Court Pub- 
 lishing Company and are included in this edition, since 
 their extraordinary significance renders it desirable that 
 they be made accessible to the English reading public. 
 
 PAUL CARUS. 
 Chicago, September, 1913. 
 
CONTENTS 
 
 LECTURE I Page 
 
 MEMORY AS A GENERAL FUNCTION - 1 
 
 LECTURE II 
 
 THE SPECIFIC ENERGIES OF THE 
 NERVOUS SYSTEM 25 
 
 LECTURE III 
 
 THE THEORY OF NERVE-ACTIVITY - 43 
 
 INDEX 71 
 
MEMORY AS A GENERAL FUNCTION OF 
 ORGANIZED MATTER 1 
 
 WHEN a scientist leaves behind him his own spe- 
 cial province of inquiry to make an excursion 
 into the realm of philosophy, he may cherish the hope 
 of solving the great problem which underlies the minor 
 questions to which he has devoted his life, but he 
 must be prepared for being secretly discredited with 
 those of his colleagues who still remain quietly at work 
 with the subjects of their specialty, and at the same 
 time must expect the mistrust of the rightful represen- 
 tatives of the empire of speculation. He runs the risk 
 of losing his reputation with the former and of gaining 
 nothing with the latter. 
 
 The subject for which I ask your attention on this 
 occasion is a most alluring one; but in accordance 
 with what I have just said, it is not my intention to 
 abandon the domain of natural science to which my 
 studies have been devoted, but only to attempt to reach 
 a higher ground from which we may enjoy a freer and 
 more general survey. 
 
 It will seem in the course of this paper as though 
 
 'An address delivered before the Imperial Academy of Sciences, at 
 Vienna, May 30, 1870. 
 
2 MEMORY. 
 
 I am not always faithful to this purpose; for I shall 
 often have occasion to tarry in the province of psy- 
 chology. Consequently, for my own justification, al- 
 low me to point out the extent to which psychological 
 inquiries form, not only an allowable, but also an in- 
 dispensable accompaniment of physiological research. 
 
 The animal human organism with its material 
 mechanism is the subject of physiology. But con- 
 sciousness is a simultaneous datum. Besides the mov- 
 ing of the atoms of the brain according to certain laws, 
 the inner life of our soul is woven of sensations and 
 conceptions, of feeling and will. 
 
 Everyone experiences this in himself ; and it is a fact 
 also which beams forth from the faces of his fellow- 
 beings. It breathes in the life of all higher organized 
 animals, and even the simplest creatures bear some 
 vestiges of it. Who can fix the limit of empsychosis 
 in the empire of organic nature? 
 
 In the face of such a dual aspect of organic life 
 jvhat can physiology best do? Shall science be blind- 
 folded on the one side, in order the better to compre- 
 hend the other? 
 
 As long as a physiologist is a mere physicist and 
 I use the word physicist now in its most comprehensive 
 sense his method of inquiring into organic nature is 
 altogether one-sided. But it is justly so. As a crystal 
 is to the mineralogist, so to the physiologist of this 
 class is a man or an animal a mere lump of matter. 
 An animal feels, of course, pleasure and pain, and with 
 the material phenomena of the human body mental 
 emotions are connected; but that is no reason why a 
 physicist should take a different view of the corporeal 
 existence of man, who to him remains a compound of 
 
MEMORY. 3 
 
 matter subject to the same irrefragable laws as stones 
 and plants; like a machine, his motions are casually 
 connected with each other and dependent upon their 
 surroundings. 
 
 Neither sensation nor conception nor conscious will 
 can form a link in the chain of the material processes 
 of which the physical life of organisms consists. When 
 I answer a question, the initial material process is con- 
 ducted from the organ of hearing by sensory nerve- 
 fibers to the brain, and must pass through it as a ma- 
 terial process in order to reach the motor nerves of the 
 organ of speech. It cannot, after having arrived at a 
 certain spot in the brain, enter into something imma- 
 terial, in order to be re-transformed, in some other 
 place of the brain, into another material process. A 
 caravan in the desert might just as well enter the oasis 
 of a mirage, to return thence after a refreshing rest 
 into the actual desert. 
 
 Such is the physiologist, so far as he is a physicist. 
 He stands behind the stage and carefully observes the 
 working of the machinery and the movements of the 
 actors, but he misses the meaning of the action which 
 a spectator readily understands. Now, should a phys- 
 iologist never be allowed to change his point of view? 
 
 True, his object is not to understand a world of 
 concepts, but a world of realities. Nevertheless, if 
 now and then he changes his point of observation and 
 looks at things from the other side, or at least accepts 
 from trustworthy observers the results of their expe- 
 rience, he will derive much benefit from such an at- 
 titude and will better comprehend both the apparatus 
 he is studying and its methods of working. 
 
 For this very reason psychology is an indispen- 
 
4 MEMORY. 
 
 sable auxiliary of physiology. If the latter science has 
 hitherto not made much use of the former, it has not 
 been wholly the fault of physiology. Psychology has 
 only lately worked her fields with the plough of induc- 
 tion, and it is only in such a soil that the fruits can be 
 raised for which the physiologist has most need. 
 
 The neurologist is thus placed between the phys- 
 icist and the psychologist. The physicist regards the 
 causal continuity of material processes as the basis of 
 his inquiry; the thoughtful psychologist seeks for the 
 laws of conscious life, and in so doing works accord- 
 ing to the rules of inductive methods, assuming the va- 
 lidity of an inalterable order. Now, if the physiologist 
 learns from simple self-observation that conscious life 
 is dependent upon his bodily functions, and vice versa 
 that his body to some extent is subject to his will, he 
 has only to assume that this interdependence of mind 
 and body is arranged according to certain laws, and the 
 link is found which connects the science of matter with 
 the science of consciousness. 
 
 Thus considered, phenomena of consciousness ap- 
 pear to be functions of material changes of organized 
 substance, and vice versa. As I wish to avoid all mis- 
 conceptions, let me mention (although it is included 
 in the term function) that the converse of this asser- 
 tion means that material processes of the cerebral sub- 
 stance also appear to be functions of the phenomena 
 of consciousness. For if two variables are dependent 
 upon each other according to certain laws, a change 
 of the one demanding a change of the other and vice 
 versa, the one is called a function of the other. 
 
 This does not mean that the two variables, matter 
 and consciousness, are connected with each other as 
 
MEMORY. 5 
 
 cause and effect; for we do not know anything about 
 that. Materialism explains consciousness as the out- 
 come of matter, idealism takes the opposite view, and 
 a third position might postulate the identity of spirit 
 and matter. The physiologist, as such, should not 
 meddle with such questions. 
 
 Aided by this hypothesis of a functional connec- 
 tion between spiritual and material facts, modern phys- 
 iology is enabled to bring the phenomena of conscious- 
 ness within the domain of its inquiry, without leaving 
 the terra firnia of scientific method. The physiolo- 
 gist, as a physicist, observes how a beam of light, a 
 wave of sound, or a vibration of heat affects the organs 
 of sensation; how they enter the nerves, are trans- 
 formed into an irritation of the nerve-fibers and con- 
 ducted to the brain-cells. Here he loses all trace of 
 them. On the other hand, he observes a spoken word 
 coming from the mouth of a speaking person ; he sees 
 the person move his limbs, and finds these movements 
 are caused by muscular contractions produced through 
 motor nerves irritated by the nerve-cells of the central 
 organs. Here again he is at his wit's end. The bridge 
 which should lead him from the irritated sensory nerve 
 to the irritated motor nerve, is indicated in the laby- 
 rinthian connections of the nerve-cells, but he lacks a 
 clue to the infinitely involved processes which are in- 
 terposed in this place. 
 
 It is here the physiologist successfully changes his 
 point of view. Here matter no longer reveals the 
 secret to his inquiring glance; but he finds it in the 
 mirror of consciousness, not directly, but indirectly and 
 figuratively yet in lawful connection with what he 
 inquires into. Here, in observing how one idea re- 
 
6 MEMORY. 
 
 places another, how conception rises from sensations, 
 how Will starts from conceptions and how emotions 
 and thoughts interweave, he will suppose that there is 
 a corresponding series of interconnected material proc- 
 esses accompanying the whole action of conscious life 
 according to the law of the functional interdependence 
 of matter and consciousness. 
 
 After this introduction I may venture to combine 
 under one point of view a long series of phenomena 
 which are apparently widely separated and belong 
 partly to the conscious, partly to the unconscious life, 
 of organic nature. These we shall consider, compre- 
 hensively, as the results (Aeusserungen) of one and 
 the same faculty of organized matter, viz., memory, or 
 the faculty of reproduction. 
 
 Memory, as generally understood, is merely the 
 faculty of voluntarily reproducing ideas or a series of 
 ideas. But if faces and events of past days appear, 
 uncalled for, and take possession of our consciousness, 
 should we not also call this with the same right, re- 
 membering? We are justly entitled to include in the 
 concept of memory all involuntary reproductions of 
 sensations, conceptions, emotions, and aspirations. In 
 doing so, memory becomes. an original faculty, being 
 at once the source and unification of all conscious life. 
 
 It is well known that sensuous perceptions, if con- 
 stantly repeated for a time, are impressed into what we 
 call the memory of the senses in such a way that often 
 after hours, and even after we have been busy with a 
 hundred other things, they suddenly return into con- 
 sciousness in the full sensuous vivacity of their original 
 perception. We thus experience how whole groups 
 of sensations, properly regulated in their spatial 
 
MEMORY. 7 
 
 and temporal connections, are so vividly reproduced as 
 to be like reality itself. This clearly shows that after 
 the extinction of conscious sensations, some material 
 vestiges still remain in our nervous system, implying 
 a change of its molecular and atomic structure, by 
 which the nervous substance is enabled to reproduce 
 such physical processes as are connected with the corre- 
 sponding physical processes of sensations and percep- 
 tions. 
 
 Every one can observe in his daily and hourly ex- 
 perience such phenomena of sense-memory, although 
 in fainter forms. Consciousness produces legends of 
 more or less faded memory-pictures of former sensu- 
 ous perceptions. They partly are called in voluntarily, 
 and partly crowd in spontaneously. Faces of absent 
 persons come and go as pale and volatile shadows, and 
 sounds of melodies which have long died away haunt 
 us, if not audibly, yet perceptibly. 
 
 Of many things and events, especially if they have 
 been perceived only once or very superficially, merely 
 single, unusually striking qualities are reproducible ; of 
 other things only those qualities are reproducible which 
 have been remarked on former occasions, our brain 
 being in this way prepared for their reception. Such 
 are responded to more strongly and enter consciousness 
 more easily and energetically. Thus their ability of 
 being reproduced increases. In this way, what is com- 
 mon to many things and hence has been most fre- 
 quently perceived, will by and by be so reproducible 
 as to be easily called forth by a slight internal impulse, 
 without any exterior and real stimulus. Such a sen- 
 sation, which is, as it were, produced internally for 
 instance, the idea of white is not of the same vivacity 
 
8 MEMORY. 
 
 as the sensation of white color externally produced by 
 white light. But it is, after all, essentially the same, 
 being a weak repetition of the same material brain- 
 process and of the same conscious sensation. Thus 
 the idea of white is an almost imperceptibly weak per- 
 ception. 
 
 In this way the qualities which are common to 
 many things are, as it were, separated from them in 
 entering our memory. They attain an independent ex- 
 istence in consciousness as concepts or ideas, and the 
 whole rich world of our concepts and ideals is con- 
 structed of these materials of memory. 
 
 It is easily seen that memory is not so much a fac- 
 ulty of conscious as of unconscious life. What was 
 conscious to me yesterday and again becomes con- 
 scious to me to-day, where has it been in the interim? 
 It did not exist as a fact of consciousness, and yet it 
 returned. Our concepts appear on the stage of con- 
 sciousness only transiently ; they quickly disappear be- 
 hind the scenes, ,to make room for others. Only on 
 the stage are they conceptions, as an actor is king only 
 on the stage. As what do they remain behind the 
 scenes ? For we know that they exist somehow ; a cue 
 only is needed to make them reappear. They do not 
 continue as conceptions, but as certain dispositions of 
 the nervous substance (Stiinmung der Nervcnsub- 
 stanz) by virtue of which the same sound that was 
 produced yesterday can again be evoked to-day. 
 
 Innumerable reproductions of organic processes in 
 our cerebral substance constantly combine with each 
 other, according to certain laws, each in its turn stimu- 
 lating another; but the phenomenon of consciousness 
 is not necessarily joined with each link of such a series 
 
MEMORY. 9 
 
 of processes. Accordingly, chains of conceptions some- 
 times seem to lack proper connections when conveyed 
 to the cerebral substance through processes unaccom- 
 pained by consciousness. Therefore, also, a long se- 
 ries of ideas may follow the correct logical order and 
 have a proper organic structure, although the differ- 
 ent premises that are indispensable to such combina- 
 tion do not become conscious at all. Some ideas 
 emerge from unconscious life into consciousness, with- 
 out being connected with any conscious conception 
 whatever; others sink into unconsciousness without 
 ever having been connected with conscious ideas. 
 
 Between what I am to-day and what I was yester- 
 day, a gap of unconsciousness lies, the nocturnal 
 sleep ; and it is only memory which spans a bridge be- 
 tween my to-day and my yesterday. Who can hope to 
 unravel the manifold and intricately infertwined tis- 
 sues of the inner life by simply following the threads 
 of consciousness ? You may as well gather your infor- 
 mation about the rich organic life of the oceanic world 
 from those few forms which now and then emerge 
 from the surface of the sea merely to disappear again 
 into the depths of the ocean. 
 
