plexity. There can be no doubt that this complexity, if not immediately, is then gradually, lessened in the animal economy. There are probably countless stages in the oxidation into urea of each particle of nitrogenous tissue, be it cell-wall or cell-protoplasm, and at each stage of the process the particle will have consequently less potential energy and greater chemical stability; that is, less usefulness for the exhibition of vital phenomena. It is therefore impossible to lay up a permanent stock of physical vigor. Even if we should keep motionless as statues, our stock would steadily disappear. Not in size, but in quality, would come the great depreciation. Nature's own tendency to replace lost organic material with new teaches us how this depreciation may be avoided. In the body there is at best only a sluggish tendency to replace poor with better material, but by destroying that poor stuff we can arouse the organism into active efforts for its replacement. This is the philosophy of the advantage to muscles of regular exercise.
The full development and the continued vigorous condition of the circulatory system are of far more importance to the general health than are similar states of the voluntary muscles and the skeleton, whose importance is mainly in relation to the respiratory and circulatory systems. If we desire to possess maximum vigor, we must have large lung capacity, and, most of all, a stout heart and elastic arteries. In two great ways are the latter needs procured by physical exercise: First, in response to unusual demands there is an accelerated destruction of degenerating substance in the involuntary muscles of the heart and arterial walls, which, as we have seen, is requisite to the substitution of newer and more useful substance in them. Second, by the increased blood-tension, the coronary arteries and the vasa vasorum, in the intervals of dilatation, will carry more nourishment to the heart and arterial walls. It is hardly conceivable that a person, accustomed to regular physical exercise, should ever suffer from a fatty degenerated heart. And, with regard to this increased blood-tension gained in exercise, it is probable that it is productive of many other valuable results. For instance, the blood is drained from the overcharged brain, not merely as might be effected by venesection, thereby requiring an increased production, but by diverting its course into previously only half dilated channels, whose sluggish currents now become swift streams of lively blood. And, again, in consequence of this heightened blood tension, both the secretions and the excretions are increased, thereby developing the capacity of the glandular organs, and also directly aiding the body, both in the riddance of waste material and in the production of the necessary fluids. Of especial advantage, then, would be this increased blood-tension in aiding digestion. The circulation of the blood is, of course, directly aided during physical exercise by the rhythmical pressure of the muscles upon the veins, whose valves allow the blood to be driven only in the right direction.
