Popular Science Monthly/Volume 21/July 1882/The Physiology of Exercise I
|←Porcelain and the Art of its Production||Popular Science Monthly Volume 21 July 1882 (1882)
The Physiology of Exercise I
By Emil du Bois-Reymond
|A Curious Burmese Tribe→|
By EMILE DU BOIS-REYMOND.
ALTHOUGH the reputation of the Romans as a civilized people has somewhat sunken of late, their army-life still awakens unbounded admiration. The Greeks called their army after the camp, the Macedonians after its formation. To the neo-Latins the army is armed power; the Germans seem to regard it as a union of the warriors into a common host. The Romans, on the other hand, as Gibbon has remarked, named their army from exercise. The Greeks aimed at the harmonious development of individuals, without any well-defined purpose. Incessant methodical drill of the manhood, a field of Mars, is essentially a Roman institution, for war was the natural condition of the Roman commonwealth.
Overthrown by the barbarian hosts, the regular army disappeared from the world's stage for a thousand years, and the greatest question of controversy for mankind, whether Christian or of Islam, was how once upon a time the quarrel of a clan over a pretty woman was decided by single combat of knights before Ilium. With the revival of ancient culture on the threshold of the new time, the drilling of troops came again into its right. No one now doubts that, other things being 3 i8 THE POPULAR SCIENCE MONTHLY.
equal, the better-schooled army prevails. Hardly any army deserves better than the Prussian- German the name JExercitus. Before a meet- ing of the physicians of that army it is not inappropriate to consider exercise somewhat in its direct physiological aspect.
Boyish excesses have brought the Darwinian doctrine into such bad repute in wide circles that I do not without consideration place myself at its point of view. Yet, whatever view of the world one may take, science, which desires to comprehend the world, will not be prevented from at once representing the world comprehensively ; since it, according to Herr Helmholtz's evident remark, must start out from this presupposition unless it is contradicted at the outset. Only me- chanical conception is science ; when supernaturalism comes in, science ceases. As the jurist takes the law, without considering equity and palliating circumstances, so the naturalist goes on to mechanical con- clusions, without regarding venerable beliefs. It is not his office to reconcile these beliefs with those conclusions.
More, the Cuvierian doctrine of repeated creations underlaid by repeated cataclysms has lost all justification since Lyell showed that goeological phenomena have proceeded without general cataclysms, and Darwin has added that species change. Now w r e can more intel- ligibly ascribe to the creating Almighty only the action of having placed a first germ of life in previously inanimate nature. Is it not, then, simpler and more worthy of that Almighty to conceive that he at once endowed matter with the power of allowing the living to arise out of itself under definite conditions, without new assistance ?
This was Leibnitz's view, and it may be said of it that even the most cautious need not be afraid of it. According to this view, it is the object of natural research to show how the living originated by mechanical processes out of the inorganic, and how, out of this doubt- less most simple life, the present organic nature has been mechanically developed. If we could succeed in filling up the scheme of the theory of descent with real contents, we should know how the series of living beings has unfolded itself during unlimited time and through numer- ous generations, according to certain norms which appear to us as laws of organization. But with this the problem would be only half solved.
Living beings are in themselves fitted to their purpose, and adapted to the external conditions of their lives ; they were always so ; and, while they transform themselves according to their surroundings, they not merely adapt themselves to their new conditions, but they also perfect themselves in our human conception. Thus, from this point of view, organic nature appears not only as a machine, but also as a self -improving machine.
This second half of the problem demands for its solution the proof that the adaptive process has gone on mechanically, and the only not wholly vague effort to give this proof that has yet been made is the
�� � THE PHYSIOLOGY OF EXERCISE. 3 i 9
theory of selection. Unfortunately, however, this theory encounters insuperable difficulties as soon as it tries to step from the free-sailing air-balloon of probabilities upon the hard ground of realities. Nothing is easier than to ridicule the doctrine of natural and sexual selection. So much the more earnestly will the seeker for truth seize any means that can contribute anything to the solution of the problem. Is it not now a most promising coincidence that the higher beings exhibit in exercise such a self-improving machinery as we have recognized in the
aggregate of life ?
