1. =Gastrocnemius= (Figs. 179, 201, 203).—This muscle extends from the
lower third of the femur to the point of the hock. It arises by two
heads.
_Origin._—(1) Outer head, from the external rough margin of the
supracondyloid fossa (Fossa plantaris); (2) inner head, from the
supracondyloid crest.
_Insertion._—The posterior part of the tuber calcis.
_Action._—To extend the hock.
_Structure._—The two bellies are thick, fusiform, and somewhat
flattened. They are covered by a strong aponeurosis and contain
tendinous intersections. They terminate toward the middle of the leg on
a common tendon (Tendo calcaneus s. Achillis), which at first lies
posterior to that of the superficial flexor, but, by a twist in both,
comes to lie in front of the latter. The deep fascia blends with the
tendon throughout its length, and the soleus muscle is inserted into its
anterior edge. A small =bursa= (Bursa tendinis calcanei) lies in front
of the insertion on the tuber calcis, and a large =bursa= is interposed
between the two tendons from the twist downward. The superficial flexor
lies between the two heads and is adherent to the outer one (Fig. 459).
_Relations._—Anteriorly, the stifle joint, the superficial flexor,
popliteus, deep flexor, popliteal vessels, and tibial nerve; internally
(above), the semitendinosus, semimembranosus, and adductor, (below) the
fascia and skin; externally (above), the biceps femoris and peroneal
(anterior tibial) nerve, (below) the fascia and skin.
[Illustration: