_Blood-supply._—Deep femoral and obturator arteries. _Nerve-supply._—Obturator nerve. 3. =Obturator internus= (Fig. 451).—This arises by two heads within the pelvic cavity, the tendon emerging through the lesser sciatic foramen. _Origin._—(1) The pelvic surface of the pubis and ischium around the obturator foramen; (2) the pelvic surface of the shaft of the ilium and the wing of the sacrum. _Insertion._—The trochanteric fossa. _Action._—To rotate the femur outward. _Structure._—The ischio-pubic head lies on the pelvic floor and covers the obturator foramen. It is thin and fan-shaped. The iliac head extends along the lateral wall of the pelvis, and is pennate, with a central tendon throughout. Both terminate on a flat tendon which passes outward through the lesser sciatic foramen to be inserted into the trochanteric fossa. A synovial bursa facilitates the play of the tendon over the external border of the ischium.[69] _Relations._—The pelvic surface is covered by the pelvic fascia and the peritoneum. The obturator vessels and nerve lie between the two heads, and the internal pudic vessels and nerve lie along the dorsal edge of the iliac head. _Blood-supply._—Obturator and internal pudic arteries. _Nerve-supply._—Great sciatic nerve. 4. =Gemellus=[70] (Fig. 455).—This is a thin, triangular muscle, which extends from the external border of the ischium to the trochanteric fossa. _Origin._—The external border of the ischium near the ischiatic spine. _Insertion._—The trochanteric fossa. _Action._—To rotate the femur outward. _Structure._—Fleshy, some fibers being inserted into the tendon of the obturator internus. _Relations._—Dorsally, the tendon of the obturator internus and the gluteus profundus; ventrally, the obturator externus. _Blood-supply._—Obturator artery. _Nerve-supply._—Sciatic nerve. III. ANTERIOR MUSCLES OF THE THIGH This group consists of the sartorius, quadriceps femoris, and capsularis. 1. =Sartorius.=—This is described on p. 281. 2. =Quadriceps femoris= (Figs. 179, 186, 200) (Crural triceps).—This constitutes the large muscular mass which covers the front and sides of the femur. It has four heads, one of which, the rectus, arises from the ilium; the other three arise from the femur. All are inserted into the patella. (1) =Rectus femoris.=—This is fusiform and rounded. It arises by two tendons. _Origin._—Two depressions on the shaft of the ilium above and in front of the acetabulum. _Insertion._—The base and anterior surface of the patella. _Action._—To extend the stifle joint and to flex the hip joint.