From these remote distances of research, which are the peculiar metaphysics of our time, come with me into a blacksmith's shop. The lad who lifts the hammer for the first time to-day soon becomes tired in spite of his splendid muscular foundation. He sweats ; and, when he takes a horseshoe from the master's hands, he burns his fingers. Two years later he can, without sweating, perform the trick illustrating the mechanical theory of heat of pounding cold iron red-hot, and is not afraid to touch the hot metal. What has happened ? First, the lad's arms have increased in compass, their muscles in tension to the highest capacity of contraction. If we could have weighed the muscles of his arms at the beginning of his apprenticeship, and could weigh them now, we should find that they had grown heavier ; as also, according to Edward Weber, the muscles of the right side of the body are heavier than those of the left. The muscles are also the most perfect power-machines not only in that when active they make the most complete use of the consumed matter ; not only in that, according to Herr Heidenhain, their strength in particular instances increases with the service demanded of them but they are distinguished above all machines made by man in that by frequent labor-service they become stronger and more capable of enduring further labor. It does not need to be proved that the effect of exercise on the muscles is imme- diate and local, and not transmitted through the favorable influence of bodily exertion on the general organism. Even the Greeks found fault with the disproportionate degree to which boxers trained their arms only, and runners their legs ; and our pugilists and ballet-dancers are illustrations of the same. Under some circumstances the local results of exercise may be destructive to the whole, as when the muscles of the heart suffer hypertrophy in consequence of excessive resistance in some part of the circulation.
On the other hand, the surgeon knows only too well that the mus- cles of a stiffened or sprained joint, or of one that has been confined with bandages, become wasted, as do likewise muscles the nerves of which have been cut or that have been otherwise disabled.
The part is known which the latter fact, falsely interpreted by the older physiologists, played in the question of what was called the Hal- lerian muscular irritability, till John Reid at a time when experi- ments on living animals were not prohibited in England showed
�� � 3 20 THE POPULAR SCIENCE MONTHLY.
that muscles deprived of their natural innervation could be kept tit for work provided they were electrically excited at sufficiently brief intervals ; an experiment which found an important application in surgery and neuropathology.
Even in the midst of health unused muscles pine away, or become pale and powerless, like the ear-muscles of most men. In general, the redness of muscles is related to greater strength in consequence of fre- quent exertions. Herr Ranvier showed that the red and pale muscles occurred together in rabbits and rooks ; that they were distinguished by their structure and by the time required for contraction without its being possible to decide that one set worked more than the other, and without any clew being given to the object of this disposition. Little is known of the microscopic qualities of used and unused muscles. In contrast with the muscles of fattening cattle, working cattle have thicker primitive bundles and coarser sarcolemma, the latter determin- ing the lesser nutritive value of the flesh. According to Herr Vir- chow's terminology, nutritive stimulation has also taken place. In muscles falling away through disuse, as the waste progresses a fatty metamorphosis sets in, against which, as is well known, its ceaseless activity does not protect the heart-muscle. Muscular contraction is accompanied by chemical changes. The blood flows darker from tense than from resting muscles ; they consume more oxygen and form more carbonic acid. An acid permanently reddening litmus is set free in them. Their watery constituents and the amount of substances soluble in alcohol increase in them, while the amount of substances soluble in water diminishes probably because glycogen is consumed in the con- traction. The albuminous constituents remain about the same, yet the derivatives of albumen known as the flesh bases appear to be richer. That to the last hard-working muscle, the heart, is for this reason a mine of such bodies to the chemist ; and the flesh of a fox that had been shot was found by Liebig to be ten times richer in creatine than that of a captive fox. We are, unfortunately, still very far from understand- ing the connection of these various processes and their relation to mus- cular contraction, that is, to the interchange of isotropic and uniso- tropic substances in the muscular fibers, and to the transformation of mechanical, thermic, and electric forces. We only know that there is involved an increase and modification of a process of change that was already going on during rest, particularly of the oxidation of nitrogen- ous substances, by which, in addition to mechanical labor-service, an apparent surplus of heat is developed. Even the muscles at rest are a seat of respiration and the development of heat in animal bodies. The muscle acts very much like the reserve-locomotive that stands ready for use on the switch, which is all the time burning a little fuel and can be attached to a train or sent to help a disabled engine at any instant, but which requires, in connection with the greater display of force it is to make, a greater consumption of material and expenditure