 Thus the cause which produces the unity of all 
 single phenomena of consciousness must be looked for 
 in unconscious life. As we know nothing of this ex- 
 cept w r hat we learn from our investigations of matter, 
 and since in a purely empirical consideration, matter 
 and the unconscious must be regarded as identical, the 
 physiologist may justly define memory in a wider 
 sense to be a faculty of the brain, the results of which 
 to a great extent belong to both consciousness and un- 
 consciousness. 
 
10 MEMORY. 
 
 Every perception of an object in space is a highl 
 complicated process. For instance, a white ball sue 
 denly looms up before my eyes. It is necessary, IK 
 only to convey to consciousness the perception of whil 
 but the circular periphery of the visible ball as wel 
 also its globular form, as it is recognized from the di: 
 tributions of light and shade; then the exact distanc 
 from my eyes must be considered, and from this \\ 
 form an estimate concerning its size. What an aj 
 paratus of sensations, perceptions, and conclusions 
 apparently necessary for accomplishing all this! An 
 yet the actual perception of the sphere is performe 
 in a few seconds, without my becoming conscious c 
 the single processes which construct the whole; tt 
 result enters my consciousness complete. 
 
 The nervous substance faithfully preserves tt 
 records of processes often performed. All functior 
 necessary for correct perception, which at first operate 
 slowly and with difficulty with the constant help of coi 
 sciousness, are afterwards reproduced summarily an 
 without an intensity sufficient to push each single lin 
 of the chain beyond the threshold of consciousnes 
 Such chains of unconscious nerve processes, which ; 
 last end in a link accompanied with consciousnes 
 have been called unconscious chains of perceptions, c 
 unconscious conclusions; a name which is justifiafr 
 from the standpoint of psychology. For psycholog 
 might frequently lose sight of the soul, if unconsciot 
 states were not taken into consideration. To a phy: 
 ical consideration, however, unconscious and materi; 
 mean the same, and a physiology of the unconscious 
 no philosophy of the unconscious. 2 
 
 2 Refers to Von Hartmann's "Philosophy of the Unconscious." 7 
 
MEMORY. 11 
 
 Almost all movements which man performs are the 
 result of long and difficult practice. The harmonious 
 co-operation of the different muscles, the exactly 
 gauged amount of work which each one must contrib- 
 ute to the common labor, must be learned for most 
 movements with great trouble. How slowly a begin- 
 ner at the piano finds the single notes, the eye direct- 
 ing his fingers to the different keys, and then how 
 marvellous is the play of the virtuoso ! With the swift- 
 ness of thought each note finds an easy passage through 
 the eye to the finger, to be performed correspondingly. 
 One quick glance at the music suffices to transform 
 into sound a whole series of chords; and a melody 
 which has been sufficiently practised may be played 
 while the player's attention is directed to other sub- 
 jects. 
 
 In such a case the will no longer directs each sin- 
 gle finger to produce the desired movements, and so 
 close attention is not needed to watch the whole execu- 
 tion carefully. The will is only commander-in-chief. 
 The will issues an order, and all the muscles act ac- 
 cordingly. They continue to work as long as they move 
 in their customary tracks, till a slight hint of the will 
 prescribes some other direction. 
 
 This would be impossible, if those parts of the cen- 
 tral nervous system which bring about the movement 
 were not capable of reproducing entire series of states 
 of irritation. When they have been previously prac- 
 tised under a constant accompaniment of conscious 
 ness, they can be called forth, as it were, independ- 
 ently on the slightest impulse of consciousness, being 
 executed more quickly and more perfectly, the oftener 
 the reproductions have been repeated. All this is pos- 
 
12 MEMORY. 
 
 sible only if they remember what they did before. Our 
 perceptive faculty would forever remain in its lowest 
 stage if we should consciously construct every single 
 perception from the given single materials of sensa- 
 tion. Our voluntary motions would never surpass the 
 awkwardness of a child, if in every case we should re- 
 incite with conscious will the different single impulses 
 and reproduce over again all our single conceptions; 
 or, to state it briefly, if the nervous motor system 
 were not endowed with memory, i. e., an unconscious 
 memory. What is called "force of habit," is the 
 strength of this memory. 
 
 It is to memory that we owe all we are and have. 
 Ideas and concepts are products of it; each percep- 
 tion, each thought, each motion is carried by it. 
 Memory unites all the innumerable single phenomena 
 of consciousness into one entirety; and as our body 
 would be dispersed into myriads of atoms if it were 
 not held together by the attraction of matter, so, but 
 for the binding power of memory, consciousness 
 would be dissolved into as many fragments as there 
 are moments. 
 
 We have seen that only a part of the reproductions 
 of organic processes, as effected by the memory of 
 nervous substance, enters our consciousness; no less 
 important parts remain unconscious. And the same 
 may be proved by numerous facts relating to parts of 
 the nervous system which are exclusively subservient 
 to the unconscious processes of life. For the memory 
 or reproductive faculty of the so-called sympathetic 
 nervous system is by no means weaker than that 
 of the brain and the spinal cord. Medical art, to a 
 great extent, makes good use of this. 
 
MEMORY. 13 
 
 In concluding this part of my investigation, let me 
 leave the subject of nervous substance for a moment 
 in order to take a cursory view of other organic mat- 
 ter, where we meet with the same reproductive fac- 
 ulty, but in a simpler form. 
 
 Daily experience teaches us that muscles grow 
 stronger the oftener they are used. Muscle-fiber, which 
 in the beginning but feebly responded to the irritation 
 of a motor nerve, works with more energy the oftener 
 it is irritated, after proper intervals of rest. After each 
 single action it becomes more capable of action; it 
 grows fitter for the repetition of the same work and 
 better adapted to the reproduction of the same organic 
 process. Pari passu, its size increases, because it as- 
 similates more than in a state of constant rest. 
 
 This is the very same faculty of reproduction whose 
 action in nervous substance is so complicated; here it 
 is observable in its simplest form, and easier under- 
 stood as a physical process. And what is more accu- 
 rately known of muscle-substance, is more or less 
 clearly demonstrable of the substances of all other 
 organs. Everywhere we find an increased activity 
 with adequate pauses of rest accompanied by an in- 
 creased strength of action ; and organs which are used 
 oftener in the animal economy also grow in size by 
 increased assimilation. But this increase of mass not 
 only means an augmentation and growth of the single 
 cells or fibers of which the organ is composed, but also 
 an augmentation of their number. A cell grown to a 
 certain size divides into filial cells, which inherit, in a 
 greater or less degree, the qualities of the parental 
 cell, and accordingly represents repetitions of it. This 
 growth and augmentation of cells is one of the differ- 
 
14 MEMORY. 
 
 ent functions which are characteristic of organized 
 matter. These functions are not only inferior phe- 
 nomena of the cell-substance, not only certain changes 
 or motions of its molecular structure, but they also be- 
 come externally visible as a modification of form, an 
 increase of size or a division of the cell. Thus the 
 reproductive function of a cell is manifested also as 
 a reproduction of the cell itself. This is most obvious 
 in plants ; the chief function of their cells is the work 
 of growth, while in animal organisms other functions 
 are predominant. 
 
 Now, let me finally consider the phenomena in 
 which the power of memory in organized matter is most 
 striking. 
 
 On the basis of numerous facts, we may justly as- 
 sume that even such qualities of an organism can be 
 transferred to its posterity as have not been inherited 
 but have been acquired under peculiar circumstances 
 of life. Thus, every organic being endows its germs 
 with some small inheritance which has been acquired 
 during the individual life of the parental organism and 
 is added to the total legacy of the race. 
 
 Considering that properties are inherited which 
 have been developed in different organs of the paren- 
 tal being, it has appeared highly enigmatic to investi- 
 gators how these same organs could have influenced a 
 germ developed in a distant place. So it has happened 
 that as a solution of this problem mystic views have 
 often been propounded. 
 
 The subject may be best comprehended from a 
 physiological standpoint, in this way. 
 
 The nervous system, in spite of its being a com- 
 pound of many thousands of cells and fibers, never- 
 
MEMORY. 15 
 
 theless forms one coherent entirety. It is in communi- 
 cation with all organs; according to recent histolog- 
 ical researches, it is believed that it is connected with 
 every cell of the more important organs, either directly 
 or at least indirectly through a living, irritable, and 
 therefore conducting cell-substance. Through this 
 connection, all organs are more or less interdependent, 
 so that the destinies of the one are re-echoed in the 
 others ; and any irritation effected in any one, is trans- 
 fused, be it ever so feebly, to the remotest parts of the 
 body. In addition to this delicate connection of all 
 parts of the nervous tissue, another, but slower and 
 more sluggish, connection is effected by means of the 
 circulating fluids. 
 
 We notice further that the developmental process 
 of the germs destined to attain independent existence, 
 exercises a powerful reaction on both the conscious 
 and unconscious life of the whole organism. And this 
 is a hint that the organ of germination is in a closer 
 and more momentous connection with the other parts, 
 especially with the nervous system, than any other or- 
 gan. Conversely, the conscious and unconscious des- 
 tinies of the whole organism, it is probable, find a 
 stronger echo in the germinal vessels than elsewhere. 
 
 This, it must be recognized, is the path on which 
 we have to look for the material link between the ac- 
 quired properties of an organism and those elements 
 of a germ that redevelop the parental qualities. 
 
 You may object that an immaterial something can- 
 not be determinative of the future development of 
 germs so like each other ; it must rather be the peculiar 
 character of its material composition. But I answer: 
 The curves and planes which a mathematician imag- 
 
16 MEMORY. 
 
 ines, or accepts as imaginable, are more numerous and 
 manifold than the shapes of the organic world. But 
 if we imagine infinitely small portions of all these pos- 
 sible curves, they will bear a closer resemblance to 
 each other than germs do. Nevertheless, the whole 
 curve is latent in each portion of it and if a mathema- 
 tician extends it in its proper directions, it will grow 
 into the peculiar curve which was determined by the 
 form of its small fragmentary part. 
 
 Therefore it is erroneous to declare that we cannot 
 imagine such minute differences in germs as must here 
 be assumed by physiology. 
 
 An infinitely minute dislodgment of a point or a 
 complex of points in one part of a curve will alter the 
 law in its entire course. In exactly the same way, an 
 evanescent influence of the parental organism upon the 
 molecular structure of its germ is sufficient to prede- 
 termine its whole future development. 
 
 Accordingly, the reappearance of properties of the 
 parental organism in the full grown filial organism can 
 be nothing else than the reproduction of such proc- 
 esses of organized matter as the germ when still in 
 the germinal vessels had taken part in; the filial or- 
 ganism remembers, so to .say, those processes, and as 
 soon as a same or similar irritation is offered, a reac- 
 tion takes place in it just as formerly in the parental 
 organism, of which it was then a part, and whose des- 
 tinies influenced it. 
 
 If in a parental organism by long habit or constant 
 practice something grows to be second nature, so as 
 to permeate, be it ever so feebly, its germinal cells, 
 and if the germinal cells commence an independent 
 life, they increase and grow till they form a new 
 
MEMORY. 17 
 
 being, but their single parts still remain the substance 
 of the parental being, they are bones of its bones, and 
 flesh of its flesh. If, then, the filial organisms repro- 
 duce what they experienced as a smaller part of a 
 greater whole, this fact is marvelous indeed, but no 
 more so than when an old man is surprised by remin- 
 iscences of his earliest childhood. Whether it still be 
 the very same organized substance which reproduces 
 old experiences, or whether it be its descendant and 
 offspring, a part of itself, which in the meantime ex- 
 panded and grew, is a difference which, apparently, is 
 one of degree, not of kind. But is it not strange that 
 we are engaged in considering how trifling inheritances 
 of the parental organism can be reproduced in the 
 filial being, as if we had forgotten that the filial or- 
 ganism is nothing but one great reproduction of par- 
 ental organism, even in its minutest details? This is 
 because we are so accustomed to accept their similar- 
 ity as granted, that we are surprised to find a child 
 who is in some respect not quite like its mother, and 
 yet the fact that it is like its parent in so many thou- 
 sand ways is much more wonderful! 
 
 If the substance of a germ is able to reproduce 
 what the parental organism has acquired during its 
 individual life, how much more will it be able to repro- 
 duce what is innate in the parental organism and has 
 been repeated through innumerable generations in the 
 same organized matter of which the germ of to-day, 
 after all, is, and remains, but a part. Is it then to be 
 wondered at, that the things which organized matter 
 has experienced on numberless occasions are impressed 
 more strongly into the memory of a germ than the in- 
 cidents of a single life? Every organic being which 
 
18 MEMORY. 
 
 lives to-day, is the latest link of an immeasurable series 
 of organic beings, of which one rose into existence 
 from the other, and one inherited part of the acquired 
 properties of the other. The beginnings of this series, 
 it must be assumed, are organisms of extremest sim- 
 plicity, like those which are known to us as organic 
 germ-cells. In consideration of this, the whole series 
 of such beings appears as the work of the reproductive 
 faculty which was inherent in the substance of the 
 first organic form with which the whole development 
 started. When this first germ divided, it bequeathed 
 to its descendants its properties; the immediate de- 
 scendants added new properties and every new germ 
 reproduced to a great extent the modi operandi of its 
 ancestors; part of which grew feebler, because under 
 altered circumstances their reproduction was no longer 
 elicited. 
 
 Thus every organized being of our present time is 
 the product of the unconscious memory of organized 
 matter. Constantly increasing and dividing, constantly 
 assimilating new and excreting waste matter, con- 
 stantly recording new experiences in its memories, 
 to be reproduced again and again, each has taken 
 richer and more perfect shape the longer it has lived. 
 
