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 activity, 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 central nerve-system.
The more easy unfolding of a frequently repeated molecular movement 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 comprehension 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 comparatively slight importance of exercise of the muscles. In the Andoyer method of teaching to write, the pupil writes over with a pen for twenty times in succession the identical letters that have been previously 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 understand 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