174 ANATOMY OF THE WOOD RAT 



basic principles must be mentioned, however, in order that 

 a logical discussion may be presented. 



Each muscle is composed of a varying number of fibers. 

 The longer the fibers, the greater is the possible distance of 

 contraction. The more numerous the fibers the stronger the 

 muscle is supposed to be. Thus, a pennated muscle is more 

 powerful than a longitudinal one. Each muscle fiber has 

 its own, separate, nerve fiber and motor ending, by which its 

 orders are received. It is a matter of controversy whether, 

 during gentle movements, each fiber of the muscle involved 

 contracts but a fraction of the distance of which it is capable, 

 or whether a small proportion of the fibers contract strongly 

 while the remainder rest inactive — in other words, that 

 selective action of the fibers is involved. Personally the 

 writer favors the former hypothesis. 



Possible movement of a muscle depends upon the condi- 

 tion both of the muscle fibers and of their nerve fibers and 

 endings. Probably upon both of the foregoing depends the 

 tone, or normal tension of a muscle. When the tone is 

 poor, a muscle is flabby and soft; when it is good, the 

 muscle is hard and springy. Exercise improves the texture 

 of the muscle fibers and also enlarges and makes more sensi- 

 tive the motor endings of the nerve fibers concerned. The 

 precise conditions involved are unknown, but a muscle may 

 gain in strength without a corresponding gain in quickness, 

 and similarly, quickness and expertness in the action of a 

 muscle may be increased without appreciably adding to its 

 strength. The truly marvelous adroitness in manual move- 

 ments exhibited by an expert in legerdemain is a result of 

 hypersensitiveness and responsiveness of the nerve endings 

 of the hand muscles, rather than of the muscles themselves — 

 to such an extent that the majority of the movements 

 involved have become in reality reflex, extraordinarily swift, 

 and complex to a surprising degree. 



In actual practice a small man of slight physique is often 