 The whole history of individual development, as 
 observed in higher organized animals, is, from this 
 point of view, a continuous chain of reminiscences of 
 the evolution of all the beings which form the ances- 
 tral series of the animal. A complicated perception 
 takes place by means of a volatile, and, as it were, 
 superficial reproduction of cerebral processes which 
 have been long and carefully practised; exactly so a 
 growing germ passes quickly and summarily through 
 
MEMORY. 19 
 
 a series of phases which were developed and fixed, 
 step by step, in the memory of organized matter in 
 the series of its ancestral beings, during a life of in- 
 calculable duration. This view was repeatedly fore- 
 shadowed. It took shape in several theories; but was 
 only rightly understood by a scientist of recent times. 
 For truth hides in different shapes before the eyes of 
 its inquisitors, until it is revealed to the elect. 
 
 Not only is there a reproduction of the outward and 
 inner conformation of body, organs, and cells, but 
 of its functions as well. A chick emerging from its 
 shell at once runs off as did its mother, when she as a 
 chick broke her shell. Think how extraordinarily 
 complicated are the motions and sensations of such 
 acts ! Only consider the difficulty involved in preserv- 
 ing equilibrium in running, and it will be conceded 
 that the supposition of an innate reproductive faculty 
 alone can serve as an explanation of these intricate 
 actions. The execution of a motion that is exercised 
 during the greater part of an individual life becomes 
 second nature, and the actions of a whole race, re- 
 peated over and over again by each member of the 
 race, must also become second nature. 
 
 The chick is not only endowed with an inborn 
 dexterity in its motions, but possesses also a strongly 
 developed perceptive faculty. Without hesitation it 
 picks up the grains which are thrown to it. This im- 
 plies that it sees them, and that it correctly judges 
 their position and their distance; moreover, it has to 
 move its head and other limbs with great precision. 
 All these things could not be learned in the egg shell ; 
 they were learned from the many thousands of beings 
 which lived before this chick, and of which it is the 
 
20 MEMORY. 
 
 direct offspring. 
 
 The memory of organized matter is here most strik- 
 ingly displayed. The gentle stimulus of the rays 
 which proceed from a grain and fall upon the retina 
 of the chicken gives occasion for the reproduction of 
 a complicated series of sensations, perceptions, and 
 motions, which in this individual have never as yet 
 been combined, and which, nevertheless, were adjusted 
 from the beginning with accuracy and precision, as if 
 the animal itself had practised them thousands of 
 times. Such surprising performances of animals are 
 generally regarded as manifestations of instinct; and 
 some philosophers have indulged in mystic explana- 
 tions of instincts. If instinct is regarded as the result 
 of the memory or reproductive faculty of organized 
 matter, if we assume that the race is also endowed 
 with memory, instinct is understood at once; and the 
 physiologist is enabled to correlate and connect in- 
 stinct with the great series of facts we have found to 
 be phenomena of the reproductive faculty. In this 
 way we have not yet gained, but we have certainly ap- 
 proached a physical explanation of the problem. 
 
 If, for instance, a caterpillar changes into a chrys- 
 alis, or a bird builds a nest, or a bee constructs a cell, 
 such animals, in obeying their instinct, act with con- 
 sciousness, and are not unconscious machines. They 
 know to some extent how to adapt their actions to 
 change circumstances and are liable to err; they feel 
 pleasure if their work proceeds, and displeasure if they 
 meet obstacles. They learn by working, it must be 
 assumed, and birds, no doubt, build their nests better 
 a second time than the first. But that animals so easily 
 find the most practical means of attaining their ends 
 
MEMORY. 21 
 
 the very first time, and that their motions are so excel- 
 lently and perfectly adapted to their purposes, is due 
 to the inherited disposition of the memory of their 
 nerve substance, which only awaits an occasion to 
 work in full conformity with the situation, and re- 
 members just what is necessary for that occasion. 
 
 It is striking how easily dexterity is acquired, if the 
 attention is sufficiently confined to its acquisition. 
 Specialization gives rise to proficiency. He who ad- 
 mires a spider for spinning its web, should bear in 
 mind how limited its other facilities are. Nor should 
 he forget that the spider did not learn its art by 
 itself, but this was acquired slowly by innumerable 
 generations of spiders, and comprises almost all they 
 learned. Man took to bow and arrows if his nets 
 failed to catch him food, but the spider starved. 
 
 Thus we see the body and, what is of greater im- 
 port, the whole nervous system of a newborn animal 
 is predetermined and predisposed for intercourse with 
 the world which it enters; it is prepared to respond 
 to stimuli and influences in the same way as this was 
 done by its ancestors. 
 
 We cannot expect that the brain and nervous sys- 
 tem of man should form an exception to this rule. 
 
 True, man learns with difficulty, while the animal 
 from its very birth is possessed of instinct. However, 
 the human brain at birth is much farther from 
 its highest development than the brain of an animal. 
 Its growth not only takes a longer time, but is much 
 more marked. The human brain, we may say, is 
 much younger when it enters the world than the ani- 
 mal brain. The animal is born precocious, and at once 
 behaves precociously. It is like a phenomenal child 
 
22 MEMORY. 
 
 whose brain is overmatured and, as it were, too old, 
 so that it is unable to develop as luxuriantly as an- 
 other brain, which is less finished, and inured to work, 
 but fresher and more youthful. The scope for in- 
 dividual development in the case of the human brain, 
 and generally of the human body, is much larger, be- 
 cause a relatively great part of its development is 
 relegated to the time subsequent to birth. It grows 
 under the influences of its surroundings, which affect 
 its senses, and acquires under such circumstances, in 
 a more individual way, what an animal has received in 
 the fixed formation of the race. 
 
 A far-reaching memory, or reproductive faculty, 
 we must assume, is to be ascribed to the whole body, 
 as well as particularly to the brain of a newborn man. 
 By the help of this memory he is able to acquire much 
 more quickly and easily the attainments which were 
 developed in his ancestors thousands of times and are 
 necessary for his life. What appears as instinct in 
 animals, in man appears, in a freer form, as a predis- 
 position. True, ideas are not inborn in an infant, 
 but the ability of ready and precise crystalization of 
 ideas from a complex mixture of sensations, is due, 
 not to the labor of the child, but to the labor of the 
 brain-substance of innumerable generations of ances- 
 tors. 
 
 Theories of individual consciousness, according to 
 which it is assumed that each human soul starts life 
 for itself and commences a development of its own, as 
 if the thousands of generations before it had not been 
 in existence, are in striking disagreement with the facts 
 of daily experience. 
 
 Those cerebral processes which elevate and dis- 
 
MEMORY. 23 
 
 tinguish man, it must be conceded, are not of such 
 antiquity as are those connected with his physical 
 necessities. Hunger and the reproductive instinct 
 have affected even the oldest and simplest forms of or- 
 ganic beings. Accordingly, organic substance has the 
 most powerful memory for these stimuli, as well as for 
 their satisfaction. The impulses and instincts rising 
 from them still exercise elemental power over the man 
 of today. Spiritual life grows slowly, and its most 
 beautiful blossoms belong to the latest epochs of the 
 evolutionary history of organized matter. It is not 
 long that the nervous system has been crowned with 
 the glory of a large and well-developed brain. 
 
 Oral .and written traditions have been called the 
 memory of mankind, and this conception is true. But 
 there is another memory, which is the reproductive 
 faculty of the cerebral substance. Without it, all 
 written and oral language would be empty and mean- 
 ingless to later generations; for, if the loftiest ideas 
 were recorded a thousand times in writings or in oral 
 traditions, they would be nothing to brains not predis- 
 posed for them. They must not only be received, they 
 must be reproduced. If increasing cerebral potency 
 were not inherited simultaneously with the inward 
 and outward development of the brain, with the wealth 
 of ideas which are inherited from generation to gen- 
 eration, if an increased faculty for the reproduction 
 of thoughts did not devolve upon coming generations 
 simultaneously with their oral and written traditions, 
 scripts and languages would be useless. 
 
 The conscious memory of man comes to an end with 
 his death; but the unconscious memory of nature is 
 true and indestructible. Whoever has succeeded in im- 
 
24 MEMORY. 
 
 pressing the vestiges of his work upon it, will be re- 
 membered forever. 
 
SPECIFIC ENERGIES 
 
THE SPECIFIC ENERGIES OF THE NERVOUS 
 SYSTEM. 
 
 JOHANNES MUELLER, the greatest physiologist 
 of our century, in his essays on the senses, estab- 
 lished a theory which is well known as "the theory of 
 the specific energies of the sensory nerves." I cannot 
 here recapitulate his doctrine in his own perspicuous 
 language, which would be intelligible only to special- 
 ists. But a few sentences will suffice to explain the 
 quintessence of his theory to any one whose occupa- 
 tion prevents him from bestowing more than that kindly 
 interest upon physiology which this most fascinating 
 science awakens in the mind of every educated man. 
 
 From the eye and from the ear, from the mucous 
 membranes of the organs of taste and smell, and from 
 the skin of the whole body, which is the organ of touch 
 and temperature, proceed thousands of delicate nerve- 
 fibers. Gradually uniting, they coalesce into steadily 
 enlarging bundles, which either lead directly to the 
 brain, or are indirectly connected with it by the spinal 
 cord. Through these nerve-fibers the sensory organs 
 communicate with the brain, that most wonderful liv- 
 ing structure which is both the origin and the product 
 of our consciousness. 
 
 When a vibration of ether stimulates the nervous 
 
26 THE SPECIFIC ENERGIES. 
 
 membrane of our eye (the retina), a process ensues, 
 whose real nature we do not yet understand. We 
 only know that the stimulation is at once transmitted 
 to the fibers of the optic nerve, and in its further prog- 
 ress acts upon those cerebral parts into which the 
 optic nerve enters. As the life of these brain-struc- 
 tures is in close connection with our consciousness, it 
 happens that when a ray of light enters the eye, it 
 causes an irritation of the nervous fibers and of the 
 cerebral cells; and thus we become conscious of the 
 sensations of light and of color. 
 
 If now these same rays, which when entering the 
 eye produced the sensation of light, fall upon the skin 
 of the hand, and there stimulate the delicate rootlets of 
 the sensory nerves, this stimulation is transmitted 
 through the nerves and the spinal cord to the brain, and 
 instead of light we are conscious of warmth. How 
 is it that the same external agent in one case produces 
 light, and in the other warmth ? 
 
 Moreover, the sensation of light can be produced 
 in a perfectly dark room by stimulating the nerves of 
 the eye by an electric current ; and if we pass the elec- 
 tric current through the auditory nerve, we hear sounds 
 and noises, though the deepest silence surround us. 
 If we apply the current to the nerves of the skin, we 
 experience the sensation of heat or cold, although not 
 in contact with any cold or warm object, and if by 
 the same current we excite the nerves of the 
 tongue, gustatory sensations are produced. Accord- 
 ingly, the nervous apparatus of each sensory organ re- 
 sponds to the same stimulus with different sensations. 
 And again we ask : How does precisely the same cause 
 produce such a variety of effects? 
 
THE SPECIFIC ENERGIES. 27 
 
 Even by the aid of the microscope the anatomist 
 has not been able to discover any essential difference 
 between the various sensory nerves. For instance, that 
 part of the brain which produces the visual sensations 
 does not, in its ultimate structure, vary noticeably 
 from those cerebral regions which produce sensations 
 of sound or temperature. But (and this is the answer 
 to the problem in question) this sameness of form is 
 not accompanied by a sameness of nature. The diverse 
 structures of the nervous system, the nerve-cells and 
 the nerve-fibers, are internally different in spite of all 
 external similarity, and the diversity of the sensations 
 produced is a manifestation of such difference. 
 
 It is the nature of the nerve substance in the visual 
 organ to produce sensations of light, and only such. 
 It is the bell which sounds, and not its tongue; and 
 similarly it is not the vibration of ether, but the nerve, 
 that produces light. No matter whether it be a ray of 
 light, whether it be a pressure or a blow upon the eye, 
 an electric current, or any stimulus whatever, that 
 affects the nervous apparatus, it invariably manifests 
 itself as light or color. In the same way we become 
 conscious of the stimulations of the auditory organ 
 in the form of sound or noise, no matter what their 
 cause, which may be aerial vibrations or any morbid 
 irritation of the inner ear, or an organism of the blood. 
 
 Johannes Miiller called the inherent function of cer- 
 tain nerves to communicate certain sensations, not 
 otherwise producible, to our consciousness, the "specific 
 energy" of those nerves. More than half a century 
 has elapsed since this great physiologist developed his 
 theory in bold and magnificent proportions thus form- 
 ulating, in scientific terms, an idea, the original germ of 
 
28 THE SPECIFIC ENERGIES. 
 
 which lies buried in the distant past, as far back as 
 Aristotle. Johannes Miiller's doctrines were re-echoed 
 in innumerable writings, but it cannot be said that the 
 seed he sowed fell upon fertile soil, or that it was de- 
 veloped in any essential feature. A few partially suc- 
 cessful attempts were made to promote Miiller's theory 
 of the sensations of color and of sound, but aside from 
 that his doctrine bore little fruit. On the contrary it 
 was suppressed even by Johannes Miiller's own dis- 
 ciples. It again became customary to regard all nerve- 
 fibers as having essentially the same nature, and to 
 suppose that the same kind of stimulation is transmit- 
 ted in all fibers of the various nerves. The question 
 why the nerves of the various sensory organs produce 
 such different sensations was either entirely abandoned, 
 or it was deemed sufficient to say that the cause should 
 be sought in the brain, although the same reasons which 
 were thought to prove that all nerve-fibers are of the 
 same nature, would hold good also in the case of the 
 brain cells and fibers. Even in some writings of the 
 present day we meet with authors who, confounding 
 philosophy and physiology, declare that the theory of 
 specific energies is one of the great aberrations of 
 physiology. 
 
