'Ohne Kochsalz kein Gedanke'; for thought would stop as quickly if the brain should dry up or lose its NaCl as if it lost its phosphorus. In America the phosphorus-delusion has twined itself round a saying quoted (rightly or wrongly) from Professor L. Agassiz, to the effect that fishermen are more intelligent than farmers because they eat so much fish, which contains so much phosphorus. All the facts may be doubted.
The only straight way to ascertain the importance of phosphorus to thought would be to find whether more is excreted by the brain during mental activity than during rest. Unfortunately we cannot do this directly, but can only gauge the amount of PO_{5} in the urine, which represents other organs as well as the brain, and this procedure, as Dr. Edes says, is like measuring the rise of water at the mouth of the Mississippi to tell where there has been a thunder-storm in Minnesota.[134] It has been adopted, however, by a variety of observers, some of whom found the phosphates in the urine diminished, whilst others found them increased, by intellectual work. On the whole, it is impossible to trace any constant relation. In maniacal excitement less phosphorus than usual seems to be excreted. More is excreted during sleep. There are differences between the alkaline and earthy phosphates into which I will not enter, as my only aim is to show that the popular way of looking at the matter has no exact foundation.[135] The fact that phosphorus-preparations may do good in nervous exhaustion proves nothing as to the part played by phosphorus in mental activity.
Like iron, arsenic, and other remedies it is a stimulant or tonic, of whose intimate workings in the system we know absolutely nothing, and which moreover does good in an extremely small number of the cases in which it is prescribed.
The phosphorus-philosophers have often compared thought to a secretion.
"The brain secretes thought, as the kidneys secrete urine, or as the liver secretes bile," are phrases which one sometimes hears. The lame analogy need hardly be pointed out. The materials which the brain _pours into the blood_ (cholesterin, creatin, xanthin, or whatever they may be) are the analogues of the urine and the bile, being in fact real material excreta. As far as these matters go, the brain is a ductless gland. But we know of nothing connected with liver-and kidney-activity which can be in the remotest degree compared with the stream of thought that accompanies the brain's material secretions.
There remains another feature of general brain-physiology, and indeed for psychological purposes the most important feature of all. I refer to the aptitude of the brain for acquiring _habits_. But I will treat of that in a chapter by itself.
FOOTNOTES: [99] I shall myself in later places indulge in much of this schematization. The reader will understand once for all that it is symbolic; and that the use of it is hardly more than to show what a deep congruity there is between mental processes and mechanical processes of _some_ kind, not necessarily of the exact kind portrayed.
Leipzig Acad. Berichte, 1875, p. 372 (Journal of Physiol., 1875). J.
Ward: Archiv f. (Anat. u.) Physiol., 1880, p. 72. H. Sewall: Johns Hopkins Studies, 1880, p. 30. Kronecker u. Nicolaides: Archiv f. (Anat.
u.) Physiol., 1880, p. 437. Exner: Archiv f. die ges. Physiol., Bd.
Thl. ii, p. 31. François-Franck: Leçons sur les Fonctions motrices du Cerveau, p. 51 ff., 339.--For the process of summation in _nerves_ and Wundt: Physiol. Psych., i, 243 ff.; Richet: Travaux du Laboratoire de Philosophique, t. xxi, p. 564. Kronecker u. Hall: Archiv f. (Anat. u.)
Physiol., 1879; Schönlein: _ibid._1882, p. 357. Sertoli (Hofmann and Centralblatt, 1883, No. 7. Grünhagen: Arch. f. d. ges. Physiol., Bd.
34, p. 301 (1884).
[101] Bubnoff und Heidenhain: Ueber Erregungs- und Hemmungsvorgänge innerhalb der motorischen Hirncentren. Archiv f. d. ges. Physiol., Bd.
26, p. 156 (1881).
(_ibid._ Bd. 28, p. 497 (1882)) that the summation here occurs in the spinal cord. It makes no difference where this particular summation occurs, so far as the general philosophy of summation goes.
