178 ANATOMY OF THE WOOD RAT 



less or actually misleading. This is unfortunate, for rela- 

 tively slight differences in the leverages exerted by many 

 muscles are productive of very great variation in actional 

 results. 



It will be seen from the foregoing statements what 

 numerous and complex factors are concerned in the actions 

 of muscles. It is necessary, therefore, to employ the utmost 

 caution and conservatism in making assertions regarding the 

 exact part that they play in the economy of any mammal. 

 For this reason any attempt to compare the strength of a 

 particular muscle in terms solely of its cubic capacity, such 

 as was done by Holliger (1916) in the case of the pocket 

 gopher, a ground squirrel, and a rabbit, is misleading. It 

 can be only after much patient research and a consideration 

 of extensive data that any such comparisons can constitute 

 anything more than guess work. 



In considering coordination between groups of muscles 

 it should be understood that muscular action is of three 

 kinds — shortening, static, and lengthening action. During 

 the act of leaping certain muscles sharply contract, it is 

 necessary that others be lengthened, while still others per- 

 form static work, as holding the body rigid or exerting force 

 to prevent undesired movements. The lengthening of the 

 muscles concerned is not passive nor caused merely by the 

 contraction of the opposing muscles, but is a true inhibitional 

 stimulus of the muscles, which removes the normal tone. 

 Every strong or especially quick movement necessitates the 

 employment of these three sorts of action. There is also 

 selective coordination between two or more actional results. 

 This is illustrated by the fact that if one reclines, he may at 

 will raise the legs vertically, or sit up with equal facility. 

 Static contractions are employed when one stands, or holds 

 a weight stationary, or holds the body rigid for the better 

 performance of some other movement. The same amount 

 of muscular energy may be accomplished as in violent 



