60 



The Phylogeny of the Crural Flexors 



muscle passes obliquely downward and inward over the knee joint 

 (Fig, 9) and shows quite distinctly a composition from two masses of 

 fibers, one of which (P^), as in the mouse, has an oblique direction, 

 while the other (P-) is more vertical. No indications of a lower portion 

 of the muscle could be found in the individual I studied, although it 

 may be noted that both in the cat and in the mouse the interosseous 

 membrane is more strongly developed than in the oppossum. 



IV. The Nerves of the Mammalian Crus. 



In the opossum, at the level where my sections began, there was a 

 main nerve stem, the internal popliteal (Fig. 13), and on one side of 

 it a stem for the internal gastrocnemius (GI) and on the other side 



IP 

 Fio. 13. Fig. 14. 



Fig. 13.— Diagram of the crural nerves of Didelphys virginiana. EP, external 

 plantar ; FF, branch to flexor flbularis ; FT, branch to flexor tibialis ; OE, branch to 

 gastrocnemius lateralis ; OI, branch to gastrocnemius medialis ; IP, internal plantar ; 

 pi, branch to plantaris ; PT, branch to pronator tibiae ; TP, branch to tibialis posticus. 



Fig. 14. — Digram of the crural nerves of the Mouse. EP, external plantar ; FF, 

 branch to flexor flbularis ; FT, branch to flexor tibialis ; Ge, branch to gastrocnemius 

 lateralis ; Oi, branch to gastrocnemius medialis ; IP, internal plantar ; P, branches to 

 popliteus ; pi, branch to plantaris ; s^ branch to soleus ; TP, branch to tibialis posticus ; 

 tp, posterior tibial. 



a branch for the external gastrocnemius (and soleus) {GE), and more 

 externally the external popliteal which wound around the head of the 

 fibula to the front of the leg. The internal popliteal descended into 

 the crus between the two gastrocnemii, and soon after divided into five 

 branches, of which tw^o {EP and IP) were quite large, two others were 

 much smaller, one of tliem {pi) passing exclusively to the plantaris, 

 while the other one {FF) was distributed to the flexor digitorum flbu- 

 laris. 



The fifth branch (Fig. 7, rp) was of moderate size and passed 



