194 ANATOMY OF THE WOOD EAT 



extended. The foot probably can be extended when the 

 leg is in any position, but only with an effort when the two 

 more proximal segments are flexed. 



The muscle action controlling this state of affairs is as 

 follows (see fig. 37) : When the long (psoas magnus) and 

 short (iliacus) flexors of the thigh operate, the two-joint 

 extensors of the lower leg (gluteus superficialis anterior and 

 rectus femoris) are relaxed, but the corresponding flexors of 

 this segment (the three adductors, two lesser glutei, and 

 quadratus femoris) are under tension, which tends to flex 

 the lower leg. At the same time, the two-joint extensors of 

 the foot (the gastrocnemii and plantaris) are relaxed. Theo- 

 retically the long extensor digitorum should in this position 

 exert a tendinous action to flex the foot, but such is not now 

 the case. Instead, the tibialis anticus has sufficient tone so 

 that it definitely flexes the foot when the normal tension of 

 the gastrocnemii is removed by means of flexion of the 

 lower leg. This tonal contraction of the tibialis anticus is 

 evidently not sufficiently pronounced so that it is im- 

 possible for the animal to extend the foot when the remainder 

 of the leg is flexed, but it is ample for the involuntary recov- 

 ery or flexion of the foot following extension of this segment 

 during the act of running. This tonal action of the anterior 

 tibial muscle is not apparent in man, he probably having 

 lost it (if he ever had it) as a result of his erect posture. It 

 may be readily seen that involuntary flexion of the foot while 

 a man is running is not of importance, but that in such an 

 animal as the wood rat, which runs in a somewhat crouching 

 posture, it is of consequence that the longer foot be auto- 

 matically and safely flexed during recovery of the leg to 

 prevent it from dragging on the ground. 



As discussed shortly, the foot is capable of no rotation 

 per se, and yet functional rotation of this member is neces- 

 sary in the economy of the animal. This is accomphshed by 

 rotation of the whole leg, and for this reason the muscles — ■ 