 In consideration of this fact, permit me, as an en- 
 thusiastic follower although not a personal disciple of 
 the great scientist, to disclose and reveal the deep 
 significance of the master's doctrine, and to show 
 that it is the application of a principle which has been, 
 or surely will be, accepted in other provinces of bi- 
 ology. 
 
 The animal kingdom exhibits an inexhaustible mul- 
 tiplicity of form, and to a layman who is not initiated 
 
THE SPECIFIC ENERGIES. 29 
 
 into the science of biology it seems almost incredible 
 that living creatures so variously different in their 
 forms and habits, should be so identical in form as 
 germs in the first stage of their development! As a 
 rule, even the most experienced eye, with the assist- 
 ance of every means of scientific analysis, would not 
 be able to recognize in a germ the animal into which 
 it is going to develop. The fish as well as the bird, 
 and the insect as well as man, so far as we can judge 
 from external appearances, all begin their lives as very 
 simple and microscopically small, spheroidal struc- 
 tures. Nor does this uniformity exist only for the eye ; 
 for chemical analysis resolves them all into the same 
 ultimate elements. 
 
 We ask, how is it possible that totally different 
 forms can develop from apparently similar germs ; and 
 the answer is, that this resemblance of the germs is 
 merely external. By the aid of the most powerful 
 microscopes we can barely discern the roughest out- 
 lines of their structures. 
 
 In the heavens whole systems of suns appear only 
 as nebulas, which even the most powerful telescopes 
 cannot resolve into single stars. As observation is 
 impossible, we can only surmise their structure. Simi- 
 larly the ultimate and exquisitely delicate framework 
 in the achitecture of the living substance of germs is 
 withdrawn from the observation of even the minutest 
 research. Could we approach nearer and nearer to 
 one of these nebulas, one star after the other would 
 emerge from the apparently homogeneous mass; we 
 should see planets revolving around their suns, and 
 satellites about the planets. Thus, if with our corporeal 
 or intellectual eye we could penetrate the minutest in- 
 
30 THE SPECIFIC ENERGIES. 
 
 ternal structure of the substance of germs if we could 
 comprehend the arrangement and motion of the mole- 
 cule and atoms we should discover that the living 
 germ-substance of each animal species has its specific 
 properties, and the substance of each single germ has 
 its individual properties by virtue of which, in a fur- 
 ther evolution, a special and peculiar type must me 
 chanically develop. 
 
 Whether these internal variations of the germs are 
 chemical or physical is at present immaterial; for 
 the physical properties of a substance are conditioned 
 by their chemical qualities, and when we inquire into 
 the molecular and atomic structure of a substance the 
 dividing line between the domains of chemistry and 
 physics entirely disappears. We cannot in the im- 
 mediate future, however, hope to find a chemical for- 
 mula for the individual germ substances. To reveal 
 the delicate secret of living matter by the compara- 
 tively crude methods of chemistry would be like try- 
 ing to explain the mechanism of a watch by melting it 
 in a crucible and examining the molten mass with re- 
 spect to its ingredients. 
 
 As we cannot at present solve the problem of the 
 internal variation of the externally similar germ-sub- 
 stances, we must be satisfied with the statement that 
 the germs of each animal species possess an inherent 
 and innate faculty i. e., a specific energy, which di- 
 rects its developments in a manner characteristic of 
 this animal and of no other. Again, each single germ 
 possesses an individual energy which in addition to 
 the normal features of its species secures an individual 
 character to its future development. 
 
 Let us now approach our problem from another 
 
THE SPECIFIC ENERGIES. 31 
 
 side. When the naked eye is not able to discern the 
 more minute organization and delicate structure of an 
 organism, the anatomist employs the microscope, and 
 a new world of discernible facts is revealed to him. 
 The apparently homogeneous form dissolves into in- 
 numerable distinct structures ; millions of the minutest 
 separately-existing beings, different in shape and in- 
 ternal structure, compose a systematically arranged 
 aggregate, thus forming the diverse organs ; and these 
 beings, in spite of the complicated interdependence, 
 lead quite separate lives, for each single being is an 
 animated center of activity. The human body does 
 not receive the impulse of life like a machine from one 
 point, but each single atom of the different organs 
 bears its vitalizing power in itself. The current of 
 life does not emanate from one special part of the 
 body, but all its minutest parts are themselves sources 
 of life. The architecture of the human body, which 
 consists of these elementary organisms, or cells as 
 they are called, has often been explained. The har- 
 monious interaction and the division of labor among 
 these innumerable particles has been compared to the 
 judiciously adapted co-operation of the individual 
 members of a well-regulated community. As in such a 
 community, so also in the human organism, a special 
 kind of work is assigned to each group of individuals, 
 and according to the various functions the elemen- 
 tary organisms are differently formed; but those ele- 
 ments which possess the properly so-called vital power, 
 in every respect exhibit the most striking resemblance, 
 although it may be hidden by and interwoven with 
 various less important solid or fluid ingredients. 
 
 In all living cells and fibers of the different organs 
 
32 THE SPECIFIC ENERGIES. 
 
 we encounter the same colorless viscous, almost fluid, 
 soft unstable substance, in the shape of highly delicate 
 threads, nets, or drops. It is the vital elements of the 
 cell. There the enigma of life lies hidden, for this 
 viscous substance is the moving and creating power in 
 the elementary organism. It produces the contrac- 
 tion of muscular fibers and transmits the irritation in 
 the nerve-fibre ; it builds up the solid and strong mass 
 of the supporting bone and the tough fibre of the ten- 
 don it shapes the feathers of the bird, the scales of the 
 fish, and the horns of the stag. 
 
 Yet it is everywhere apparently the same, and if 
 it is isolated from its proper sphere and surroundings, 
 and considered by itself, the most experienced eye can- 
 not tell which of the different functions was performed 
 by it. 
 
 Again we ask, how is it possible that apparently 
 equal causes produce such different effects. Here no 
 one will doubt that in spite of external similarity the 
 living substance in the cells of the individual or- 
 gans is internally different, and a difference of func- 
 tion necessarily results from this difference of internal 
 structure. It is an innate function. The specific 
 energy of the living substance in the liver produces 
 bile, as the specific energy of the root of a hair builds 
 up the horny mass of hair. 
 
 All the innumerable elementary beings or cells of 
 an organism are the offspring of one single germ-cell 
 in which the development commenced. By division, 
 the first cell was split in two. Although both were 
 intimately connected with each other, they were never- 
 theless, to a certain extent, independent cells. These 
 two cells divided again and formed other cells, and so 
 
THE SPECIFIC ENERGIES. 33 
 
 on. Thus, by a constantly renewed formation of more 
 living substance the number of the elementary struc- 
 tures increases in almost inexhaustible multiplicity. 
 But in the progress of multiplication the form and 
 arrangement of the cells are also changed. They 
 separate into various homogeneous groups, each of 
 which differs from the others in character in so far 
 as it performs a special function. The living substance 
 is specialized in the process of development according 
 to its function and destination. All the united differ- 
 ent specific energies which later on will separately de- 
 velop to full life in its descendants, lie concealed, al- 
 though only potentially, in the substance of the germ. 
 
 In the light of these considerations the diversity of 
 function in the nervous substance can no longer sur- 
 prise us. Its external similarity prevents us from 
 considering it as internally different and from claim- 
 ing for it specific energies according to the doctrine 
 of Johannes Miiller. 
 
 The specific energies of the living substance in the 
 different organs are characterized by their chemical or 
 physical functions ; while in the present state of science 
 the energies of the nervous substance can be recog- 
 nized only by the different sensations which they pro- 
 duce in our consciousness. Our sensations and all the 
 phenomena of consciousness are the psychological ex- 
 pressions of physiological processes or the stimulations 
 of our nerves, especially of our brain. Vice versa, 
 these stimulations are the material expression of the 
 processes in our soul. 
 
 The soul does not act unless the brain is in action 
 at the same time. Whenever the same sensation or the 
 same thought recurs, a certain physical process which 
 
34 THE SPECIFIC ENERGIES. 
 
 belongs to this special sensation or thought is re- 
 peated ; for both are inseparably connected. They are 
 conditioned by and productive of each other. Accord- 
 ingly, from the course of our sensations we can draw 
 inferences concerning the simultaneous and corre- 
 sponding course of processes in the brain. The reso- 
 lution of our sensations into their various elements is 
 at the same time an analysis of the involved interac- 
 tions of the various elementary cerebral functions or 
 stimulations. 
 
 For instance, let us suppose that the great variety 
 of the sensations of light and color can be reduced to 
 a few simple or elementary sensations, to those of the 
 principal colors, which by combining in different pro- 
 portions can produce innumerable different sensations. 
 This fact, if proved, would justify the conclusion that 
 different kinds of elementary stimulations can take 
 place also in the nerve substance of the visual organ. 
 Each of them corresponds to one of the elementary 
 sensations, and the elementary stimulations can be ar- 
 ranged in a manner analogous to that of the elemen- 
 tary sensations. Or similarly, if we succeed in reduc- 
 ing all the many and various gustatory sensations to a 
 few simple sensations, we may again justly infer that 
 a corresponding number of elementary irritations can 
 be produced in the nerve-substance of the tongue. 
 
 Consequently the analysis of our sensations leads 
 us to recognize the fact that what Johannes Miiller 
 summarily called the specific energy of a sensory nerve 
 may be resolved into a certain number of elementary 
 irritations. But we need not assume that a distinct 
 nerve-element is a medium for each simple irritation. 
 The same nerve-cell can produce the sensation of heat 
 
THE SPECIFIC ENERGIES. 35 
 
 or of cold according to the direction in which its spe- 
 cific energy is stimulated. The same fiber of the visual 
 organ can be stimulated in different ways and thus con- 
 vey correspondingly different sensations of color. 
 
 Each single kind of stimulus, therefore, does not 
 necessarily correspond to one and the same nerve 
 substance. The specific energy of a certain nerve- 
 element is not merely a simple property, it is not a 
 faculty which causes only one kind of function, it is a 
 multiform potency. 
 
 The power of specializing and individualizing its 
 functions is an inborn quality of living substance, and 
 bears its richest and most wonderful fruit in the ner- 
 vous system. In this respect the nervous system far 
 surpasses all other organs. 
 
 One fiber of a muscle performs the same function 
 as all its other fibers, and even the fibers of different 
 muscles possess essentially the very same energy. One 
 liver-cell works as all the other liver-cells do, and it 
 cannot work otherwise. The intensity of a function 
 may be different in the different fibers or cells of such 
 an organ, but the kind of function is common to all. 
 
 Not so in the nervous system. The various ener- 
 gies in the various groups of the nervous elements are 
 innate. By an innate faculty the optic nerve of the 
 new-born babe responds to the ray of light which en- 
 ters the eye with a sensation of light, and the nerve of 
 the skin responds to an increase of temperature with 
 a sensation of warmth. 
 
 The specific energy of almost all other organs is 
 definitely fixed at the time of birth and will change in 
 the further development of life in degree only but 
 never in character. 
 
36 THE SPECIFIC ENERGIES. 
 
 The muscle-fiber of a babe contracts in the same 
 way, and thus exhibits the same energy, as does the 
 muscle-fiber of an adult person. The liver-cell of an 
 old man produces bile just as does the liver-cell of a 
 child. The muscle and the liver grow with the entire 
 man, but the fibers and cells added can always per- 
 form only one and the same function. Some fibers 
 and cells perish in the course of life, but those which 
 take their place merely perform the functions of the 
 replaced fibers and cells. 
 
 Thus the innate energy of almost all organs re- 
 mains unchanged throughout life. The small individu- 
 al cell-organisms of which the organs consist, come 
 and go, one generation follows another, in some organs 
 more rapidly and in others more slowly. The living 
 substance of each single element is consumed and then 
 replaced by nutrition, but their character and activity 
 always remain the same. In the nervous system all 
 this is very different. Although, as a rule, the innate 
 energies of many regions, especially in the peripheral 
 nervous system, remain unchanged throughout life, 
 there is in the nervous system of a new-born babe 
 some living substance which is ready to be moulded 
 for the performance of this or that function and for the 
 development of this or that individual energy. 
 
 Above all, the brain of a new-born babe is not a 
 completed structure. It grows and develops ; and when 
 the externally visible growth has reached its limits, the 
 internal process of formation continues. Up to the 
 moment of birth the nervous system with the brain is 
 developed according to its own inner law. Until then, 
 neither light nor sound nor any other sensory stimulus 
 has affected the nerves, and the brain has been asleep. 
 
THE SPECIFIC ENERGIES. 37 
 
 After birth thousands of new stimuli at once intrude 
 from the external world upon the nervous system. The 
 eye is opened to the vibrations of ether and sound- 
 waves obtrude upon the ear ; pressure and impact, cold 
 and warmth affect the skin thus placing the brain 
 which heretofore was left to itself, under the influence 
 and discipline of the external world. 
 
 Before birth the chemical processes of the nervous 
 system, its change of matter and its growth, depended 
 upon internal conditions of life. After birth the stimuli 
 of the external world excite the brain and produce 
 a more vigorous exchange of matter for further de- 
 velopment an'd increase of the living substance. The 
 further development, the inner formation and cultiva- 
 tion henceforth depend upon occurrences in the ex- 
 ternal world which the brain experiences. 
 
