_Structure._—The long or vertebral head is small at its origin, but becomes larger by the accession of fibers arising on the intermuscular septum. Below the tuber ischii it is joined by the short head, which arises partly by fleshy fibers, partly by a common tendon with the biceps. The muscle then passes downward on the back of the thigh, and terminates on a wide tendon on the inner surface of the proximal third of the leg. A distinct band passes forward to be inserted on the tibial crest (a =bursa= lying between the tendon and the tibia), part fuses with the fascia of the leg, while the remainder joins the biceps tendon and concurs in the formation of the tendinous band, which, as before described, terminates on the tuber calcis (Fig. 458). A _bursa_ may occur under the long head where it passes over the tuber ischii. _Relations._—Externally, the skin and fascia, the biceps, and the internal head of the gastrocnemius; internally, the coccygeal fascia, the sacro-sciatic ligament, the semimembranosus; anteriorly, the biceps femoris, branches of the femoral artery, and the great sciatic nerve. _Blood-supply._—Posterior gluteal, obturator, and femoro-popliteal arteries. _Nerve-supply._—Posterior gluteal and great sciatic nerves. 7. =Semimembranosus=[65] (Figs. 179, 186, 199, 451).—This muscle lies on the inner surface of the preceding muscle and the gastrocnemius, and has two heads of origin. _Origin._—(1) The posterior border of the sacro-sciatic ligament; (2) the ventral surface of the tuber ischii. _Insertion._—The internal epicondyle of the femur, behind the lateral ligament. _Action._—To extend the hip joint and to adduct the limb. _Structure._—The long head, small and pointed above, extends toward the root of the tail, fusing with the sacro-sciatic ligament. Passing downward, it becomes larger and covers in part the posterior aspect of the tuber ischii. A =bursa= may be found here. Below this it joins the short head, which is much larger. The large belly so formed passes downward and forward, covered in great part by the gracilis, and terminates on a short, flat tendon of insertion at the distal end of the femur. _Relations._—The upper part of the muscle assists in forming the lateral boundary of the pelvic outlet. It is related posteriorly and externally to the skin and fascia and the semitendinosus; internally, to the anus and its muscles, the vulva in the female, and the internal pudic artery and nerve (Figs. 452, 453). Below the pelvis the chief relations are: externally, the semitendinosus, biceps, and gastrocnemius, branches of the obturator, femoral, and femoro-popliteal arteries, and the great sciatic nerve and its chief branches; internally, the crus penis and ischio-cavernosus muscle (in the male), and the gracilis; in front, the adductor and the femoral vessels; behind, the skin and fascia. _Blood-supply._—Obturator and femoral arteries. _Nerve-supply._—Great sciatic nerve.