280 FASCIA AND MUSCLES OF THE HORSE 



posterior one assists in the formation of the posterior contour of the limb. The 

 aponeurosis blends with the deep layer of the fascia cruris, as already descriljed. 

 A synovial bursa occurs under the patellar insertion. 



Relations. — Superficially, the skin and fascia; deeply, the sacro-iliac and sacro- 

 sciatic ligaments, the coccygeal fascia, the femur, the obturator, gemellus, quadratus 

 femoris, adductor, semimembranosus, vastus externus, and gastrocnemius muscles, 

 branches of the lateral sacral, gluteal, obturator, femoral and deep femoral vessels, 

 the great sciatic, tibial, peroneal, and posterior gluteal nerves; in front, the super- 

 ficial and middle glutei; behind and internally, the semitendinosus. 



Blood-supply. — Gluteal, obturator, and femoro-popliteal arteries. 



Nerve-supply. — Posterior gluteal and great sciatic nerves. 



6. Semitendinosus (Biceps rotator tibialis) . — This is a long muscle which 

 extends from the first two coccygeal vertebrae to the proximal third of the inner 

 surface of the tibia. It lies at first behind the biceps, then passes downward on the 

 back of the thigh, between that muscle and the semimembranosus (Figs. 178, 179, 

 199). It has two heads of origin. 



Origin. — (1) The transverse processes of the first and second coccygeal ver- 

 tebrae, the coccygeal fascia, and the intermuscular septum between this muscle and 

 the biceps femoris; (2) the ventral surface of the tuber ischii. 



Insertion. — (1) The tibial crest; (2) the crural fa.scia and the tuber calcis. 



Action. — To flex the stifle and rotate the leg inward; also to extend the hip 

 and hock joints, acting with the biceps and semimembranosus in propulsion of the 

 trunk, rearing, etc. 



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, jjranches 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^ (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 boun- 



1 This muscle was incorrectly designated the adductor magnus by Percivall and Strange- 

 ways. Its name, however, is not at all descriptive of its structure in the domesticated animals. 