 All living substance, especially nerve-matter, has 
 the peculiarity that every stimulation produced in a 
 limited region at once spreads to the adjoining parts. 
 It continues spreading as long as it meets with any sub- 
 stance which is capable of being similarly stimulated 
 and which, so to speak, responds to such stimulation. 
 
 The specific stimulation awakened in the sensory 
 nerves by external causes, -is thus transmitted to the 
 virgin parts of the brain. Here the stimulation ter- 
 minates in the most youthful and most docile living 
 substance, and here every kind of stimulation finds 
 its echo. For this substance which possesses no innate 
 and definitely specialized energy, has not yet lost its 
 susceptibility for all other stimulations through the 
 frequent repetition of a certain kind of stimulation. 
 
 If the virgin substance of the brain is excited and 
 internally agitated by a stimulation which has been 
 
38 THE SPECIFIC ENERGIES 
 
 transmitted through the nerve-fibres of the sensory 
 organs, an increased ability to reproduce the same kind 
 of stimulation is acquired by a permanent change of its 
 internal structure. If the sensory nerve again trans- 
 mits the same stimulation, the cerebral substance re- 
 sponds to it more easily. The oftener it is repeated, 
 the stronger will grow the inclination to reproduce just 
 this kind of stimulation. Through frequent repetition, 
 one particular kind of function becomes, as it were, 
 the second nature of a single cerebral cell, i. e., the 
 cell acquires this special ability or energy. In this way 
 the individual energies of the cerebral cells and fibers 
 are developed by education on the basis of the inher- 
 ited dispositions. Also the additional energy which 
 the cells acquire during life is transmitted by inheri- 
 tance to the new-formed cells generated by partition. 
 These new cells can in their turn develop, evolve, or 
 modify the inherited energy. 
 
 The anatomical arrangement of the brain is such 
 as to place (single) parts of the so-called gray sub- 
 stance into a particularly intimate relation with special 
 sensory nerves. The stimulation of a sensory nerve- 
 fiber will necessarily seize upon and affect those cere- 
 bral cells first which are in closest connection with it. 
 But each cerebral cell is connected with other cerebral 
 cells by a net-work of most delicate nerve-fibers. 
 
 The stimulation which enters from the sensory nerve- 
 fibers into the gray substance can advance (through 
 those cerebral elements which are excited first) in all 
 directions farther and farther into the labyrinth of the 
 cerebral cells and fibers, until at last it dies out and 
 ceases sooner or later, or calls forth new stimulations 
 in exchange, which start from the brain and return to 
 
THE SPECIFIC ENERGIES. 39 
 
 the peripheral nervous system. 
 
 Every cerebral element is subject to the educating 
 influence of those sensory nerve-fibers with which it is 
 anatomically connected and whose energies are most 
 closely related to it. But these single cerebral ele- 
 ments can also receive stimulations, although in a 
 weaker degree, from the adjoining fibers of the same 
 sensory nerve and even from those nerve-fibers which 
 enter the gray substance in more remote parts and 
 originate in other sensory organs. 
 
 In this way the cerebral substance is constantly 
 permeated with many diverse stimulations crowding 
 upon it from all the sensory regions. The cerebral cell 
 will be particularly educated for the qualities of these 
 stimulations according to its opportunity of easily and 
 repeatedly receiving stimulation from this or that sen- 
 sory organ and from such or such a sensory nerve- 
 fiber. It will acquire the faculty of reproducing them 
 vigorously, as often as any stimulus is offered, no mat- 
 ter how weak it may be. 
 
 Consequently, every single cerebral element attains 
 an individual character in the course of its develop- 
 ment and under the influence of sensory experience. 
 And it may be asserted that not even two of the nu- 
 merous cerebral cells are alike in kind and degree 
 of individual energy. If one cerebral cell is destroyed, 
 there would of course be many others which possess 
 in all essential points the same energy, and can by 
 their functions compensate its loss, but no other cere- 
 bral element could do exactly the same work with ex- 
 actly the same individual ability, with the same ease 
 and exactness; just as no man can in all respects en- 
 tirely replace another man. 
 
40 THE SPECIFIC ENERGIES. 
 
 Experience and practice rest upon this specializa- 
 tion and individualization of the functions in the differ- 
 ent cerebral elements, and the energies of the nerve 
 substance which are developed in the course of our 
 life are the organic expression of individual memory. 
 
 The nervous system, and above all the brain, is the 
 great tool-house of consciousness. Each one of the 
 cerebral elements is a particular tool. Consciousness 
 may be likened to a workman whose tools gradually 
 become so numerous, so various, and so specialized 
 that for every detail of his work he has a tool specially 
 adapted to perform just this kind of work most easily 
 and accurately. If he loses one of his tools, he still 
 possesses a thousand other tools to do the same work 
 although with more difficulty and loss of time. Should 
 he lose these thousands also, he might retain hundreds, 
 with which he can possibly do his work still, but the 
 difficulty increases. He must have lost a very large 
 number of his tools, if certain actions became abso- 
 lutely impossible. 
 
 The knowledge of the tools alone does not suffice 
 to ascertain what work is performed by the tools. The 
 anatomist, therefore, will never understand the laby- 
 rinth of cerebral cells and fibers, and the physiologist 
 will never comprehend the thousand-fold intertwined 
 actions of its stimulations, unless they succeed in re- 
 solving the phenomena of consciousness into their ele- 
 ments in order to obtain from the kind and strength, 
 from the progression and connection of our percep- 
 tions, sensations, and conceptions, a clear idea about 
 the kind and progression of the material processes in 
 the brain. Without this clue the brain will always be 
 a closed book to us. 
 
THEORY OF NERVE ACTIVITY. 41 
 
 We can indeed compare the brain to a book. A 
 book is anatomically a number of rectangular white 
 leaves, bound on one side, and marked on their pages 
 with numerous black spots of different form and size. 
 Under a microscope, the leaves will be seen to consist 
 of delicate fibers, and the black spots of minute black 
 granules. A chemical analysis will show that the 
 leaves are cellulose, the spots carbon and resinous oil. 
 If all this has been investigated and ascertained with 
 the utmost accuracy, we do not know in the least 
 why the black spots are arranged just in this and in 
 no other way, why some spots are large and others 
 small, why some occur frequently, others rarely, why 
 the single leaves follow one another in this and in no 
 other order, and altogether what the whole book really 
 means. 
 
 Whoever wishes to know what the book signifies, 
 must know the function of the specific energy of each 
 single letter and of the individual energy of each 
 single word in short, he must know how to read. 
 
 Nothing can be fully explained by analogy, and it 
 is perhaps dangerous to attempt to adorn the dry lan- 
 guage of science with allegories. 
 
 But let the scientist wear his working apparel while 
 ploughing the field of his science; and when, on a 
 festive occasion he offers the fruits of his labor to 
 others, he should be welcome in holiday attire. 
 
NERVE ACTIVITY 
 
ON THE THEORY OF NERVE-ACTIVITY 1 
 
 44 A THEORY of nerve-energy would have to 
 JLJL show how precisely those properties which 
 are characteristic of the activity of the nerves result 
 with necessity from the multifarious aggregate of the 
 conditions constituting them." 
 
 I have taken these words from the Manual of Physi- 
 ology of Carl Ludwig, 2 that memorable man who 
 achieved so much for physiology during his long con- 
 nection with the university of Leipsic. And now that 
 I am about to develop a special view of my own relating 
 to nerve-activity, it is both appropriate and requisite 
 that I should apply the criterion involved in Carl Lud- 
 wig's words to the theory which I am to advance. 
 
 If I were asked whether this view could be consid- 
 ered as a contribution to the theory of nerve-activity in 
 the sense indicated by that great master of experimental 
 physiology, I should have to confess that such is not 
 the case ; for neither from their constitution nor from 
 their form am I compelled to deduce that property 
 which I am going to attribute to the nerves, however 
 much I may be inclined to assent to Ludwig's dictum 
 
 ^Academic Discourse delivered before the University of Leipsic, 
 May 21, 1898. 
 
 2 Second Edition, page 141. 
 
44 THE SPECIFIC ENERGIES. 
 
 "that a nerve is indebted for its energy to its constitu- 
 tion and structure," and for a change in its energy to a 
 corresponding change in them. 
 
 The source from which I have derived my views 
 relative to the mode of activity of the nerve-fibers, 
 lies quite remote from all the knowledge we now have 
 of their structure and of their chemical and physical 
 properties, and I am therefore perfectly willing that 
 the view which I have put forward as a contribution 
 to the theory of nerve-activity should be regarded 
 only as a conjecture. 
 
 In justification of my position, however, I may add 
 that even at best we are in possession only of con- 
 jectures concerning the real inner nature of nerve- 
 activity. We know, thanks to Helmholtz, that any 
 sudden alteration in the condition of the nerve-fiber 
 caused by some stimulus, is propagated along the fiber 
 with a measurable speed, and we know further how 
 great this speed approximately is. But the exact na- 
 ture of that alteration, and the exact character of the 
 process propagated along the fiber, we do not know. 
 Dubois-Reymond's classical investigations have made 
 us acquainted with the electro-motor property of the 
 nerve in its various conditions. But as little as a gal- 
 vanic current gives us an explanation of the peculiari- 
 ties of the chemical process to which it owes its origin, 
 so little does the current derived from a nerve give 
 us information concerning the peculiarities of the 
 chemical change in the nervous substance. The as- 
 sumption that chemical phenomena are the gist of the 
 process which we are wont to designate as the activity 
 of the nerve, is, of course, no more than a mere con- 
 jecture. But in making such a conjecture we assert 
 
THEORY OF NERVE ACTIVITY. 45 
 
 concerning the nerve nothing more than what might 
 just as well be asserted of all living substances, and 
 nothing is said which would characteristically dis- 
 tinguish the life of the nerve from the life of any 
 other organ. 
 
 In fact, life is still as much of an unsolved riddle 
 as it was when the so-called mechanical conception 
 of vital phenomena overthrew the vitalistic, and 
 awakened by its brilliant achievements the most san- 
 guine expectations, foreshadowing results far beyond 
 what has even yet been fulfilled, valuable and fruitful 
 as this has been. 
 
 To whatever point the physical or chemical inves- 
 tigation of the animal organism has penetrated, it has 
 always, sooner or later, come upon the mysterious 
 action of the living substance of those elementary 
 organisms of which the animal and the human body 
 are composed. We have now learned modesty, and 
 from having once believed that we had entered the 
 holy of holies, we now acknowledge that we have as 
 yet scarcely passed the portico of the temple. Can 
 it surprise us then that to-day the old fallacious doc- 
 trine of vital force which we imagined had been defi- 
 nitely vanquished, should again rear its head under 
 new names? Let us confess that we ourselves are 
 to blame for this because in the first exultation of 
 success we promised more than we were able to 
 fulfill. 
 
 Let us cease considering physiology merely as a 
 sort of applied physics and chemistry and thus avoid 
 arousing the justifiable opposition of those who be- 
 lieve it to be an idle task to seek an exhaustive ex- 
 planation of the living from the dead. Life can be 
 
46 THEORY OF NERVE ACTIVITY. 
 
 fully understood only from life, and a physics and a 
 chemistry which have sprung solely from the domain 
 of inanimate nature, and which therefore apply solely 
 to inanimate nature, are adequate only to the explana- 
 tion of such things as are common to the living and 
 the dead. 
 
 This is very much, but it is not all, and I am fain 
 to paraphrase here the words of a brilliant physicist 3 
 who has used them in an analogous connection, and 
 say: If the assertion that physiology is only applied 
 physics and chemistry be taken to mean that the 
 laws discovered in the domain of physics and chem- 
 istry are sufficient without extension and generaliza- 
 tion to explain fully the phenomena of life, "we 
 are, in my opinion, confronted with a view which 
 is in all respects comparable to that of Thales, who 
 endeavored to explain everything from the proper- 
 ties of water. Think of the improbability that a wide 
 domain of experience can be absolutely exhausted 
 by a narrower one previously known." If every- 
 thing that took place in nature could be designated 
 outright as physical or chemical, whether it was sub- 
 sumable under the present known laws of physics 
 or chemistry or not, then naturally all phenomena 
 of life would in such a case fall within the domain 
 of physics and chemistry. But we cannot hoist our 
 flag over a territory where we have never as yet set 
 foot, much less explored. 
 
 At bottom, it was not the mere negation of vital 
 force at this juncture that secured for physiology its 
 brilliant successes, but rather the concomitant intro- 
 duction into biology of the rigorous methods of phys- 
 
 E. Mach, Die Principien der Warmelehre, p. 351. 
 
THEORY OF NERVE ACTIVITY. 47 
 
 ical science and of the great mass of apparatus and 
 appliances which had been created by those methods; 
 and only in so far as vitalism persisted in employing 
 unproductive methods for treating biological problems 
 did it do real harm and was its overthrow of real 
 benefit. In fact, every .observed attribute of life 
 which was susceptible of immediate physical explana- 
 tion had already been physically explained in the 
 heyday of vitalism. The laws of the lever were 
 applied to the movement of the human members, and 
 the movement of the blood attributed to the con- 
 traction of the muscles of the heart, even at a time 
 when the muscular activity itself was still conceived 
 in characteristically vitalistic fashion. Even to-day 
 we cannot explain this last-named activity, although it 
 is one of the most palpable and most obtrusive of the 
 actions of living things. 
 
