192 ANATOMY OF THE WOOD RAT 



the flexors of the femur and extensors of the lower leg, need 

 not be so efficient, for there is ample time during long leaps 

 for slow recovery between impulses. In a relatively short- 

 legged mammal, however, which normally takes short steps, 

 quick recovery is important, and the muscles concerned 

 must be powerful. Such is the case — especially in view of the 

 normal position of the femur — with the psoas magnus, ihacus, 

 gluteus superficialis anterior and rectus femoris which are 

 the muscles of chief use in flexing the thigh, and with the 

 extensor muscles of the lower leg, especially the vasti, or 

 short extensors. 



In the normal standing position of the rat the femur and 

 lower leg are sharply flexed, to a degree rather comparable 

 to the corresponding segments of a man when he is squatting 

 or crouched. This static posture gives great leverage to 

 the muscles extending from the long ilium, and from the 

 caudally extended inferior tuberosity of the ischium through 

 the more superficial flexors extending far down the lower 

 leg, such as the biceps femoris, gracilis, and semitendinosus. 

 A similar position for the last three muscles is entirely im- 

 possible for a mammal with static posture comparable to that 

 of man. 



In rapid locomotion of the wood rat the arc described by 

 the femur probably does not much exceed 90, and is certainly 

 less than 120 degrees. As the static position of this bone is 

 almost parallel to the vertebral axis, its rearward swing 

 would be but little more than vertical to this axis, and, as 

 previously mentioned, its flexor and extensor muscles are 

 excellently placed for efficacy in this position. As is neces- 

 sary, the extensors of the femur are much more powerful 

 and numerous than are those of the humerus, or than the 

 actionally comparable flexors of the latter, for the hind leg 

 is much the more important in locomotion. 



In the case of the fore leg, inhibitional, tendinous action 

 of the two-joint muscles is not marked. The forearm may 