�� � THE PHYSIOLOGY OF EXERCISE. 321
of heat. Ludwig and Sadler showed on this point that, aside from mechanical hindrances, the blood flows freer and richer through the vessels of the working muscle. This is not only in the sense that new combustible matter is introduced, but also in that the ashes are at the same time swept away from the muscle-hearth ; since, according to the discovery of Herr Johann Ranke, followed out by Hermann Roeber, the acids formed by muscular activity depreciate the mechan- ical and electrical capacity of the muscle, exhaust it chemically as we are accustomed to say, without being able to conceive any other than a chemical exhaustion of the muscle. Still less than of the chemical mechanism of muscular contraction, have we a conception of what takes place chemically in the strengthening of the muscle by exercise, of how it becomes better fitted for work through a higher degree of oxidation, and of why it falls away on the cessation of the changes that take place in it in activity. It seems most natural to think that these effects depend alone upon the increase and diminution of the flow of blood during activity and during rest ; yet this of itself only makes the case darker than if we had not ventured to decide the question in such a way. Chemistry throws the manifold varieties of muscle-flesh which our taste distinguishes with so much refinement generally into one pot ; and the old statement, established in knowledge, that English park-deer tastes flat, is still far from being explained.
A subjective explanation is finally to be mentioned. The tired muscle, as long as it is becoming stronger through exercise, gives pain for several days when it is used and when it is pressed upon. Even a muscle which has been once or oftener hardened, or thoroughly trained, gives pain when it is again put to work after a long idleness, as we soon learn when we begin a journey on foot or on horseback. Whoever, after a long interruption in gymnastic exercises, feels no more pain, will make no further progress. The muscles hurt after epileptic spasms. Even if we attempt to ascribe the feeling of the muscles to the nerves of the tendons, joints, and skin, and the Vater- Pacini bodies, we still should not imagine that they bring on the pains in tetanus and trichinosis. Notwithstanding Sach's labors, we are not yet in the light concerning the pathic nerves that bring about these pains. Wherever and however they do it, they also produce muscular aches after exertions.
The improvement of the muscles by exercise, little as we know of it, has* been established from antiquity, and, being relatively more familiar, the best case of improvement, is fitted to serve as an ex- ample for similar processes in other tissues. Indeed, the question now is, whether other tissues than the cross-striped muscles are by frequent exercise of their office in the animal household made bet- ter fitted for that office. After what has been said above, we can
x . . .
with some justification add to this question the many times more
vnr. ttt't 9,1
�� � 322 THE POPULAR SCIENCE MONTHLY.
easily answered one, whether other tissues diminish in consequence of a failure to exercise them in their office.
A physiological proof that the smooth muscles are strengthened by exercise is wanting. The adaptation of the eye to near vision dimin- ishes from childhood to age according to a regular law, notwithstand- ing the constant exercise of the faculty ; but it does not follow from this that Briicke's muscle does not gain strength, for its gain may be more than compensated by the growing stiffness of the tissue and the diminished elasticity of the crystalline lens. The fact that men see imperfectly at close range what their occupation gives them little occasion to regard, indicates that Briicke's muscle loses strength when it is not used. The uterus has no occasion to be exercised, for it is active only after long pauses, and gains a portion of new fibers every time for that purpose. We know nothing of the movements of the muscle-maw of the bird, which forms a transition to the cross-striped muscles. On the other hand, such pathological facts as the hyper- trophy of the muscles of the bladder and the pylorus under circum- stances of extraordinary resistance leave no doubt that the smooth muscles, like the cross-striped ones, are strengthened by labor. Thus an empirical basis is given to Herr Rosenthal's supposition that the immunity against cold conferred by cold-bathing depends upon the exercise of the smooth muscles of the skin and their vessels, which are intrusted with the lowering of the co-efficient of cooling of the body in the cold. Cold washing and bathing are the gymnastics of the smooth muscles.
