THE PHYSIOLOGY OF EXERCISE. 435 



lation, by exercise of the central nerve-system is signified, in the first 

 place, the making fluent certain molecular movements, partly through 

 regulation and suitable re-enforcement of the impulses producing them, 

 partly by the elimination of the obstacles originally opposing them. 

 It must not, however, be said with this, that the highly vascular gray 

 substance is not also nutritively stimulated by the activity incumbent 

 upon it. Every thing indicates that, without a proper degree of ac- 

 tivity, gray substance wears away as muscle does. But every increase 

 in the fluency of definite forms of motion with a definitely enduring 

 course is a newly acceding moment, indicative of exercise of the cen- 

 tral nerve-system. 



The more easy unfolding of a frequently repeated molecular move- 

 ment in the ganglion-cells may be represented under the figure of a 

 water-channel or a stone-slide, in which the path originally built 

 roughly is so worked out and polished by the continuous passage of 

 water, snow, and stones through it, that thenceforth water, snow, and 

 stones reach the bottom surely and quickly, almost as soon as they 

 begin to fall upon the ways converging toward it. All machines are 

 improved with time through the wearing away of little roughnesses, 

 so that their course becomes a more or less evenly or periodically 

 varying one. Since they afterward become shackling through usage, 

 they have, it seems, an age of development, one of bloom, and one 

 of decay ; and Tiede speaks of his chronometer as of a living being 

 with a definite period of life. In order to bring nearer to comprehen- 

 sion the facilitating of the molecular movements in the ganglion- 

 cells, it will be well to remember that the tone of a violin becomes 

 softened by long use, as inversely India-rubber, that is not stretched 

 at intervals, becomes brittle. The instructiveness of this comparison 

 lies in its poverty. It shows us the utterly hopeless insufficiency of 

 our knowledge in the face of such mysteries. 



Herr Fechner has mentioned a particularly curious case of exercise 

 of the central nerve-system, which sets anew in a clear light the com- 

 paratively slight importance of exercise of the muscles. In the An- 

 doyer method of teaching to write, the pupil writes over with a pen 

 for twenty times in succession the identical letters that have been pre- 

 viously written with a pencil, and the hand returns with a swing from 

 the end of the line to its beginning, in order to write it over again 

 without a pause. Ernst Heinrich Weber has observed in the case of 

 his children that the left hand learned to write some at the same time 

 with the right, but it wrote as in a looking-glass. We do not under- 

 stand how the right side of the brain gained by exercise without the 

 left hand moving during the exercise. 



But equally whether we understand it or not, man is adapted to 

 self -improvement by means of exercise. It makes his muscles stronger 

 and more enduring ; his skin becomes fortified against all injury ; 

 through exercise his limbs become more flexible, his glands more 



