THE NF.UROMUSCULAR SYSTEM I FATIGUU 111 



v 



One fact is impressed upon the mind of the man in the 

 street: that muscles grow with use. We shall not be in- 

 clined to make this the foremost consideration, but it is 

 well worth emphasizing. This is not the place to enter 

 upon any discussion of the causes underlying this well- 

 known growth. It may be due in part to improved cir- 

 culation and in part to chemical conditions arising when 

 contractions take place. It may be said to be a principle 

 of rather wide application in biology that a destructive 

 process, during vigorous life, will be followed not only by a 

 process of repair, but by overcompensation. This is held 

 to be true of antitoxin production and acquired im- 

 munity to disease. It is simply illustrated in callous for- 

 mation: when friction destroys the surface layers of the 

 skin they are not merely replaced, but their number is in- 

 creased. The increase in the mass of a muscle consequent 

 on its regular use is also a reaction from a decomposition 

 process, for every contraction is executed at the cost of 

 some substance disintegrated. But the growth of the 

 muscle is not wholly analogous to the formation of a 

 callus, because in the first case it is believed that we have 

 no multiplication of fibers, but only an increase in volume 

 on the part of those present. 



One may make proper allowance for the greater size of 

 trained, as compared with untrained, muscles, and he will 

 be convinced that the gain in power is out of all proportion 

 to the gain in bulk. Betterment of quality must certainly 

 be admitted. The service of exercise to a man may be 

 rather to give him full command of the tissue which he has 

 than to put more at his disposal. Superior quality in 

 muscle may mean an actual chemical change by which the 

 fuel is made more available or stored in larger quantity, 

 or the chief improvement may be in the efficiency of the 

 circulation. Much will depend upon this, for the supply 

 of oxygen to the muscle protoplasm is an imperative 

 requirement. 



In the third place, it is conceivable that facility of 

 end-plate transmission is one of the gains. Difficulty in 