The young blacksmith, of whom we spoke a short time ago, had gained another advantage from exercise besides greater strength in the muscles of his arm : he ceased to burn his fingers. Every one knows that the epidermis thickens on those parts of the skin that are frequently subjected to pressure, rubbing, and the touching of hot things and caustic fluids. Handling of tools, rowing, vaulting on the rack and bars, produce a callus chiefly at the ends of the middle-hand bones or in the palms ; glass-blowing produces callus on the inside of the fingers. Recurrent blisters often result in callus. Under the pressure of hard shoes the form of skin-thickening known as corns takes the place of callus. Callus and corns have been histologically investigated, yet we can not tell why the useful callus is formed here, the painful corn there, to say nothing of our having a theory of the processes. They fall in the category of what Herr Virchow calls for- mative stimulation of the cell-complex, and regards, like the nutritive stimulation, as the result of a general and fundamental property of the elementary organisms. An increased supply of matter, immediately conditioned on an increased flow of blood, also takes place here. Since we can not well predicate a vis a fronts, enlargement of the vessels re- mains the only yet possible step toward an understanding, and with this we reach a closed gate before which many other problems are
�� � THE PHYSIOLOGY OF EXERCISE. 323
already encamped, in the question how inflammation and vascular paralysis are distinguished. Our case is also distinguished by the fact that the skin, protected by callus like the practiced muscles, now af- fords better service under similar circumstances. The callus, m par- ticular cases, represents an improvement in the grasping organ. For- mative stimulation also occurs in the muscles ; the contents of the primitive bundle are moved to nucleation through local stimulation, yet . the advantageous stimulation by exercise seems to be almost en- tirely, or chiefly, of a nutritive sort.
In like manner as the skin fortifies itself against the repeated touch of hot bodies by means of local calluses, it adapts itself to the heat of the sun through erythema and a change consequent upon it which is accompanied with the development of pigment, although pigment favors the absorption of the sunbeams. The fact is, perhaps, con- nected with this, that it is advantageous to animals to have the side that is turned toward the light of a dark color. Hence, as Moseley observed on the Challenger, JEcheneis rernora has the belly dark, the back light. Heat from artificial sources of a relatively lower temper- ature, which is deficient in refrangible rays, has a remarkably differ- ent effect from sunlight. Workers by the fire are pale. It is still to be seen whether the electric light will take the place of the sunlight in its effect on the skin as it does in the case of plants.
Horny structure becomes unfit for its purpose with insufficient use. A remarkable example of this is the cessation of the growth of the hoofs of horses and cattle on the soft turfs of the Falkland Islands, mentioned by Darwin. On the other hand, the hoofs of horses harden on dry, stony soils, as Xenophon teaches in his school for horsemen ; and colts brought up on such soils need no protection.
The so-called rider's bones, the exercise-bones, which have not be- come rarer since the introduction of the new armor and the modified drill, but have moved from the left to the right, may be considered as a kind of inner callus, the development of which affords a new ex- emplification of the Osteo-blasten theory. These bones hardly bring any advantage to their possessor, and can not be included among the instances of self-improvement through exercise. It would be too far fetched and groping in a too dark quarter for me to do more than mention here that Ludwig Fick believes that the well-adapted form of the joints may have been derived from exercises during the fetal pe- riod and the earliest days of life. Is it not possible that the splendid formation of the spongy bone-substance in the epiphyses, which was discovered by Hermann Meyer, and further investigated by Julius Wolff, depends on nutritive and formative stimulus in the direction of the greatest pressure and strain ? The injurious effect of insufficient use is shown in this region by the non-growth of the teeth of rodents when they are fed on too soft food, or after the trigeminus has been cut.
�� � 324 THE POPULAR SCIENCE MONTHLY.