 The impulse to resort to analogy and to carry over 
 propositions abstracted from one domain into others 
 is so great that there can be no fear that any phe- 
 nomenon of life will long remain exempt from phys- 
 ical or chemical explanation after physics and chem- 
 istry have supplied the requisite means. To-day, 
 the danger of precipitate and therefore of insuffi- 
 cient physico-chemical explanation of vital phenom- 
 ena is perhaps greater than the danger of that vital 
 force should continue to be employed (to use a cele- 
 brated saying) "as the comfortable couch where rea- 
 son is quieted upon the pillow of obscure qualities." 
 
 Even the mechanical theory of life has not been 
 able to prevent the substitution of a new dogmatism 
 for the old vitalistic creed, and in adducing a strik- 
 ing instance of this fact I reach the real subject of 
 
48 THEORY OF NERVE ACTIVITY, 
 
 my present discussion. 
 
 I have already stated that the process which is 
 propagated along a nerve-fiber in the shape of a so- 
 called excitation is in its real nature unknown to us. 
 Nevertheless, it is accepted as an established fact 
 by most physiologists to-day that this excitation is 
 always of exactly the same kind, not only in one 
 and the same nerve-fiber but in all nerve- fibers; and 
 that it can consequently undergo alteration only with 
 regard to the strength and time of its propagation, 
 but not as regards its quality; and that, therefore, 
 all functional differentiation of the nerves is exclu- 
 sively resident in either their central or their peri- 
 pheral terminal apparatus. So thoroughly convinced 
 are many physiologists of the truth of this view, that 
 they absolutely refuse to consider any theory which 
 assumes a qualitative variability in the excitations 
 which pass through the nerve-fibers. And they do 
 this in the consciousness that they are supported by 
 the authority of a Helmholtz, a Dubois-Reymond, 
 and a Bonders. 
 
 But let us examine for a moment the meaning 
 of the assertion that the excitations are absolutely 
 alike in all nerve-fibers. 
 
 If it were possible to insert a portion of a sensory 
 nerve into the path of a motor nerve, and to con- 
 nect the former with the latter, fiber for fiber, a 
 cerebral excitation of the motor nerve would pass 
 unaltered through the inserted piece of sensory nerve 
 to the muscle, which it would forthwith set into 
 activity. Or, if we could cut out a piece of the optic 
 
THEORY OF NERVE ACTIVITY. 49 
 
 nerve and insert in its place a piece of a motor nerve, 
 and combine every fiber of the former with every 
 fiber of the latter, functional continuity would be 
 restored along with the anatomical continuity, and 
 the perception of light and color would be possible 
 just as before. 
 
 Finally, let us imagine the optic nerve and the 
 auditory nerve severed, and each brought into con- 
 junction with the other crosswise. According to Du- 
 bois Reymondi, we should in such a case hear the 
 lightning with the eye as a noise, and see the thun- 
 der with the ear as a succession of light-impressions. 
 
 Of such a character are the consequences that flow 
 from the assumption that the functions of the nerve- 
 fibers are all absolutely alike. The impossibility of 
 actually realizing the hypothetical cases which have 
 been adduced affect in no wise the correctness of 
 the conclusions drawn from them. 
 
 What, now, are the considerations that could have 
 led to the enunciation of a definite opinion regard- 
 ing processes which are still involved in so much 
 mystery for us? Carl Ludwig himself once consid- 
 ered the question of the likeness or unlikeness of the 
 nerve-fibers, and did so with all his wonted caution, 
 but he finally left the question undecided, as did also 
 Johannes Muller in his time. Since then, no new 
 facts have been brought to light which could have 
 tended to confirm the theory of the homogeneity of 
 the nerve-fibers, as Ludwig called it. For the ex- 
 periments in the grafting of the peripheral termi- 
 nals of severed nerves, with the central terminal of 
 
50 THEORY OF NERVE ACTIVITY. 
 
 a nerve having different functions, even if they had 
 been successful, could not have decided our ques- 
 tion. And yet, by the majority of physiologists the 
 theory of homogeneity is regarded to-day as an estab- 
 lished truth. 
 
 By his Investigations in Animal Electricity Dubois 
 Reymond 4 believed he had "awakened into lifelike 
 reality the hundred years' dream of the physicists and 
 physiologists regarding the identity of nervous energy 
 and electricity, even though in a slightly changed 
 form." The facts which he adduced have been con- 
 firmed in all their essential points, and although their 
 interpretation has turned out to be different from 
 what he conjectured, yet the opinion which he cher- 
 ished, according to which the essential nature of nerv- 
 ous activity found its expression in electrical phe- 
 nomena, is still shared by many physiologists to-day. 
 Now this view was in its time, and is to-day, so far 
 as I can see, the principal foundation of the con- 
 viction which so generally obtains in the scientific 
 world regarding the homogeneity of the nerve-fibers 
 and of their excitations. 
 
 When we set a nerve in excitation at any point 
 of its course by an artificial stimulus, the propaga- 
 tion of the process of excitation can be followed 
 step by step along the path of the nerve by means 
 of a galvanometer, for the reason that the electric 
 behavior of the nervous substance changes precisely 
 as the condition of the nervous substance itself 
 changes in passing to the excited state. The gal- 
 vanometer then shows us the current of action, or 
 at the severed terminus of the nerve the so-called 
 
 'Preface, p. 15. 
 
THEORY OF NERVE ACTIVITY. 51 
 
 oscillation of the current of injury. It was rendered 
 highly probable by Dubois Reymond (and later 
 investigations have only confirmed the conjecture) 
 that these electrical phenomena are characteristic of 
 all nerves, and that they accompany not only the 
 excitations which have been produced by artificial 
 stimuli, but also such as are disengaged in the natu- 
 ral way from the central or peripheral terminal appa- 
 ratus of the nerve-fiber. If now the electric phe- 
 nomena were the expression of the real inner char- 
 acter of the excitation passing along the nerve, the 
 homogeneity of the latter would follow immediately 
 from the homogeneity of the former; and different 
 nerve-fibers could exhibit different behaviors only 
 with regard to the intensity and time of action of 
 the excitation. 
 
 It was afterwards discovered that the electric phe- 
 nomenon in question admitted of scarcely any other 
 explanation than that of a chemical process propagated 
 along the excited nerve. Yet even this discovery could 
 not shake the theory of homogeneity. 
 
 After that theory had once taken root, the identity 
 of the chemical process was deduced immediately 
 from the identity of the electrical behavior, and the 
 idea never suggested itself that we might with just 
 as good reason infer the identity of two chemical 
 processes from the identity of their thermal effects, 
 or the identity of the chemical transformation in two 
 galvanic elements from the identity of their currents. 
 
 The electrical phenomena which accompany the ex- 
 citation of the nerve furnishing no adequate founda- 
 tion for the the'ory of homogeneity, the next natural 
 support likely to be resorted to for this theory is 
 
52 THEORY OF NERVE ACTIVITY 
 
 that of the morphological and chemical homogeneity 
 of the nerve-fibers. It is quite true, we are as yet 
 unable to distinguish by the microscope and by chem- 
 ical reactions every motor fiber from every secretory 
 fiber, and every optic fiber from every auditory fiber. 
 But the cases are numberless in which living ele- 
 mentary structures having different functions exhibit 
 absolutely the same behavior when subjected to known 
 optical or chemical tests. 
 
 The germs of quite different species of animals 
 are frequently so much alike as to be confounded 
 with one another; and the germs of different indi- 
 viduals of the same species naturally show a still 
 greater degree of similarity. Yet no biologist has 
 the least hesitation in ascribing to each individual 
 germ some quite specific individuality, some personal 
 idiosyncrasy, so to speak, of inward structure or mol- 
 ecular constitution by virtue of which a perfectly 
 definite path of future development is marked out 
 for it. 
 
 It is regarded as almost certain that the different 
 functions of the secretory cells of the various glands 
 are attributable to the physical and chemical differ- 
 ences of their vital substance, and yet in many cases 
 it would be quite impossible by the microscope and 
 micro-chemical methods at our command, to deter- 
 mine the actual function of any single glandular cell. 
 
 It was shown by Max Schultze many years ago, 
 and recent inquiries have confirmed his results, 5 that 
 while the pseudopodia of the same rhizopod merged 
 and fused perfectly on contact, the pseudopodia of 
 
 "Paul Jensen, "Ueber individuelle physiologische Unterschiede 
 zwischen Zellen der gleichen Art." Pfliiger's Archiv fur die ges. 
 Physiologic. Bd. LXIL, p. 175. 1895. 
 
THEORY OF NERVE ACTIVITY. S3 
 
 different individuals of the same species would not 
 fuse on being brought together. Now, if the pro- 
 toplasm of two individuals of the same species were 
 absolutely of the same composition, it would be diffi- 
 cult to see why their pseudopodia should not behave 
 with regard to one another precisely as the pseudo- 
 podia of the same individual do. 
 
 We are obliged, accordingly, to attribute to the 
 living substance of every single one of these minute 
 and inferior creatures, specific individual properties, 
 by virtue of which their substance is distinguished 
 from the substance of every other individual of the 
 same species; although there cannot be, even in so 
 patent a case as the present, the remotest thought 
 of directly demonstrating these inferred differences 
 by the experimental means of investigation at our 
 command. 
 
 But if a distinctive individual stamp must be im- 
 puted to every one of the countless members of the 
 same rhizopod species, why shall some such distinc- 
 tive mark be refused to the elementary organisms 
 of which the nervous system is composed? The fact 
 that the rhizopods lead an independent life, while the 
 nerve-elements are rigorously subordinated parts of 
 a more highly developed organism, is no reason 
 whatever for our not doing so, seeing that each indi- 
 vidual nerve-fiber is connected with a vast and most 
 varied host of vital functions, and that consequently 
 it can be highly differentiated according to the prin- 
 ciple of the division of labor; whereas no such divi- 
 sion of labor can come into consideration in the case 
 of the individuals of a rhizopod species. 
 
 In this division of labor of our organism lies, in 
 
54 THEORY OF NERVE ACTIVITY. 
 
 fact, part of the answer which I should give to 
 the advocates of the theory of homogeneity, if they 
 were to ask why I am anxious to assume heterogeneity 
 in places where the assumption of homogeneity ap- 
 pears to be adequate to explain the function of nerve- 
 fibers as simple organs of conductivity. The argu- 
 ment that I have adduced, they would contend, proved 
 at most that the sameness of all the fibers cannot now 
 be directly demonstrated, that there are even reasons 
 for suspecting them to be not the same, but that in 
 any event no counterproof to their theory is furnished. 
 Even the wires which conduct electricity, they would 
 say further, may be put to very different uses, and 
 yet the electrical phenomenon in the conducting wires 
 is qualitatively the same. 
 
 The fact that the excitation of a glandular nerve 
 produces a secretion, while that of a muscle-nerve 
 produces a movement, is sufficiently explained by a 
 difference in the terminal apparatus upon which those 
 nerves act; and there is just so much ground and 
 no more for supposing a qualitative difference of 
 excitation to result from a difference in the stimuli 
 by which the nerves of the several sense-organs are 
 excited. The further fact, they would continue, that 
 sound is able to excite the auditory nerve and light 
 the optic nerve is due entirely to the different struc- 
 ture of the apparatus at the peripheral end of these 
 nerves, one apparatus being especially adapted for 
 the reception of sound-waves and the other for the 
 reception of light-waves and for their transformation 
 into nervous excitation. If the sound were to strike 
 directly upon the auditory nerve itself, and not on 
 the peripheral apparatus, if the light were to fall di- 
 
THEORY OF NERVE ACTIVITY. 55 
 
 rectly upon the optic nerve itself, these nerves would 
 not be excited, a proof that the so-called specific 
 excitability of the various sensory nerves is not due 
 to any dissimilarity of the latter, but entirely to the 
 differences of the terminal apparatus which receive 
 the impression. 
 
 In answer to this oft-repeated argument, it must 
 be frankly confessed that so far as our present knowl- 
 edge goes, no conclusion whatever follows from the 
 varying behavior of the sense-organs toward differ- 
 ent sensory stimuli in favor of the assumption of a 
 specific heterogeneity of the sensory nerve-fibers; but 
 neither does anything follow in contravention of such 
 an assumption. And it must be further admitted that 
 considering the dissimilarity in results which follow 
 upon the excitation of nerves that act centrifugally 
 according as they set a muscle or a gland in activity, 
 no conclusive argument is forthcoming either in favor 
 or in contravention of the homogeneity of these 
 nerves. 
 
 But the situation assumes a different aspect when 
 we consider the manifold results to which the excita- 
 tion of such sensory nerves leads as have not their 
 terminus in such heterogeneous organs as the mus- 
 cles or the glands, but all end in the brain. For in- 
 stance, we are compelled to inquire how it is that 
 the excitation of one nerve brings with it the sensa- 
 tion of light and color, that of another the sensa- 
 tion of sweet and sour, that of a third the sensation 
 of heat and cold, while at the same time all these 
 nerves carry to the brain excitations which are abso- 
 lutely the same in quality. To this the theory of 
 homogeneity has always answered without the least 
 
56 THEORY OF NERVE ACTIVITY. 
 
 ado that it is because the like nerve-fibers of the 
 tongue and of the eye lead to unlike nerve-cells in 
 the brain, some of them to cells whose specific char- 
 acter enables them to take up the excitatory condi- 
 tions that correspond to the sensations of taste, others 
 to cells which, conformably to their specific function, 
 respond to the excitation of the fiber with precisely 
 that physiological process whose psychical correlate 
 is a sensation of light. 
 
 And here finally we are met with the frank conces- 
 sion that interior commotion of the nervous substance 
 which we call excitation or activity is, at least in the 
 various sensory centers of the brain, specifically dif- 
 ferentiated, and that here at last the functional homo- 
 geneity reaches its termination. 
 
