_Structure._—It is almost entirely fleshy, and is composed of parallel bundles united rather loosely. It is usually possible to separate from the principal mass a small anterior short portion,[67] which is inserted into the femur behind the pectineus. The principal mass[68] is perforated below its middle by the femoral vessels, and is thus divided into two branches. The outer branch is inserted into the back of the femur with the short portion, while the inner branch is attached to the internal epicondyle and lateral ligament. There is often a superficial slip which reaches the internal straight patellar ligament. Some fibers pass under the lateral ligament and end on the tendon of the semimembranosus. _Relations._—Internally, the gracilis, and branches of the femoral artery and of the obturator nerve; externally, the femur, the obturator externus, quadratus femoris, biceps femoris, and gastrocnemius, and the femoral, deep femoral, and obturator arteries; anteriorly, the pectineus, vastus internus, and a large branch of the obturator nerve; posteriorly, the semimembranosus and the great sciatic nerve. _Blood-supply._—Femoral, deep femoral, and obturator arteries. _Nerve-supply._—Obturator nerve. 3. =Semimembranosus.=—Described on p. 280. THIRD LAYER 1. =Quadratus femoris= (Ischio-femoralis).—This is a narrow, flat muscle, which lies under cover of the upper part of the adductor (Figs. 455, 456). _Origin._—The ventral surface of the ischium, just in front of the semimembranosus. _Insertion._—An oblique line on the posterior surface of the femur, near the lower part of the internal trochanter. _Action._—To extend the hip joint, and to adduct the thigh and rotate it outward. _Structure._—It is composed of parallel bundles of fibers directed downward, forward, and outward. _Relations._—Internally, the adductor, semimembranosus, and the obturator vessels; externally, the obturator externus and biceps femoris, the deep femoral artery, and the great sciatic nerve. _Blood-supply._—Deep femoral and obturator arteries. _Nerve-supply._—Great sciatic nerve. 2. =Obturator Externus= (Fig. 456).—This is a pyramidal muscle which extends across the back of the hip joint from the obturator foramen to the trochanteric fossa. _Origin._—The ventral surface of the pubis and ischium, and the margin of the obturator foramen. _Insertion._—The trochanteric fossa. _Action._—To adduct the thigh and to rotate it outward. _Structure._—It is almost entirely fleshy, the muscle-bundles being rather loosely connected. The insertion is pointed, flattened, and partly tendinous. The origin is perforated by the obturator vessels and nerve. _Relations._—Internally, the adductor and quadratus femoris and the deep femoral vessels; externally, the gemellus, the tendon of the obturator internus, the biceps femoris, and the great sciatic nerve; anteriorly, the hip joint, the pectineus, and the external pudic vein.