8 Thaddeus L. Bolton and Eleonora T. Miller 



muscle fibers increase both in size and in contractile force. It is 

 not known whether new muscle cells or fibers actually develop in 

 the arm of the baseball pitcher or the blacksmith, but there is an 

 enormous increase in size which may be due only to the enlarge- 

 ment of the muscle fibers themselves, but a simple enlargement 

 must be accompanied by a multiplication of nuclei at least. It is 

 probable that after exercise under a given degree of nervous 

 stimulation the muscle will contract with greater force. There 

 is an elimination of fatty and other superfluous tissues which 

 hinder the contraction of the muscles and so decrease the resist- 

 ance within the member itself. Also, there is less friction in 

 moving joints, and tendons slide more easily upon their bearings 

 as a result of the callouses that develop on them. 



In the nerve centers controlling the muscles changes no doubt 

 take place. From Professor Hodge's experiments upon nerve 

 cells we learn that the energy-producing compounds are broken 

 down during functional activity, which is indicated by a shrink- 

 ing of the nucleus. This follows the normal daily activity of 

 the nerve cells as well as electrical stimulation of them. 

 During repose and sleep these compounds are again rebuilt, and 

 it is probable that the anabolic level to which they rise is higher 

 after exhaustion than before. Each exhaustion seems to add an 

 increment of energy-producing substance above the usual level 

 to which rebuilding rises. Again, as the result of further activ- 

 ity, the catabolic changes may be carried to a point lower than 

 before, from which full recovery is possible and speedy, so that 

 in the center which is properly exercised the limits of metabolic 

 change are increased at both extremes. Thus it is possible, on 

 the theory that each discharge of the center destroys an equal 

 amount of the nervous compound, for the center to discharge a 

 greater number of times as a result of practice. Permit us to 

 offer the further hypothesis that a given discharge in the center 

 will, on account of decreased resistance, send out a more intense 

 impulse over the nerve fibers. This would make it possible for 

 a weaker discharge to accomplish the same result, and thus 

 exhaustion of the nerve center would go on more slowly. Recu- 

 perative processes between successive discharges would become 



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