 While this doctrine was taking its development, a 
 doctrine according to which all the nerve-fibers are 
 exactly the same in kind, but the terminal apparatus 
 which are functionally connected with them are differ- 
 ent in kind, the idea necessarily suggested itself of 
 separating the nerve-fibers as a particular group of 
 elementary structures from the nerve-cells; but, even 
 at the period of the development in question, it would 
 have been quite admissible, in view of the anatomical 
 continuity which had been established in numerous 
 instances between cells and fibers, to have inquired 
 by what right physiologists ascribed to the cells func- 
 tions which varied in character, whereas they denied 
 to the fibers connecting them all specific dissimilarity 
 whatever, and did so in spite of the fact that chem- 
 ical and physical research had not been able to dis- 
 
THEORY OF NERVE ACTIVITY. 57 
 
 cern any more characteristic differences in the sub- 
 stance of the cells than it had in the axial cylinder 
 of the fibers. 
 
 Since that time, a different conception of the ele- 
 mentary structures of the nervous system has found 
 almost universal acceptance. According to this the- 
 ory, every nerve-fiber is associated in such wise with 
 a nerve-cell as to form with it a single elementary 
 organism only ; and the living substance of that organ- 
 ism, which is collected in more abundant quantities 
 about the nucleus of the nerve-cell, is continued into 
 the fibers. Accordingly, the nerve-fibers would be 
 integral constituents of these elementary structures of 
 the nervous system, or "neurons" as they are called. 
 On this theory, the idea is quite natural to ascribe 
 to the fibers which continue the cells the same specific 
 differences which physiologists were obliged to assign 
 to the nerve-cells of the various cerebral centers. 6 But 
 as soon as we do this, we no longer have before us 
 cells which, though capable of performing different 
 tasks, are yet connected by filaments having quite the 
 same functions, but we have elementary organisms 
 whose specific or individual dissimilarity extends to 
 their remotest filar prolongations. A nerve-trunk is 
 no longer a mere bundle of conducting wires disen- 
 gaging different sorts of effects, according to the 
 kind of apparatus with which they are connected at 
 their termini and being at the same time in their own 
 specific function absolutely of the same kind; but it 
 is a bundle of living arms which the elementary organ- 
 isms of the nervous system send forth for the pur- 
 
 6 I offer no opinion as to the correctness of the histological doctrine 
 of neurons. I lay it at the foundation of the present discussion be- 
 cause my theory needs some definite histological substratum. 
 
58 THEORY OF NERVE ACTIVITY. 
 
 pose of entering into functional connection with one 
 another, or of permitting the phenomena of the out- 
 side world to act upon them, or of exercising control 
 over other organs like the muscles and the glands. 
 And in each one of these arms a quite special kind 
 of life is active, corresponding precisely to the neuron 
 to which the nerve-fiber belongs. The conducting 
 path which unites a sense-organ with the cerebral cor- 
 tex, or the latter with a muscle, appears as a chain 
 of living individuals of which every member, although 
 always dependent upon its neighbors, still leads a sep- 
 arate life, the specific character of which is generally 
 different in the different parts of the nervous system. 
 Even in the neurons of the same group it is not abso- 
 lutely the same, but bears in each of them a more or 
 
 less individual stamp. 
 
 * 
 
 * * 
 
 The theory of the homogeneity of the excitatory 
 process in all the nerve-fibers involves the further 
 assertion that this process remains qualitatively the 
 same in one and the same fiber, and that it is variable 
 only as to intensity and time of propagation. 
 
 I once advanced views- relating to the processes in 
 the nerve-apparatus of the visual organs which pre- 
 supposed that in one and the same retinal element 
 different processes could be set up by light-waves 
 of different rates of vibration. The physiologists de- 
 clared such a view inadmissible inasmuch as more 
 than one process in the same structural element would 
 presuppose more than one conduction-process in the 
 corresponding fiber, a view which physiology was 
 
THEORY OF NERVE ACTIVITY. 59 
 
 forthwith compelled to repudiate. 7 And this dogma, 
 according to which every nerve-fiber is held to be 
 capable of only one kind of excitation, had actually 
 been extended also to the nerve-cells. True, as al- 
 ready said, physiologists were even then compelled 
 to ascribe to the nerve-cells of the different sensory 
 centers different functions; but in one and the same 
 cell the excitation was said to be of unalterable quality. 
 
 It was no less an authority than Helmholtz who 
 introduced this conception into the physiology of sen- 
 sation, and to-day his disciples still esteem the doc- 
 trine of cerebral cells sensitive only to red, green, 
 or violet, as the rarest fruit of Johannes Miiller's doc- 
 trine of the specific energies of the sensory nerves. 
 But I am convinced that Johannes Miiller would not 
 have accepted such a view, for his conception of biol- 
 ogy was thoroughly monistic, and he would not have 
 denied to the cell of the brain what must be conceded 
 to the lowest unicellular organism, namely, a more or 
 less wide qualitative variability of its inner life. 
 
 Who can deny that the chemical changes in the 
 substance of an infusorian vary qualitatively accord- 
 ing to the changes in its external life-conditions; for 
 example, in food material, or in other stimuli acting 
 upon its body? And even granting that the progres- 
 sive division of labor which accompanies the increas- 
 ing structural complexity of the animal organism, in 
 general brings more structural uniformity into the life 
 of the individual cells, still there is no ground for 
 the view that just in the nerve-cell this uniformity 
 can become as complete as the homogeneity theory 
 
 'Bonders, "Ueber Farbensysteme," Arch. f. Ophthalmol., XXVII. 
 1881. 
 
60 THEORY OF NERVE ACTIVITY. 
 
 demands. 
 
 Granting, however, the possibility of qualitatively 
 different excitation-conditions in the same cell (and 
 in the fiber springing from it), then there is opened 
 for the theory of nerve-life a series of points of 
 view which are quite excluded by the homogeneity 
 theory. 
 
 At the outset it follows that a neuron which is capa- 
 ble of various kinds of activities, will possess a corre- 
 spondingly complex excitability, that is to say, it will 
 vary in its reactions according to the nature of the 
 stimuli, it will respond by one or another of its habit- 
 ual special activities; in brief, the activity of the neu- 
 ron and of its fiber may vary, not merely, as has been 
 supposed, in intensity but also in quality, according 
 to the nature of the stimulus, whether this be exer- 
 cised by an external sense-organ, or by an adjacent 
 neuron. A further consequence of our view would 
 be that the effects in those non-nervous elementary 
 organs with which centrifugal fibers stand in functional 
 connection, might also vary with the kind of excita- 
 tion conducted in the fiber. 
 
 For the motor fibers, of course, there is, owing to 
 the uniform action of the muscle-fiber, no apparent 
 reason for such a view. It is different, however, with 
 the nerve-fibers which govern the activity of a gland. 
 The view now generally accepted is that, in conform- 
 ity with their unalterable homogeneity of excitation, 
 these secretory fibres can influence the activity of 
 their dependent gland-cells only quantitatively. But 
 how would it be if, according to the nature of the 
 excitation given by the nerve-fiber, the chemical proc- 
 esses in the secretory cells were different, and con- 
 
THEORY OF NERVE ACTIVITY. 61 
 
 sequently the nervous system could affect, within cer- 
 tain definite limits, the quality ol the secretion fur- 
 nished by one and the same cell? 
 
 And would not similar views hold in regard to the 
 so-called trophic activities of centrifugal nerves? In- 
 asmuch as these activities do not, as in the case of 
 the motor or secretory nerves, immediately reveal 
 themselves by easily demonstrable movements, they 
 have hitherto remained rather within the domain of 
 conjecture than in that of safely ascertained facts. 
 If, however, the nervous system exerted an imme- 
 diate influence, not merely on the motor and secre- 
 tory, but also on the other elementary organs of the 
 body (e. g., on certain epithelia, on the developing 
 cells of the ovary, etc.), then here, too, the answer 
 to the question whether such an influence were varia- 
 ble only according to quantity or also according to 
 quality, would be of far-reaching influence. 
 
 But let us turn back to the activities which the 
 excitation of a neuron exercises on those other neu- 
 rons with which it is connected. 
 
 If, as histology teaches us, a sensory fiber enter- 
 ing the spinal cord divides into an ascending branch 
 and a descending branch, and from these latter there 
 branch off the so-called collaterals which finally stand 
 in relation with other neurons (it matters not whether 
 through direct contact or through connection) ; if 
 finally every one of these neurons by its branching 
 stands in functional relation with still others, and 
 so on; then, from a purely anatomical standpoint, 
 there exists for an incoming excitation an incalculable 
 multiplicity of paths through the central nervous sys- 
 tem. If now an excitation which has seized a neuron, 
 
62 THEORY OF NERVE ACTIVITY. 
 
 should pass on indifferently to all the other neurons 
 with which the first is functionally connected, there 
 would result an exceedingly widespread diffusion of 
 the excitation entering through a sensory fiber or 
 coming from a neuron to the cerebral cortex, a dif- 
 fusion such as is not known to exist, or, at the most, 
 is only approximated to in motor effects under patho- 
 logical conditions. 
 
 It must consequently be assumed that the excita- 
 tion of a fiber does not take indifferently all the 
 anatomically given ways, but selects predominantly 
 certain determined paths, diffusing itself in the others 
 only in a more or less enfeebled manner or only ex- 
 ceptionally; and the question hence arises: What is 
 the determining factor for this choice, and for the 
 varied ratio of the strengths with which the excita- 
 tion propagates itself along the many possible paths? 
 
 The homogeneity theory finds the answer in the dif- 
 ferent degree of excitability and conductivity of the 
 individual paths, and in the different amounts of 
 resistance offered to passage from one neuron to 
 another. As for the further diffusion this is made 
 to depend entirely on the strength of the incoming 
 stimulus. 
 
 Numerous facts from the sensory and motor terri- 
 tories of nerve-life teach us that the innervation issu- 
 ing from a neuron can diffuse itself in the nervous 
 system very differently according as certain other 
 excitations take place at the same time. For exam- 
 ple, though the stimulus be the same, a reflex move- 
 ment is sometimes reinforced and sometimes inhibited 
 by excitations issuing simultaneously from another 
 place on the periphery of the body or from the brain. 
 
THEORY OF NERVE ACTIVITY. 63 
 
 Accordingly, the homogeneity theory accepts the view 
 that resistance to conduction in a neuron can some- 
 times be increased and sometimes lessened by excita- 
 tions which reach the same neuron from another 
 direction, in other words that sometimes a so-called 
 path-breaking, sometimes an arrestation takes place; 
 but it does not further explain how excitations which 
 are said to be always of the same kind and to differ 
 only quantitatively, can at one time operate on the 
 neuron so as to promote, and at another time so as 
 to arrest, its function. 
 
 Everything is seen in a different light, however, 
 as soon as a qualitative variability of the conducted 
 excitation, and also a qualitatively different excita- 
 bility of the conducting paths, are admitted. If the 
 individual nerve-fiber is suited for the taking up 
 and transmission of definite qualities of excitation, 
 either preferentially or exclusively, then the path 
 which is taken by an excitation is coincidently deter- 
 mined by the quality of that excitation. 
 
 As a report is principally taken up and circulated 
 by those who take a special interest in it, that is 
 to say, the paths of its spreading depend on the 
 nature of its contents, so to a definite afferent excita- 
 tion those neurons will react preponderatingly whose 
 peculiar nature precisely corresponds to that excita- 
 tion. 
 
 The mutual relations of neurons will then depend 
 not merely upon their anatomical arrangement, but 
 also on their degree of internal structural affinity. 
 And as one and the same fiber need not be adapted 
 merely to one kind of excitation, but may be adapted 
 to a certain number (though within a narrow range), 
 
64 THEORY OF NERVE ACTIVITY. 
 
 so not only will the same path be able to conduct 
 various kindred qualities, but the excitation issuing 
 from the same neuron will be able, according to its 
 particular quality, to penetrate various parts of the 
 nervous system. 
 
 If, further, excitations should be simultaneously 
 brought to the same neuron by two of its neigh- 
 bors, then according to the homogeneity theory these 
 excitations could on meeting only strengthen or 
 weaken each other. According to our view, how- 
 ever, the two excitations could be of different kinds, 
 and from their meeting in the same substance might 
 originate a new quality, which would indeed be closely 
 related to the two single excitations, but not like 
 either. 
 
 And generally the whole life and being of the nerv- 
 ous system, its ontogenesis and phylogenesis, appear 
 in a totally different light as soon as we give up the 
 dogma of the absolutely homogeneous function of all 
 nerve-fibers, and ascribe to the individual fiber-groups 
 characters. For that dogma excludes from the neu- 
 ron every capability for development and improve- 
 ment, in so far as such capability is not already inborn 
 and in so far as it is not a mere augmentation of its 
 excitational or vital process, which latter is from the 
 beginning to the end of its life supposed to be peculiar 
 and unalterable. 
 
 Rightly have the opponents of the doctrine of the 
 specific energies of the sensory nerves pronounced 
 against the view of a life-long unalterable constancy 
 of function of the nerves, but they went too far, as 
 I conceive, when they contested the congenitally dif- 
 ferent and special nature of the individual sensory 
 
THEORY OF NERVE ACTIVITY. 65 
 
 nerves, accepted the indifference of function of all 
 sensory fibers of the new-born, and regarded all func- 
 tional differences of nerve-fibers, which are the phylo- 
 genetic acquisition of innumerable generations, merely 
 as the result of an adaptation to heterogeneous, indi- 
 vidual sensory stimuli during the post-embryonic 
 period. 
 