[103] G H. Lewes: Physical Basis of Mind, p. 479, where many similar examples are given, 487-9.
[104] Romanes: Mental Evolution in Animals, p. 168.
[105] See a similar instance in Mach: Beiträge zur Analyse der Empfindungen, p. 36, a sparrow being the animal. My young children are afraid of their own pug-dog, if he enters their room after they are in bed and the lights are out. Compare this statement also: "The first question to a peasant seldom proves more than a flapper to rouse the torpid adjustments of his ears. The invariable answer of a Scottish peasant is, 'What's your wull?'--that of the English, a vacant stare. A second and even a third question may be required to elicit an answer."
(R. Fowler; Some Observations on the Mental State of the Blind, and Deaf, and Dumb (Salisbury, 1843), p. 14.)
[106] The reader will find a great deal about chronographic apparatus in J. Marey: La Méthode Graphique, pt. ii, chap. ii. One can make pretty fair measurements with no other instrument than a watch, by making a large number of reactions, each serving as a signal for the following one, and dividing the total time they take by their number. Dr. O. W. Holmes first suggested this method, which has been ingeniously elaborated and applied by Professor Jastrow. See 'Science' for September 10, 1886.
[107] See, for a few modifications, Cattell, Mind, xi, 220 ff.
[108] Physiol. Psych., ii, 221-2. Cf. also the first edition, 728-9.
I must confess to finding all Wundt's utterances about 'apperception' both vacillating and obscure. I see no use whatever for the word, as he employs it, in Psychology. Attention, perception, conception, volition, are its ample equivalents. Why we should need a single word to denote all these things by turns, Wundt fails to make clear. Consult, however, his pupil Staude's article, 'Ueber den Begriff der Apperception,' etc., in Wundt's periodical Philosophische Studien, i, 149, which may be supposed official. For a minute criticism of Wundt's 'apperception,' see Marty: Vierteljahrschrift f. wiss. Philos., x, 346.
[109] By Exner, for example, Pflüger's Archiv, vii, 628 ff.
[110] P. 222. Cf. also Richet, Rev. Philos., vi, 395-6.
[111] For instance, if, on the previous day, one had resolved to act on a signal when it should come, and it now came whilst we were engaged in other things, and reminded us of the resolve.
[112] "I need hardly mention that success in these experiments depends in a high degree on our concentration of attention. If inattentive, one gets very discrepant figures...This concentration of the attention is in the highest degree exhausting. After some experiments in which I was concerned to get results as uniform as possible, I was covered with perspiration and excessively fatigued although I had sat quietly in my chair all the while." (Exner, _loc. cit._ vii, 618.)
[113] Wundt, Physiol. Psych., ii, 226 [115] In short, what M. Delbœuf calls an '_organe adventice_.' The reaction-time, moreover, is quite compatible with the reaction itself being of a reflex order. Some reflexes (sneezing, e.g.) are very slow.
The only time-measurement of a reflex act in the human subject with which I am acquainted is Exner's measurement of winking (in Pflüger's Archiv f. d. gesammt. Physiol., Bd. viii, p. 526, 1874). He found that when the stimulus was a flash of light it took the wink 0.2168 sec.
to occur. A strong electric shock to the cornea shortened the time to 0.0578 sec. The ordinary 'reaction-time' is midway between these values. Exner 'reduces' his times by eliminating the physiological process of conduction. His 'reduced minimum winking-time' is then (_ibid._ vii, 637). These figures have really no scientific value beyond that of showing, according to Exner's own belief (vii, 531), that reaction-time and reflex-time measure processes of essentially the same order. His description, moreover, of the process is an excellent description of a reflex act. "Every one," says he, "who makes reaction-time experiments for the first time is surprised to find how little he is master of his own movements, so soon as it becomes a question of executing them with a maximum of speed. Not only does their energy lie, as it were, outside the field of choice, but even the time in which the movement occurs depends only partly upon ourselves. We jerk our arm, and we can afterwards tell with astonishing precision whether we have jerked it quicker or slower than another time, although we have no power to jerk it exactly at the wished-for moment."--Wundt himself admits that when we await a strong signal with tense preparation there is no consciousness of any duality of 'apperception' and motor response; the two are continuous (Physiol.
Psych., ii, 226).--Mr. Cattell's view is identical with the one I defend. "I think," he says, "that if the processes of perception and willing are present at all they are very rudimentary...The subject, by a voluntary effort [before the signal comes], puts the lines of communication between the centre for" the stimulus "and the centre for the co-ordination of motions...in a state of unstable equilibrium.
When, therefore, a nervous impulse reaches the" former centre, "it causes brain-changes in two directions; an impulse moves along to the cortex and calls forth there a perception corresponding to the stimulus, while at the same time an impulse follows a line of small resistance to the centre for the co-ordination of motions, and the proper nervous impulse, already prepared and waiting for the signal, is sent from the centre to the muscle of the hand. When the reaction has often been made the entire cerebral process becomes automatic, the impulse of itself takes the well-travelled way to the motor centre, and releases the motor impulse." (Mind, xi, 232-3.)--Finally, Prof. Lipps has, in his elaborate way (Grundtatsachen, 179-188), made mince-meat of the view that stage 3 involves either conscious perception or conscious will.
[116] Physiol. Psych., 3d edition (1887), vol. ii, p. 266.
[117] Philosophische Studien, vol. iv, p. 479 (1888).
[121] Lange has an interesting hypothesis as to the brain-process concerned in the latter, for which I can only refer to his essay.
[122] The reader who wishes to know more about the matter will find a most faithful compilation of all that has been done, together with much original matter, in G. Buccola's 'Legge del Tempo,' etc. See also chapter xvi of Wundt's Physiol. Psychology; Exner in Hermann's Hdbch., Bd. 2, Thl. ii, pp. 252-280; also Ribot's Contemp. Germ. Psych. chap.
viii.
[123] The nature of the movement also seems to make it vary. Mr. B. I.
Gilman and I reacted to the same signal by simply raising our hand, and again by carrying our hand towards our back. The moment registered was always that at which the hand broke an electric contact in _starting_ to move. But it started one or two hundredths of a second later when the more extensive movement was the one to be made. Orchansky, on the other hand, experimenting on contractions of the masseter muscle, found (Archiv f. (Anat. u.) Physiol., 1889, p. 187) that the greater the amplitude of contraction intended, the shorter grew the time of reaction. He explains this by the fact that a more ample contraction makes a greater _appeal to the attention_, and that this shortens the times.
[124] Physiol. Psych., ii, 223.
[125] François-Franck, Fonctions Motrices, Leçon xxii.
[127] Ueber den Kreislauf des Blutes im menschlichen Gehirn (1881), chap. ii. The Introduction gives the history of our previous knowledge of the subject.
[128] In this conclusion M. Gley (Archives de Physiologie, 1881, p.
742) agrees with Professor Mosso. Gley found his pulse rise 1-3 beats, his carotid dilate, and his radial artery contract during hard mental work.
[129] Address before Med. and Chirurg. Society of Maryland, 1879.
[130] See his book; "Experimental Researches on the Regional Temperature of the Head" (London, 1879).
[132] The most convenient account of Schiff's experiments is by Prof.
Hierzen, in the Revue Philosophique, vol. iii, p. 36.
[133] A New Study of Cerebral Cortical Localization (N. Y., Putnam, 1880), pp. 48-53.
[134] Archives of Medicine, vol. x, No. 1 (1883).
[135] Without multiplying references, I will simply cite Mendel (Archiv f. Psychiatrie, vol. iii, 1871), Mairet (Archives de Neurologie, vol.
ix, 1885), and Beaunis (Rech. Expérimentales sur l'Activité Cérébrale, 1887). Richet gives a partial bibliography in the Revue Scientifique, CHAPTER IV.[136] HABIT.
When we look at living creatures from an outward point of view, one of the first things that strike us is that they are bundles of habits.
In wild animals, the usual round of daily behavior seems a necessity implanted at birth; in animals domesticated, and especially in man, it seems, to a great extent, to be the result of education. The habits to which there is an innate tendency are called instincts; some of those due to education would by most persons be called acts of reason. It thus appears that habit covers a very large part of life, and that one engaged in studying the objective manifestations of mind is bound at the very outset to define clearly just what its limits are.
The moment one tries to define what habit is, one is led to the fundamental properties of matter. The laws of Nature are nothing but the immutable habits which the different elementary sorts of matter follow in their actions and reactions upon each other. In the organic world, however, the habits are more variable than this. Even instincts vary from one individual to another of a kind; and are modified in the same individual, as we shall later see, to suit the exigencies of the case. The habits of an elementary particle of matter cannot change (on the principles of the atomistic philosophy), because the particle is itself an unchangeable thing; but those of a compound mass of matter can change, because they are in the last instance due to the structure of the compound, and either outward forces or inward tensions can, from one hour to another, turn that structure into something different from what it was. That is, they can do so if the body be plastic enough to maintain its integrity, and be not disrupted when its structure yields.
The change of structure here spoken of need not involve the outward shape; it may be invisible and molecular, as when a bar of iron becomes magnetic or crystalline through the action of certain outward causes, or India-rubber becomes friable, or plaster 'sets.' All these changes are rather slow; the material in question opposes a certain resistance to the modifying cause, which it takes time to overcome, but the gradual yielding whereof often saves the material from being disintegrated altogether. When the structure has yielded, the same inertia becomes a condition of its comparative permanence in the new form, and of the new habits the body then manifests. _Plasticity_, then, in the wide sense of the word, means the possession of a structure weak enough to yield to an influence, but strong enough not to yield all at once. Each relatively stable phase of equilibrium in such a structure is marked by what we may call a new set of habits.
Organic matter, especially nervous tissue, seems endowed with a very extraordinary degree of plasticity of this sort; so that we may without hesitation lay down as our first proposition the following, that _the phenomena of habit in living beings are due to the plasticity[137] of the organic materials of which their bodies are composed_.
But the philosophy of habit is thus, in the first instance, a chapter in physics rather than in physiology or psychology. That it is at bottom a physical principle is admitted by all good recent writers on the subject. They call attention to analogues of acquired habits exhibited by dead matter. Thus, M. Léon Dumont, whose essay on habit is perhaps the most philosophical account yet published, writes: "Every one knows how a garment, after having been worn a certain time, clings to the shape of the body better than when it was new; there has been a change in the tissue, and this change is a new habit of cohesion. A lock works better after being used some time; at the outset more force was required to overcome certain roughnesses in the mechanism. The overcoming of their resistance is a phenomenon of habituation. It costs less trouble to fold a paper when it has been folded already. This saving of trouble is due to the essential nature of habit, which brings it about that, to reproduce the effect, a less amount of the outward cause is required. The sounds of a violin improve by use in the hands of an able artist, because the fibres of the wood at last contract habits of vibration conformed to harmonic relations. This is what gives such inestimable value to instruments that have belonged to great masters. Water, in flowing, hollows out for itself a channel, which grows broader and deeper; and, after having ceased to flow, it resumes, when it flows again, the path traced by itself before. Just so, the impressions of outer objects fashion for themselves in the nervous system more and more appropriate paths, and these vital phenomena recur under similar excitements from without, when they have been interrupted a certain time."[138] Not in the nervous system alone. A scar anywhere is a _locus minoris resistentiæ_, more liable to be abraded, inflamed, to suffer pain and cold, than are the neighboring parts. A sprained ankle, a dislocated arm, are in danger of being sprained or dislocated again; joints that have once been attacked by rheumatism or gout, mucous membranes that have been the seat of catarrh, are with each fresh recurrence more prone to a relapse, until often the morbid state chronically substitutes itself for the sound one. And if we ascend to the nervous system, we find how many so-called functional diseases seem to keep themselves going simply because they happen to have once begun; and how the forcible cutting short by medicine of a few attacks is often sufficient to enable the physiological forces to get possession of the field again, and to bring the organs back to functions of health.
Epilepsies, neuralgias, convulsive affections of various sorts, insomnias, are so many cases in point. And, to take what are more obviously habits, the success with which a 'weaning' treatment can often be applied to the victims of unhealthy indulgence of passion, or of mere complaining or irascible disposition, shows us how much the morbid manifestations themselves were due to the mere inertia of the nervous organs, when once launched on a false career.
* * * * * Can we now form a notion of what the inward physical changes may be like, in organs whose habits have thus struck into new paths? In other words, can we say just what mechanical facts the expression 'change of habit' covers when it is applied to a nervous system?
Certainly we cannot in anything like a minute or definite way. But our usual scientific custom of interpreting hidden molecular events after the analogy of visible massive ones enables us to frame easily an abstract and general scheme of processes which the physical changes in question _may_ be like. And when once the possibility of _some_ kind of mechanical interpretation is established, Mechanical Science, in her present mood, will not hesitate to set her brand of ownership upon the matter, feeling sure that it is only a question of time when the exact mechanical explanation of the case shall be found out.
If habits are due to the plasticity of materials to outward agents, we can immediately see to what outward influences, if to any, the brain-matter is plastic. Not to mechanical pressures, not to thermal changes, not to any of the forces to which all the other organs of our body are exposed; for nature has carefully shut up our brain and spinal cord in bony boxes, where no influences of this sort can get at them. She has floated them in fluid so that only the severest shocks can give them a concussion, and blanketed and wrapped them about in an altogether exceptional way. The only impressions that can be made upon them are through the blood, on the one hand, and through the sensory nerve-roots, on the other; and it is to the infinitely attenuated currents that pour in through these latter channels that the hemispherical cortex shows itself to be so peculiarly susceptible.
The currents, once in, must find a way out. In getting out they leave their traces in the paths which they take. The only thing they _can_ do, in short, is to deepen old paths or to make new ones; and the whole plasticity of the brain sums itself up in two words when we call it an organ in which currents pouring in from the sense-organs make with extreme facility paths which do not easily disappear. For, of course, a simple habit, like every other nervous event--the habit of snuffling, for example, or of putting one's hands into one's pockets, or of biting one's nails--is, mechanically, nothing but a reflex discharge; and its anatomical substratum must be a path in the system. The most complex habits, as we shall presently see more fully, are, from the same point of view, nothing but _concatenated_ discharges in the nerve-centres, due to the presence there of systems of reflex paths, so organized as to wake each other up successively--the impression produced by one muscular contraction serving as a stimulus to provoke the next, until a final impression inhibits the process and closes the chain. The only difficult mechanical problem is to explain the formation _de novo_ of a simple reflex or path in a pre-existing nervous system. Here, as in so many other cases, it is only the _premier pas qui coûte_. For the entire nervous system _is_ nothing but a system of paths between a sensory _terminus a quo_ and a muscular, glandular, or other _terminus ad quem_. A path once traversed by a nerve-current might be expected to follow the law of most of the paths we know, and to be scooped out and made more permeable than before;[139] and this ought to be repeated with each new passage of the current. Whatever obstructions may have kept it at first from being a path should then, little by little, and more and more, be swept out of the way, until at last it might become a natural drainage-channel. This is what happens where either solids or liquids pass over a path; there seems no reason why it should not happen where the thing that passes is a mere wave of rearrangement in matter that does not displace itself, but merely changes chemically or turns itself round in place, or vibrates across the line. The most plausible views of the nerve-current make it out to be the passage of some such wave of rearrangement as this. If only a part of the matter of the path were to 'rearrange' itself, the neighboring parts remaining inert, it is easy to see how their inertness might oppose a friction which it would take many waves of rearrangement to break down and overcome. If we call the path itself the 'organ,' and the wave of rearrangement the 'function,' then it is obviously a case for repeating the celebrated French formula of '_La fonction fait l'organe._' So nothing is easier than to imagine how, when a current once has traversed a path, it should traverse it more readily still a second time. But what made it ever traverse it the first time?[140] In answering this question we can only fall back on our general conception of a nervous system as a mass of matter whose parts, constantly kept in states of different tension, are as constantly tending to equalize their states. The equalization between any two points occurs through whatever path may at the moment be most pervious. But, as a given point of the system may belong, actually or potentially, to many different paths, and, as the play of nutrition is subject to accidental changes, _blocks_ may from time to time occur, and make currents shoot through unwonted lines. Such an unwonted line would be a new-created path, which if traversed repeatedly, would become the beginning of a new reflex arc. All this is vague to the last degree, and amounts to little more than saying that a new path may be formed by the sort of _chances_ that in nervous material are likely to occur. But, vague as it is, it is really the last word of our wisdom in the matter.[141] It must be noticed that the growth of structural modification in living matter may be more rapid than in any lifeless mass, because the incessant nutritive renovation of which the living matter is the seat tends often to corroborate and fix the impressed modification, rather than to counteract it by renewing the original constitution of the tissue that has been impressed. Thus, we notice after exercising our muscles or our brain in a new way, that we can do so no longer at that time; but after a day or two of rest, when we resume the discipline, our increase in skill not seldom surprises us. I have often noticed this in learning a tune; and it has led a German author to say that we learn to swim during the winter and to skate during the summer.
Dr. Carpenter writes:[142] "It is a matter of universal experience that every kind of training for special aptitudes is both far more effective, and leaves a more permanent impress, when exerted on the _growing_ organism than when brought to bear on the adult. The effect of such training is shown in the tendency of the organ to 'grow to' the mode in which it is habitually exercised; as is evidenced by the increased size and power of particular sets of muscles, and the extraordinary flexibility of joints, which are acquired by such as have been early exercised in gymnastic performances...There is no part of the organism of man in which the _reconstructive activity_ is so great, during the whole period of life, as it is in the ganglionic substance of the brain. This is indicated by the enormous supply of blood which it receives...It is, moreover, a fact of great significance that the nerve-substance is specially distinguished by its _reparative_ power. For while injuries of other tissues (such as the muscular) which are distinguished by the _speciality_ of their structure and endowments, are repaired by substance of a lower or less specialized type, those of nerve-substance are repaired by a complete reproduction of the normal tissue; as is evidenced in the sensibility of the newly forming skin which is closing over an open wound, or in the recovery of the sensibility of a piece of 'transplanted' skin, which has for a time been rendered insensible by the complete interruption of the continuity of its nerves. The most remarkable example of this reproduction, however, is afforded by the results of M.
Brown-Séquard's[143] experiments upon the gradual restoration of the functional activity of the spinal cord after its complete division; which takes place in a way that indicates rather a _reproduction_ of the whole, or the lower part of the cord and of the nerves proceeding from it, than a mere _reunion_ of divided surfaces. This reproduction is but a special manifestation of the reconstructive change which is _always_ taking place in the nervous system; it being not less obvious to the eye of reason that the 'waste' occasioned by its functional activity must be constantly repaired by the production of new tissue, than it is to the eye of sense that such reparation supplies an actual _loss_ of substance by disease or injury.
