50 



The Phylogeny of the Crural Flexors 



the soleus. It becomes a question then what shall be termed a soleus 

 and what a gastrocnemius, and since the human arrangement is the 

 type with which all other mammalia are directly or indirectly com- 

 pared, it will be advisable to base the definitions of the two muscles on 

 that arrangement, and this is, essentiall}^ that the gastrocnemius takes 

 its origin from the femur and is a two- joint muscle, while the soleus 

 has its origin from the bones of the crus and is a one-joint muscle. A 

 more satisfactory distinction could be made by referring the two muscles 

 to their respective primary layers, but for the present that given above 

 may suffice. 



Fig. 7. Fig. 8. 



Fig. 7. — Transverse section through the lower part of the crus of Didelphys virgini- 

 ana. ep, external plantar nerve ; i*'^ fihula ; FF, flexor flhularis ; ff, nerve to flexor flbu- 

 laris ; FT, flexor tibialis ; Oe, gastrocnemius lateralis ; Oi, gastrocnemius medlalis ; ip, 

 internal plantar nerve ; PI, plantaris ; PT, pronator tibiae ; rp, ramus profundus ; 

 T, tibia ; TP, tibialis posticus. 



Fig. 8.- — Transverse section through the upper part of the crus of Didelphys virgini- 

 ana. F, fibula ; FF, flexor flbularis ; Oe and Ge/. outer and inner portions of gastroc- 

 nemius lateralis ; Gi, gastrocnemius medialis ; PI, plantaris ; PT, pronator tibise ; s, 

 soleus ; T, tibia ; TP, tibialis posticus. 



The medial head of the gastrocnemius is a practically constant ele- 

 ment of the mammalian crus and presents little variation except in 

 relative size. In the opossum (Figs. 7 and 8, Gi) it arises from the 

 internal condyle and quickly passes over into a flattened tendon which 

 descends the leg, gradually verging toward its outer border, nntil near 

 the ankle joint it comes to lie to the outer side of the tendon of the 

 gastrocnemius lateralis, in close proximity to which it is inserted into 

 the OS calcis. No union occurs between the two tendons except immedi- 

 ately at their insertion. In both the mouse (Fig. 10, Gi) and the cat 

 (Fig. 9, Gi) the muscle -unites with the gastrocnemius lateralis high up 

 in the crus and the conjoined tendon inserts into the os calcis. 