The self -improvement of the series of connecting tissues by exer- cise in other members, takes place in a more peculiar manner, rather mechanical than chemical and physiological. The motions of the joints are made easier by exercise ; and making stiff joints movable is one of the most grateful objects of orthopedy. Herr Henke ex- plains the unusual suppleness of the so-called India-rubber men as the result of relaxation of the ligaments, a disappearance of edge-surfaces of bone, and a diminished radius of curvature of the sliding surfaces, but particularly of a prolongation of the flesh-fibers at the expense of the tendons. Possibly an elastic tissue is formed in the ligaments of their limbs. "Whether such a tissue grows in the vocal cords after ex- ercise in vibrating them is still uncertain.
To make our statement complete, the increased ease in labor bought, it is true, at the cost of greater danger of secondary bleedings of those who have borne many times, belongs here. We may also regard as a self -improvement, although belonging to another region, the relaxing after-birth, and the accompanying reflex action of the breast upon the uterus.
The glands are another class of tissues the efficiency of winch is raised by exercise. The sexual glands the milk- glands and testicles are known to be capable of remaining at rest for years and even for life, while their tissues are subject to a considerable diminution, as is also normally the case with animals during the intervals between the periods of heat. Inversely the sexual glands attain a wonderful degree of production by means of alternations of rest and activity, as is exemplified by stallions, milch-cows, sheep, and goats. If the breast- glands are not kept exercised by sucking, the udder by milking, they dry up and sink to rest till they are newly excited in sympathetic action with the uterus. The same can not be immediately proved of the fluids of secretory glands, but it is hardly doubtful that a digestive vessel that is kept active by two meals a day, with its glandular attach- ments, will dispose of a larger quantity of the various digestive fluids than that of a penitent. The kidneys of the practiced beer-drinker give passage to an incredible quantity of fluid. Finally, one who reads in the sketches of the manners of the last century of the continual weep- ing of the sentimental men and women of the time, will hardly be able to restrain the presumption that their tear-glands were brought up to the work by practice. We are as ignorant concerning the mech- anism of the self-improvement of the glands through exercise as con- cerning the process of secretion in them. Since this process is different in nearly every gland, according to what nerves are introduced in one the secreted matter increases ; in another becomes fatty ; in an- other persists unchanged, but receives and gives out matter, or under- goes changes in itself the problem appears twice as intricate and the information doubly scanty, so that at last we have to do again only with an increased accession of matter and more frequent innervation.
�� � THE PHYSIOLOGY OF EXERCISE. 325
The idea of exercise as we have regarded it passes so gradually over to hardening against frequently repeated injuries, that I am tempted to place here also the adaptation of the organism to accustom itself to endure poisons. Without going back as far as King Mithri- dates, many men have by habit made themselves comparatively proof against alcohol, nicotine, and the alkaloids of opium. The North-Ger- mans are only too proof against Pettenkofer's man-poison (my an- thropotoxine) in badly ventilated assembly-halls, railway-carriages, etc., to which fire-place people, like the English, are so sensitive. This inurement can hardly be called self -improvement.
You have, perhaps, gentlemen, been waiting in impatient expect- ancy for me to speak on the subject you first thought of when you heard that my address was to be on exercise. By exercise we under- stand commonly the frequent repetition of a more or less complicated action of the body with the co-operation of the mind, or of an action of the mind alone, for the purpose of being able to perform it better. Not without a purpose have I deferred the consideration of this kind of exercise to this point, for it is quite different from the kinds pre- viously spoken of, although those kinds may be connected with it. This fundamental difference has not as yet been duly considered. We seek in vain in most physiological text-books for instruction respect- ing exercise ; if it is given, only the so-called bodily exercises are generally considered, and they are represented as merely exercises of the muscular system ; therefore it is not strange that laymen in medi- cine, professors of gymnastics, and school-teachers generally believe that. Yet it is easy to show the error of this view, and demonstrate that such bodily exercises as gymnastics, fencing, swimming, riding, dancing, and skating are much more exercises of the central nervous system, of the brain and spinal marrow. It is true that those move- ments involve a certain degree of muscular power ; but we can con- ceive of a man with muscles like those of the Farnesian Hercules, who would yet be incompetent to stand or walk, to say nothing of his ex- ecuting more complicated movements. For that we have only to add to our conception the power of arranging the motions suitably, and of causing them to work harmoniously.