 Of course after birth the influences of the external 
 world belong to the conditions of the further normal 
 development of the whole body, and the sensory stim- 
 uli especially are indispensable conditions of devel- 
 opment of the nervous apparatus of our sense-organs. 
 But light, for example, does not find in the eye of 
 the new-born babe a nerve-substance from which, 
 so to speak, anything whatever can be made; in other 
 words, a substance which, if it could be transposed 
 from the eye into the ear or into the tongue could 
 be educated by the sound-waves to a medium of audi- 
 tory sensations, or by gustatory stimuli to be a me- 
 diurri of i taste-sensation. 
 
 As the- germ sprouting from the earth needs light 
 to become a green plant, so in the new-born babe 
 the' neuron in the eye needs light, and the neuron 
 in the ear the sound stimulus, to complete its course 
 of development; but just as light never makes the 
 fungus green, so it could never make the neurons 
 of the ear to see if they should be transplanted into 
 the eye. As I take it, the neurons of our eye are 
 not merely educated, but are born for seeing, and like- 
 wise those of our ear for hearing. 
 
 This, however, does not exclude the fact that, 
 within their own narrow congenital limits of exist- 
 ence and action, they are capable of further indi- 
 
66 THEORY OF NERVE ACTIVITY. 
 
 vidual development. And the same holds good (some- 
 times more, sometimes less) of all parts of our nerv- 
 ous system. It is true, the farther back in the im- 
 measurably long developmental series of the animal 
 organism a given part of it can be traced, the more 
 fast and sharp is the congenital stamp of its func- 
 tion, and the less capable does it appear of trans- 
 formation and development in the course of its fur- 
 ther life. But the cerebral cortex is reckoned among 
 the phylogenetically youngest parts of our nervous 
 system, and its neurons belong, as it seems, to those 
 elementary bodily organs which in post-natal life are 
 afforded the relatively widest sphere of individual 
 action under the influence of impinging stimuli. Now, 
 how is such a development conceivable if the internal 
 activity of a nerve-fiber or a nerve-cell, in brief a 
 neuron, is to be always of one and the same kind? 
 
 What is it then that a neuron of our cortex under 
 normal circumstances experiences? In other words, 
 of what do the stimuli which encounter it and deter- 
 mine its internal activities, consist? Under constant 
 conditions of nutrition, these are mainly the excita- 
 tions conducted to it from those other neurons which 
 stand in relation to it by means of the nerve-fibers. 
 If, however, these nerve-fibers always conduct to it 
 only the same kind of excitation, have, so to speak, 
 only one note to their lyre, and therefore the stimuli 
 which the neurons experience throughout life are the 
 same and are variable only in quantity and time, then 
 the reaction of the neuron will also always be one 
 of the same kind, and the afferent stimulus can only 
 liberate in the nerve-cell forever the same activity. 
 And if, as the homogeneity theory concedes, the lat- 
 
THEORY OF NERVE ACTIVITY. 67 
 
 ter could be different in the different nerve-cells, still 
 in one and the same cell, and in so far as it depends 
 upon the afferent excitations, it would remain the 
 same its whole life long. 
 
 It will be altogether different, however, if these 
 excitations vary qualitatively, according to the nature 
 of the neighboring neurons from which they come, 
 or if the afferent excitation from one and the same 
 neuron may vary within certain qualitative limits. 
 Then a more or less rich multiplicity of excitations 
 will take the place of the just-mentioned monotony 
 that the neuron experiences from its neighbors, and, 
 as this neuron is in its turn capable of heterogeneous 
 activities, there is also opened to it the possibility of 
 reacting to different impulses in different ways. 
 
 The nature of this reaction will, of course, also be 
 determined by the inborn individuality of the neuron ; 
 but of the entire stock of innate qualities which it 
 brings with it from birth those will be most fully 
 developed in the course of its life to whose develop- 
 ment the neuron is most frequently or most strongly 
 excited by its neighbors. Or, briefly stated, the neu- 
 ron will possess the capability of qualitative and not 
 merely of quantitative development, which last would, 
 according to the homogeneity theory, alone be possible 
 to it. 
 
 According to its place in the nervous system, its 
 more or less manifold relations to other neurons and 
 its inborn structure, this subsequent development will 
 be more or less many-sided, and the doctrine of the 
 homogeneity-theory that the sameness of excitation 
 which the neurons experience conditions a correspond- 
 ing uniformity in their further development, may per- 
 
68 THEORY OF NERVE ACTIVITY, 
 
 haps approximately hold for entire large groups of 
 neurons. On the other hand, however, all experience 
 or training both sensory and motor, in brief, every- 
 thing that can be called conscious or unconscious mem- 
 ory in the widest sense of the word, is to my mind 
 not conceivable unless the living substance of the 
 nerve-cells and fibers is capable of a qualitatively vari- 
 able development. 
 
 I have sought in vain in the writings of those sup- 
 porters of the homogeneity theory who have occu- 
 pied themselves with considerations concerning the 
 physiological foundations of nutrition and exercise, 
 for a satisfactory conception of the subject. For 
 an explanation of the development of the central 
 nervous system corresponding to the physical devel- 
 opment, we are with reason directed to the possi- 
 bility of the origin of new connections between the 
 neurons, to alterations of excitability and conductiv- 
 ity in the paths already provided, to the opening and 
 the increasing erosion of certain old paths by use, 
 etc. On all these, old or new, pervious or impervious 
 paths, however, that which is conducted through the 
 nerve-fibers still remains (according to the homo- 
 geneity-theory) always the same, and everywhere it 
 is a matter only of moreness or lessness and of vari- 
 able velocity. The whole nervous system, according 
 to this theory, appears like a land whose numerous 
 communities are connected by a richly developed net- 
 work of road, on which latter, however, always and 
 everywhere, only one and the same kind of wares is 
 transported. 
 
 Writers love to compare the nerve-fibers with tele- 
 graph or telephone wires, and they will consequently, 
 
THEORY OF NERVE ACTIVITY. 
 
 69 
 
 perhaps, point to the endless multiplicity of things 
 which can be transmitted through wires of exactly 
 the same kind. The comparison is seductive, for if 
 spun out farther it seems suited to solve all difficul- 
 ties at a single stroke. 
 
 In place of the undoubtedly "obscure" specific or 
 individual "qualities" of the nervous processes of 
 which I have spoken, appears a multiplicity of oscil- 
 lations of different temporal and spatial form, of 
 which the nerve-substance is the mere vehicle. But 
 all come finally to exactly the same result with this 
 comparison as I do. For it must be admitted that 
 neither in all nerve-fibers is the same oscillation-form 
 always transmitted, nor is every individual nerve-fiber 
 susceptible of all the oscillation-forms which are pos- 
 sible to the nerve-substance generally, but only of 
 those to which it can respond. 
 
 What we have named the specific energy of the 
 fibers or cells reappears here as special resonance- 
 capacities corresponding to definite oscillation-forms. 
 What we formerly called an inborn, acquired or indi- 
 vidual characteristic, becomes here the pitch; and, as 
 with us, the specific excitabilities, so here the reso- 
 nance and previous attunement of the neurons deter- 
 mine the paths in the nervous system which a given 
 oscillation-form shall enter upon. 
 
 We accordingly acquire only another mode of ex- 
 pression for one and the same thing; and, as I con- 
 ceive, one not corresponding so well to the facts, 
 inasmuch as it takes no account of that which in an 
 entirely unique way characterizes all life, namely, 
 metabolism, that chemical change in the living sub- 
 
70 THEORY OF NERVE ACTIVITY. 
 
 stance whose qualitative differences it is at present 
 (and may be forever) impossible to express by purely 
 quantitative spatial and temporal terms. 
 
 From all that has been said it immediately follows 
 that I not only agree with the teaching of Johannes 
 Muller, but would like to see the scope of his concep- 
 tion much broadened. The specific energies are in 
 my opinion a phylogenetically acquired heirloom, not 
 merely of the sensory nerves, but more or less of 
 all neurons, of their fibers as well as of their cells. 
 But I consider that the inheritance allotted to the 
 individual neuron is by no means so sparing and 
 uniform as was assumed in the case of the cells of 
 the sensory centers, and further that it was not be- 
 queathed with a codicil that the heir should not add 
 any newly acquired riches to his inheritance. 
 
INDEX 
 
 Acquired energy of the nervous 
 system, 70; properties of an or- 
 ganism, 15. 
 
 Analogy, 41, 47. 
 
 Animals born precocious, 21; not 
 unconscious machines, 20. 
 
 Apparatus of memory, 10; ter- 
 minal nerve, 55; sense-organs, 
 65. 
 
 Aristotle, 28. 
 
 Attunement of the neurons, 69. 
 
 Babe, new-born, not a completed 
 structure, 36. 
 
 Biology, 28. 
 
 Body and mind, functional inter- 
 dependence of, 3 et seq. 
 
 Book, the brain a closed, 40. 
 
 Brain, human, at birth virgin sub- 
 stance, 37. 
 
 Central nervous system, develop- 
 ment of, 68. 
 
 Cerebral cortex, 62, 66; potency, 
 23. 
 
 Commander-in-chief, the will, 11. 
 
 Concepts or ideas, 8. 
 
 Congenital limits of neurons, 65. 
 
 Conjecture, 44. 
 
 Consciousness and material mech- 
 anism, 2; phenomena of, 4; 
 functional interdependence of 
 matter and, 6; individual and 
 race, 22; compared to a work- 
 man, 40. 
 
 Determining factor in the activi- 
 ties of neurons, the, 62. 
 
 Dogma and the homogeneity the- 
 ory, 59 and 64. 
 
 Donders, 48 . 
 
 Dubois-Reymond, 44, 48. 49, 50 
 and 51. 
 
 Electric current, 26; phenom- 
 enon and chemical process, 51. 
 
 Egg shell, things learned in the, 
 19. 
 
 Elemental memory, 23. 
 
 Empsychosis, 2. 
 
 Enigma of life, 32. 
 
 Evolution and memory, 18. 
 
 Excitability, complex, 60. 
 
 Filial organism, 16. 
 
 Fixed formation of the race, 22. 
 
 Force of habit, unconscious mem- 
 ory, 12. 
 
 Function denned, 4. 
 
 Functional differentiation of the 
 nerves, 48. 
 
 Galvanometer, 50. 
 Germ memory, 17. 
 Germinal vessels, 15; cells and 
 memory, 16. 
 
 Helmholtz, 44, 48, 59. 
 Homogeneity theory explained, 
 48 et seq.; 62. 66' 
 
INDEX 
 
 Idiosyncrasy of inward structure, 
 52. 
 
 Immortality as unconscious mem- 
 ory, 23. 
 
 Individual consciousness, 22; en- 
 ergy, 36; and cerebral cells, 39; 
 properties, 53. 
 
 Inherited energy, 38. 
 
 Instinct and race memory, 20; 
 as predisposition, 22. 
 
 Innate function, 32. 
 
 Language, the memory of man- 
 kind, 23. 
 
 Lever, laws of a, 47. 
 Light and color, 26, 34. 
 Ludwig, Carl, 43, 49. 
 
 Mach (note), 46. 
 
 Machine, unconscious, 20; the 
 human body not a, 31. 
 
 Material process, 3; of memory, 8. 
 
 Mechanical conception of vital 
 phenomena, 45. 
 
 Memory, 6, 7, 8, 10, 12, 16, 23, 68. 
 
 Mirror of consciousness, 5. 
 
 Molecular and atomic structure, 
 30. 
 
 Monistic conception of biology, 59. 
 
 Mueller, Johannes, 25, 27, 28, 33, 
 34, 49, 59, and 70. 
 
 Multiform potency and specific en- 
 ergy, 35. 
 
 Mutual relations of neurons, 63. 
 
 Nervous system, 14. 
 
 Neurologist, 4. 
 
 Neurons, 57, 58, 60; congenital 
 
 limits of, 65. 
 Nocturnal memory, 9. 
 Nutrition, 36. 
 
 Ontogenesis, 64 
 Optic nerve, 26. 
 Perception, 10. 
 
 Phylogenesis of the nervous sys- 
 tem, 64. 
 
 Physiology, 25, 45. 
 Psychical correlate of physiolog- 
 ical process, 56. 
 Psychological inquiries, 2, 3. 
 
 Quality and quantity of excita- 
 tion of a neuron, 61, 63; in- 
 nate, 67. 
 
 Race inheritance, 14; memory and 
 instinct, 20. 
 
 Reminiscences in old age, 17. 
 
 Repetition of function and sec- 
 ond nature, 38. 
 
 Reproduction of organic process, 
 13. 
 
 Reproductive faculty of the ce- 
 rebral substance, 23. 
 
 Resonance-capacities of the neur- 
 ons, 69. 
 
 Schultz, Max, 52. 
 
 Scientific analysis, 29. 
 
 Second nature, 19. 
 
 Sensation, 3. 
 
 Sense memory, 6, 7. 
 
 Soul and brain, the, 33; inner life 
 
 of the, 2. 
 
 Sound and light, 54. 
 Specialization and memory, 21. 
 Specific energy of the sensory 
 
 nerves, 25, 27, 30, 35, 58, 64, 69. 
 Spiritual and material facts, 5. 
 
 Telegraph or telephone wires, an- 
 alogy of, 68. 
 
 Thales, 46. 
 
 Theory of nerve energy, 43, 
 
 Tool-house of consciousness, 40. 
 
 Traditions, the memory of man- 
 kind, 23. 
 
 Truth revealed, 19. 
 
 Unconscious memory, 8, 18, 23. 
 
 Virgin substance of the brain, 37. 
 
 Visual organs, nerve-apparatus of, 
 58. 
 
 Vital phenomena, mechanical con- 
 ception of, 45 
 

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