DISCUSSION 191 



In such a mammal as the wood rat the primary function 

 of the hind limb is locomotion, necessitating chiefly flexion 

 and extension of that member. In reviewing the action 

 of running it is seen that the first step is the flexion of 

 the femur, accompanied by slight flexion of the lower 

 leg, foot and toes. In man, just before the culmination 

 of the forward thrust of the knee, there is extension of 

 the lower leg, foot and toes, rapidly followed by extension 

 of the femur, flexion of the lower leg that is slight in degree 

 but strong in action through the hamstring muscles, exten- 

 sion of the foot and flexion of the toes. At the culmination 

 of the rearward motion, there is again a tendency toward 

 extension of the segments below the knee. In man, however, 

 the muscles are adaptationally modified to fit his normal 

 posture. The latter, it must be recollected, comprises ex- 

 tension of the femur to a greater degree than it is perhaps 

 possible for the rat to assume, and full extension of the lower 

 leg. In such a position, could the rat adopt it, the muscles 

 extending from the ilium to the leg would have but little 

 leverage, while those from the inferior tuberosity of the 

 ischium and the superficial muscles in this region that pass to 

 the lower leg would lose most of their efficacy. 



In running, a lack of power, or more properly speed, in the 

 flexors of the femur and extensors of the lower leg may be 

 almost as fatal to great swiftness as a similar lack in the 

 driving muscles of the leg. Some fast sprinters are such by 

 grace of the power of their driving muscles, enabling them to 

 cover an unusual distance at each stride. Others are so 

 by reason of the strength of the antagonist muscles, allowing 

 phenomenally quick recovery after each step, and so enabling 

 them to take unusually rapid, though short, strides. And 

 we can all remember, when we were boj'^s, running down hill 

 and attaining such speed that a fall ensued because of our 

 inability to work our legs faster. In a mammal exhibiting 

 saltatorial adaptations, the recovery muscles, comprising 



