82 THE POPULAR SCIENCE MONTHLY. 



plexity. There can be no doubt that this complexity, if not immedi- 

 ately, 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 motion- 

 less 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 depre- 

 ciation 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 replace- 

 ment. 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 sys- 

 tems. 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 inter- 

 vals of dilatation, will carry more nourishment to the heart and arte- 

 rial walls. It is hardly conceivable that a person, accustomed to regu- 

 lar 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 circu- 

 lation of the blood is, of course, directly aided during physical exer- 

 cise by the rhythmical pressure of the muscles upon the veins, whose 

 valves allow the blood to be driven only in the right direction. 



