82 ANATOMY OF THE WOOD RAT 



This group is unusually strong in the present genus, as it 

 is in many rodents. 



M. gastrocnemius medialis (figs. 7, 17, 33) has origin 

 by muscle fibers and narrower superficial aponeurosis from 

 the medial sesamoid bone and the adjoining portion of the 

 femur. A sheet of aponeurosis extending distad from its 

 ental belly is twisted, and passes to the tendo calcaneus 

 upon its lateral border. The tibial nerve enters the calf 

 between this muscle and the plantaris. 



M. gastrocnemius lateralis (figs. 6, 17, 33) is more 

 complicated. Superficially it is with difficulty separable 

 into three portions — a disto-medial one, situated disto- 

 laterad of the visible part of the plantaris; a proximo- 

 medial one; and a lateral, lying superficial to the last and 

 having origin by aponeurosis. Entad these three parts 

 are curiously involved, the fibers joining a much crumpled 

 sheet of aponeurosis. The common origin is from the 

 lateral sesamoid and adjoining portion of the femur, and 

 insertion is onto the tendo calcaneus. 



In Neotoma and Teonoma the ental aponeurosis of the 

 entire muscle is somewhat simpler and less folded, which 

 probably means a weaker muscle. 



M. plantaris (figs. 6, 7, 17, 32). The origin of this 

 muscle, situated upon the caudal border of the calf and 

 visible between the two gastrocnemii, is from the caudal 

 portion of the lateral epicondyle of the femur and the 

 tendinous tissue about the lateral sesamoid bone medio- 

 proximad to that of the gastrocnemius lateralis. Its 

 deep fibers are much involved with the ental aponeurosis of 

 the last-mentioned muscle, and indeed, in HomodontomySj 

 it is no more distinct, entail}^, than are the two medial 

 portions of its lateral neighbor. It maj^ readily be iden- 

 tified, however, by the fact that its tendon extends beyond 

 the tendo calcaneus, passing over the heel to the sole. 

 It there divides into two layers, although, in fact, the more 



