404 HUMAN PHYSIOLOGY 



peculiar distinction by several facts. They form about 

 half the entire body and they are the seat of a very large 

 share of the metabolism. In the second place, they are 

 under voluntary control; it remains for us to say how 

 they shall be employed. We can drive or spare other 

 organs also for example, the digestive and the sweat 

 glands but there are none which we can so slight and 

 neglect to exercise as these. 



In a book like this we cannot enter into questions of the 

 particular kinds of muscular activity appropriate to 

 people of various ages and habits. We must limit 

 ourselves to broad statements. It will be convenient to 

 speak first of the effects of exercise upon the neuro- 

 muscular mechanism itself and then to show in how many 

 ways its influence is extended to other systems. The 

 object of exercise is sometimes training for special accom- 

 plishments and sometimes simply the preservation of the 

 general health. 



The most familiar fact in the mind of the schoolboy 

 is that muscles grow with use. The increase is said not 

 to be in the number but in the size of the fibers. The 

 changes which accompany contraction are of a destruc- 

 tive kind, but it seems to be commonly true in biology 

 that the compensation for a wasting process is, in a vigor- 

 ous tissue, more than equal to the original loss of sub- 

 stance. It does not merely recover but it becomes 

 larger than it was before. Of course an increase in the 

 size of muscles means a gain in strength, but we should be 

 very much in error if we were to overlook certain other 

 factors. 



It is altogether probable that betterment of quality 

 is a more important result of training than sheer gain in 

 mass. We can easily think of persons whose muscles 

 are insignificant in appearance but whose endurance is 

 remarkable. Several conditions can be suggested which 

 they probably exemplify. First, their muscles are supe- 

 rior in a chemical sense. Second, the circulation is 

 advantageously directed to give them support. Third, 

 the blood itself comes to them with a favorable com- 



