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 Mithridates, many men have by habit made themselves comparatively proof against alcohol, nicotine, and the alkaloids of opium. The North-Germans are only too proof against Pettenkofer's man-poison (my anthropotoxine) 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 expectancy 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 understand 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 previously 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 respecting 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 medicine, 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 movements involve a certain degree of muscular power; but we can conceive 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 executing 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 direction. 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