MUSCULAK SYSTEM OF AVES. 105 



origin, becomes firmly attaclied to the strong ligamentous loop 

 attached by one end to the femur above the preceding tendon, 

 and by the other to the outer ridge of the fibula. This trochlear 

 loop is lined by synovial membrane, and sujoports the tendon of 

 the biceps cruris, which glides through it. The carneous fibres 

 of the external gastrocnemius come ofi" from the outer side of the 

 tendon, and from the fascia covering the outer surface of the 

 muscles of the leg : they are continued in a somewhat penniform 

 arrangement two-thirds down the leg, upon the inner surface of 

 the muscle, where they end in a strong subcompressed tendon. 

 This joins its fellow-tendon, from the internal gastrocnemius, 

 behind the ankle-joint, and both expand into a thick, strong liga- 

 mentous aponeurosis, which extends over three-fourths of the 

 posterior part of the tarso-metatarsal bone. The lateral margins 

 of this fascia are bent down under the flexor tendons behind the 

 joint, and become continuous with a strong ligamentous layer 

 gliding upon the posterior surface of the distal condyles of the 

 tibia, and attached to the tendons of the yeroneus and tibialis 

 anticus : the conjunction of the thickened tendons of the gastro- 

 cnemii with this deeper-seated layer of ligamento-tendinous sub- 

 stance constitutes a trochlear sheath lined by synovial membrane, 

 through which the flexor tendons of the toes glide. The synovial 

 membrane of the ankle-joint is continued upward, half an inch 

 above the articvdar surface of the bone, between it and the fibro- 

 cartilaginous pulley. Below the joint the margins are inserted 

 into the lateral ridges of the tarso-metatarsal bone, becoming 

 gradually thinner as they descend, and ending below in a thin 

 semilunar edge directed dowuAvard. 



The gastrocnemius internus ^ has two powerful heads, one from 

 the femur, the other from the tibia ; the first arises fleshy from 

 the internal condyle of the femur, expands as it descends, and 

 receives additional fibres from the lower edge of the accessorius 

 semitendinosi. About one-fifth down the tibia this muscular origin 

 in the right leg terminated in a flattened tendon which became 

 attached to the inner side of the tibial portion of the gastrocnemius 

 internus. The second head, which is separated from the pre- 

 ceding by the insertion of the semitendinosus, arises partly from 

 the internal and anterior part of the strong fascia of the knee- 

 joint by short tendinous fibres, which almost immediately become 

 fleshy, and partly from a well-defined triangular surface on the 

 inner and anterior aspect of the head of the tibia : the fleshy fibres 



' xr. vol. iii. j 1. 35, e. 



