The gastrocnemius - named from Greek terms meaning the belly, and the calf, because it forms the enlargement of that part of the leg - is double-headed, each head consisting of a fusiform muscle, the lower part of which blends with its fellow so as to form a common tendon of insertion. It forms the femoral origin of the great triceps feurae muscle.
Outer head: a well-marked impression upon the upper and posterior part of the outer surface of the external condyle and the adjacent part of the posterior surface of the femur just above the external condyle.
Inner head: an oval impression placed transversely across the posterior surface of the femur above the internal condyle, and reaching inwards to the back of the adductor tubercle.
By the tendo Achillis (so named from the legend that the heel, into which this tendon is inserted, was the only vulnerable part of the hero Achilles) into the lower part of the posterior surface of the calcaneum.
The two heads arise by short strong tendons; that of the inner is the stronger and thicker. These tendons converge downwards towards one another, and ire succeeded by large fleshy expansions which unite at the upper part of the middle third of the leg. Near this point the tendon of insertion begins as an intermuscular septum between the two bellies of the muscle. This becomes thicker and stronger, and expands into a broad aponeurosis which covers the anterior surface of the united muscle. Just below the middle of the leg the fleshy fibres terminate upon the back of this aponeurosis in two curves the convexity of which is downwards, that of the inner portion of the muscle descending about half an inch lowers than the outer. The strong aponeurosis becomes narrower and at the same time thicker, and after receiving the fibres of the soleus muscle is known by the name of the endo Achilhs.
From the internal popliteal branch of the great sciatic nerve, which sends sural branches to the adjacent portions of the anterior surfaces of the two heads, in the upper third of the leg.
Action- Its action will be described with that of the soleus, which forms a part of the same muscle.
Behind, the deep fascia, the external saphenous vein and nerve, and the communicans fibularis nerve. Between the two heads above is the ilantaris muscle. In front lie the knee joint, the tendon of the semi-membranosus, he popliteus, the plantaris tendon, the soleus, the popliteal vessels, and internal opliteal nerve. On the outer side, above, are the biceps tendon, and external lophteal nerve ; on the inner side, above, are the tendons of the semi-tendinosus, racilis, sartorius, and adductor magnus. A bursa lies beneath its inner head, eparates it from the tendon of the semi-membranosus, and communicates with the knee joint. ,
The most common variation is the addition of a third head from the posterior surface of the lower end of the femur. This may cross, or even run between, the opliteal vessels.