176 ANATOMY OF THE WOOD RAT 



supremacy in strength are due to the fact that the nervous 

 tone is far better developed than that of man could ever 

 be — in short, that the nervous stimuli received by the mus- 

 cles of certain very "strong" mammals are more comparable 

 to those of the man who exhibits ''demoniacal strength" 

 during seizures of one sort or another. 



Still another aspect of muscle action must be considered. 

 A wisp of a man, with spindly legs, may leap well over six 

 feet high, although in other ways he may be woefully lacking 

 in strength. Another man, strong, well-muscled, and weigh- 

 ing 175 pounds may jump just as high. The former indivi- 

 dual exhibits no general indications of superior nervous 

 tone. It is possible that the nervous tone of his leg muscles 

 may be superior to the average while that of his other mus- 

 cles is not, but it is more probable that the chief reason for 

 his proficiency in jumping is due to the better coordination 

 of all muscles that can be of help in swift extension of the 

 leg and foot. 



Analogous cases concerning a variety of mammals may 

 be called to mind. The hind legs of a jumping mouse of 

 the genus Zapus can administer little sledge-hammer blows 

 as if the hmbs were released by a trigger, while those of the 

 mole, although actually stronger, can kick but feebly, 

 because the animal has used its hind limbs merely for slow 

 progression and for bracing its body while digging. There 

 is no physical quality that we can see to account for such 

 great variation in the muscular action of these two mammals, 

 although there is naturally great difference in conformation, 

 that in the length of the bones being more properly a result — 

 not a cause. The muscles are essentially the same in 

 texture, but in one there is coordination of the nerve im- 

 pulses and consequent action for speed, and in the other, for 

 strength most efficient for certain work to be performed. 



In man the limbs are normally parallel to the body axis, 

 and the muscles, at rest and in action, have become adapted 



