Action.--Extends tarsometatarsus and may abduct it slightly.
Comparison.--No significant differences noted among the species studied.
_+Musculus gastrocnemius+_ (Figs. 1, 4).--The largest muscle of the pelvic appendage, it covers superficially all of the posterior surface, most of the medial surface, and half of the lateral surface of the crus. The muscle originates by three distinct heads.
The _pars externa_ covers the posterolateral surface of the crus, is intermediate in size between the other two heads, and arises by a short, strong tendon from a small bony protuberance on the posterolateral side of the distal end of the femur immediately proximal to the fibular condyle. The tendon is intimately connected with the distal arm of the loop for the _m. biceps femoris_.
The _pars media_ is the smallest of the three heads and lies on the medial surface of the crus. The head of the _pars media_ is separated from the _pars interna_ by the tendon of insertion of the _m. semimembranosus_ and originates by a short, strong tendon from the posteromedial surface of the proximal end of the internal condyle of the femur. The proximal portion of the _pars media_ has tendinous connections with the tendon of the _m. semitendinosus_ and with the _pars posticus_ of the _m. adductor longus et brevis_.
The _pars interna_ is the largest of the three heads and covers most of the medial surface of the crus. This head in its proximal portion is distinctly divided into anterior and posterior parts, the former overlapping the latter medially. The origin of the posterior part is fleshy from the anterior half of the tibial head. Some of the fibers of the anterior part arise directly from the inner cnemial crest while its remaining fibers arise from the patellar tendon (Fig. 1) and form a band that extends around the anterior surface of the knee, covering the insertion of the _m. sartorius_.
Approximately half way down the crus, the three heads give rise to the tendon of insertion, the _tendo achillis_, which passes over and is tightly bound to the posterior surface of the tibial cartilage. The insertion is tendinous on the posterior surface of the hypotarsus and along the posterolateral ridge of the tarsometatarsus. This tendon seems to be continuous with a fascia which forms a sheath around the posterior surface of the tarsometatarsus holding the other tendons of this region firmly in the posterior sulcus.