J. Playfair McMurrich 



51 



In contrast to the extreme simplicity of structure presented by the 

 gastrocnemius medialis is the complexity of the gastrocnemius lateralis 

 in all three forms here under consideration. In the opossum the 

 muscle near its origin was found to consist of four bundles. Two of 

 these (Fig. 8, Ge) arose close together from the outer surface of the 

 lateral sesamoid cartilage of the knee joint and from the ligament 

 extending from this to the external condyle, and were distinguishable 

 not only by being separated by a band of connective tissue, but also by 

 a difference in the direction of their fibers. A third portion (Ge^) 

 took its origin from the inner (tibial) surface of the lateral sesamoid 

 cartilage, while the fourth portion (s) arose from the posterior sur- 

 face of the head of the fibula, or, to be more precise, from the posterior 

 surface of a tendon which arises from the posterior surface of the head 

 of the fibula and is continued downwards to beyond the middle of 

 the crus upon the deep surface of the compound muscle. 



Fig. 9. — Transverse section through the upper part of the crus of the Cat. F, fibula ; 

 Ge? and Ge"»> lateral and medial portions of the gastrocnemius lateralis; Oi, gastroc- 

 nemius medialis ; P^ and P-, oblique and vertical portions of popliteus ; PI, plantaris ; 

 pt, posterior tibial nerve; rm, ramus superficialis medialis; rp, ramus profundus; 

 s, soleus ; T, tibia. 



Below these four bundles became more or less confused, the connec- 

 tive tissue partition between the portions from the outer and inner 

 surfaces of the sesamoid cartilage persisting for a greater distance than 

 the others, and before its disappearance a tendon appears in the center 

 of the outer sesamoid portions (Fig. 7) and gradually increases in size 

 to become the tendon of the muscle. This is continued down the leg 

 quite independent of the tendon of the gastrocnemius medialis, with 

 which it is inserted into the tuberosity of the os calcis. 



In the cat (Fig. 9) the lateral gastrocnemius arises together with the 

 plantaris (PI) from the patella by a strong aponeurotic sheet which is 

 continued backward from the lateral border of that bone, and also 

 from the downward continuation of this sheet which forms an invest- 