Thus it becomes clear, if proof were needed, that every action of our body as a motive apparatus depends not less, but more, upon the proper co-operation of the muscles than upon the force of their contraction. In order to execute a composite motion, like a leap, the muscles must begin to work in the proper order, and the energy of each one of them (in Helmholtz's sense) must increase, halt, and diminish according to a certain law, so that the result shall be the proper position of the limbs, and the proper velocity of the center of gravity in the proper direc- tion. We know little as yet of the way in which we impart a definite duration to the energy of the muscles, for our researches have so far informed us upon little else than the convulsions following extremely
�� � 326 THE POPULAR SCIENCE MONTHLY.
brief excitations, and upon tetanus. Since the nerves only trans- mit the impulses coming from the motor-ganglion cells, it is evident that the peculiar mechanism of the composite movements resides in the central nerve-system, and that, consequently, exercise in such move- ments is really nothing else than exercise of the central nerve-system. This possesses the invaluable property that the series of movements (if we may speak thus), which take place in it frequently after a definite law, are readily repeated in the same order, with the same swell and ebb and intricacy, whenever a singly felt impulse of the will demands it. Thus, all the bodily exercises we have mentioned above are not mere muscle-gymnastics, but also, and that pre-eminently, nerve-gymnastics, if for brevity we may apply the term nerves to the whole nervous system. Johann Miiller, whose explanations, in the second volume of his " Hand-book of Physiology," still appear to me the best that have been written on the theory of movement, has recog- nized this double nature of bodily exercises, but has not sufficiently in- sisted upon it. On this, he makes a remark which strikingly enforces our view; that is, that improvement in exercises of the body often con- sists nearly as much in the suppression of unessential by-motions as in acquiring dexterity in necessary motions. Observe the active boy who for the first time raises himself upon a ladder with his hands. Al- though it is of no use to him, his arms and his legs shake at every grasp. After a few weeks he holds the hips, knee and foot joints of his closely locked legs tautly extended. The suppression of by-motions furnishes unconsciously to us a mark of the pleasing appearance of the well-drilled soldier, of the skilled gymnast, and of the cultivated man ; chorea begins when they are let loose. We know nothing of the mechanism of the suppression of by-motions, yet it is evident that, when muscles remain at rest in the course of exercise, the result of the exercise is not to strengthen them.
Under continuous severe exertions, as in mountain-climbing and long walks, the heart begins to beat faster and more strongly, and op- pression of the breath is felt, because, according to Johann Miiller, the heart participates in a by-motion ; in Traube's opinion, because it is stimulated by the excess of carbonic acid formed in the laboring muscles. How is it, then, that exercise diminishes these palpitations ? Is it by means of the vagus nerve ?
Perspiration under exertion may also be regarded as a by-secre- tion as well as the greater secretion of saliva in speaking and chewing ; and the diminished perspiration of our blacksmith when taught would then be the suppression of this by-secretion, which might be compared to a by-movement, through exercise. The beating of the -heart and perspiration are, however, involuntary, and it is very questionable whether we can refer the stopping of them by means of exercise to such processes.
Still, something else than the control of the muscles by the motor-
�� � THE PHYSIOLOGY OF EXERCISE. 327
nervous system comes into consideration in most composite movements. The sight, the sense of pressure, and the muscular sense, and finally the mind, must be prepared to take in the position of the body at each instant, so that the muscles may be in a proper state of adjustment ; this is plainly shown in the exercises of fencing, playing billiards, rope-dancing, vaulting on horses in motion, or leaping down a mountain- slope. Thus not only the motor, but the sensor nervous system also, and the mental functions, are capable of being exercised and need it ; and the muscles again appear to acquire a deeper importance in gym- nastics. What is said here of the coarser bodily movements applies equally to all skilled work, of the highest as well as of the lowest kind. Although a Liszt, or a Rubinstein, without an iron muscularity of arm, can not be thought of, and although, likewise, the movements of Joachim's bow during a symphony may correspond to many kilo- grammemetres, still their power as virtuosos resides in their central nerve-system. The readiness of the turner, the machinist, the watch- maker ; of the glass- blower and glass-polisher ; the skill of the anat- omist and surgeon ; writing and drawing ; womanly labors like sewing and knitting, crocheting and lace-making ; finally, the hardly consid- ered yet more or less artful performances of daily life, dressing and undressing, the use of the sponge, comb and brush, knife and fork what are they all at last but acquired concatenations of the actions of ganglion cells which, after they have often run on in an appointed course, now succeed each other in the same manner with qualified facil- ity, catching into each other, pausing and resuming again, like the voices in an artfully composed concert ? When Lessing asked whether Ra- phael would have been any the less a great painter if he had been born without hands, he perceived this truth. Is it necessary to add that the same principle applies to all the movements as well as to those of the hands ; that, for example, vocal culture rests upon no other one ? Singers need not only flexible vocal cords, strong respiratory and laryngeal muscles, ringing resonance of the air-passages all these would in themselves alone be of no more use to them than a Stradi- varius violin to a wood-cutter their talent has its root in the gray sub- stance at the base of the fourth ventricle. Here also is concealed, but awaiting a higher command, exercising its functions through the hinder third of the left third convolution, the machinery of the speech mechanism, as bulbar paralysis sadly teaches.
It is very remarkable in all these processes that the more any com- posite movement is practiced, the more unconscious is the act of the nervous system directing it, until at last the latter can not be distin- guished from spontaneous nervous mechanisms like the involuntary re- flex and by-movements. Erasmus Darwin remarked that, when any one learns to turn, each movement of the hands seems at first to be directed by the will, but that at last the action of the hands becomes so at one with the effect that the turner's will seems to reside in the
�� � cutting of his knife—that is, that he unconsciously puts it in the right position.
Practice further exhibits its influence upon the nervous system on its purely sensory side, abstracted from all movement. It sharpens and corrects the musical ear, and teaches it to perceive over-tones, in-exact intervals, and slight dissonances. The local sense and the color sense of the eye are improved by practice. It teaches the wonderful arts of quick reading, of taking in fleeting phenomena like the vibrations of the magnetic needle, of bringing the sight of the gun to bear on the black of the target. It teaches to distinguish copies and all kinds of subjective appearances, to comprehend at a glance microscopic pictures that pass before the beginner in superficial confusion, in such a way that it is very hard to draw the line between exercise of the sense and that exercise of the judgment over the impressions of sense that is called visus eruditus. As exercise induces the discontinuance of unused muscles, it also teaches us to neglect unused images, such as the double images of the points of the picture outside of the horopter; or, in looking through an optical instrument, the impressions made upon the unengaged eye. Yet no practice appears to break through the law according to which the points of the retina in indirect vision receive attention only transiently and with a certain effect. Although it is hardened against bad smells, the nose of the chemist is the rival of spectrum analysis in delicacy. It would be unjust to say that the wine-connoisseurs of Bordeaux can discriminate concerning the place of the growth of a vintage, while only its age is in question with them. Not less susceptible of cultivation are the perceptions of temperature, pressure, and locality. The last, especially, measured according to the least distance at which two bodies, nearly in contact, still separate, may be distinguished, become sharpened by practice in the course of a few days—giving one of the arguments which oppose a purely anatomical definition of the range of feeling.
As exercise refines the senses, neglect stupefies them, and that not merely in consequence of the apathy of the organ. After destroying the eyes and ears of new-born puppies, Herr Hermann Munk observed that what he had recognized as the visory and auditory spheres of the brain borders were backward in development. According to Huguenin, blindness of many years' duration results in waste of the visory spheres.
- An address at the anniversary of the Institute for Military Surgeons, Berlin, August 2, 1881.