328 FASCIA 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 extern us, 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. 



Third 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 ischixun, 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 entirelj- 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 isohio-femoralis. 



