328 FASCLE AND MUSCLES OF THE HORSE 
branches. The lateral branch is inserted into the back of the femur with the short 
portion, while the medial branch is attached to the medial epicondyle and col- 
lateral ligament. There is often a superficial slip which ends partly on the femoro- 
patellar joint capsule and may reach the accessory cartilage or medial ligament of 
the patella. Some fibers pass under the collateral ligament and end on the tendon 
of the semimembranosus. 
Relations—Medially, the gracilis, and branches of the femoral artery and 
of the obturator nerve; laterally, the femur, the obturator externus, quadratus 
femoris, biceps femoris, and the femoral, deep femoral, and obturator arteries; an- 
teriorly, the pectineus, vastus medialis, 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. 324. 
Tuirp LAYER 
1. Quadratus femoris.'—This is a narrow, three-sided muscle, which lies under 
cover of the upper part of the adductor (Fig. 581). 
Origin.—The ventral surface of the ischium, just in front of the semimembrano- 
sus. 
Insertion.—An oblique line on the posterior surface of the femur, near the 
lower part of the trochanter minor. 
Action.—To extend the hip joint and to adduct the thigh. 
Structure.—It is composed of parallel bundles of fibers directed downward, 
forward, and outward. 
Relations.—Postero-medially, the adductor, semimembranosus, and the ob- 
turator vessels; antero-laterally, the obturator externus and biceps femoris, the 
deep femoral vessels, and the great sciatic nerve. 
Blood-supply.—Deep femoral and obturator arteries. 
Nerve-supply.—Great sciatic nerve. 
2. Obturator externus (Fig. 581).—This is a pyramidal muscle which extends 
across the back of the hip joint from the ventral surface of the pelvis around 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.—Medially, the adductor and quadratus femoris and the deep 
femoral vessels; laterally, 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. 
Blood-supply.—Deep femoral and obturator arteries. 
Nerve-supply.—Obturator nerve. 
3. Obturator internus (Fig. 288).—This arises within the pelvic cavity by two 
heads, 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. 
1 Also known as the ischio-femoralis. 
