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-