"Now, in this constant and active reconstruction of the nervous system, we recognize a most marked conformity to the general plan manifested in the nutrition of the organism as a whole. For, in the first place, it is obvious that there is a tendency to the production of a _determinate type_ of structure; which type is often not merely that of the species, but some special modification of it which characterized one or both of the progenitors. But this type is peculiarly liable to modification during the early period of life; in which the functional activity of the nervous system (and particularly of the brain) is extraordinarily great, and the reconstructive process proportionally active. And this modifiability expresses itself in the formation of the mechanism by which those _secondarily automatic_ modes of movement come to be established, which, in man, take the place of those that are _congenital_ in most of the animals beneath him; and those modes of sense-perception come to be _acquired_, which are elsewhere clearly _instinctive_. For there can be no reasonable doubt that, in both cases, a nervous mechanism is _developed_ in the course of this self-education, corresponding with that which the lower animals inherit from their parents. The _plan_ of that _rebuilding_ process, which is necessary to maintain the integrity of the organism generally, and which goes on with peculiar activity in this portion of it, is thus being incessantly modified; and in this manner all that portion of it which ministers to the _external_ life of sense and motion that is shared by man with the animal kingdom at large, becomes at adult age the expression of the habits which the individual has acquired during the period of growth and development. Of these habits, some are common to the race generally, while others are peculiar to the individual; those of the former kind (such as walking erect) being universally acquired, save where physical inability prevents; while for the latter a special training is needed, which is usually the more effective the earlier it is begun--as is remarkably seen in the case of such feats of dexterity as require a conjoint education of the perceptive and of the motor powers. And when thus developed during the period of growth, so as to have become a part of the constitution of the adult, the acquired mechanism is thenceforth maintained in the ordinary course of the nutritive operations, so as to be ready for use when called upon, even after long inaction.
"What is so clearly true of the nervous apparatus of animal life can scarcely be otherwise than true of that which ministers to the automatic activity of the mind. For, as already shown, the study of psychology has evolved no more certain result than that there are uniformities of mental action which are so entirely conformable to those of bodily action as to indicate their intimate relation to a 'mechanism of thought and feeling,' acting under the like conditions with that of sense and motion. The psychical principles of _association_, indeed, and the physiological principles of _nutrition_, simply express--the former in terms of mind, the latter in terms of brain--the universally admitted fact that any sequence of mental action which has been frequently repeated tends to perpetuate itself; so that we find ourselves automatically prompted to _think, feel,_ or _do_ what we have been before accustomed to think, feel, or do, under like circumstances, without any consciously formed _purpose_, or anticipation of results. For there is no reason to regard the cerebrum as an exception to the general principle that, while each part of the organism tends to _form itself_ in accordance with the mode in which it is habitually exercised, this tendency will be especially strong in the nervous apparatus, in virtue of that _incessant regeneration_ which is the very condition of its functional activity. It scarcely, indeed, admits of doubt that every state of ideational consciousness which is either _very strong_ or is _habitually repeated_ leaves an organic impression on the cerebrum; in virtue of which that same state may be reproduced at any future time, in respondence to a suggestion fitted to excite it...The 'strength of early association' is a fact so universally recognized that the expression of it has become proverbial; and this precisely accords with the physiological principle that, during the period of growth and development, the formative activity of the brain will be most amenable to directing influences. It is in this way that what is early 'learned by heart' becomes branded in (as it were) upon the cerebrum; so that its 'traces' are never lost, even though the conscious memory of it may have completely faded out. For, when the organic modification has been once _fixed_ in the growing brain, it becomes a part of the normal fabric, and is regularly _maintained_ by nutritive substitution; so that it may endure to the end of life, like the scar of a wound."
Dr. Carpenter's phrase that _our nervous system grows to the modes in which it has been exercised_ expresses the philosophy of habit in a nutshell. We may now trace some of the practical applications of the principle to human life.
The first result of it is that _habit simplifies the movements required to achieve a given result, makes them more accurate and diminishes fatigue_.
"The beginner at the piano not only moves his finger up and down in order to depress the key, he moves the whole hand, the forearm and even the entire body, especially moving its least rigid part, the head, as if he would press down the key with that organ too. Often a contraction of the abdominal muscles occurs as well. Principally, however, the impulse is determined to the motion of the hand and of the single finger. This is, in the first place, because the movement of the finger is the movement _thought of_ and, in the second place, because its movement and that of the key are the movements we try to _perceive_, along with the results of the latter on the ear. The more often the process is repeated, the more easily the movement follows, on account of the increase in permeability of the nerves engaged.
"But the more easily the movement occurs, the slighter is the stimulus required to set it up; and the slighter the stimulus is, the more its effect is confined to the fingers alone.