THE LATERAL MUSCLES OF THE HIP AND THIGH 323 
the stifle and hip joints and abduct the limb. The middle part, being inserted 
chiefly on the tibial crest and the lateral patellar ligament, would extend the hip, 
and may, with the semitendinosus, flex the stifle. The posterior part, by virtue 
of its attachment to the tuber calcis, assists in extending the hock. It is to be noted, 
however, that extension of the hock joint can occur only when the stifle is also ‘ex- 
tended, and vice versa. 
Structure-—The muscle has two heads of origin. The long or vertebral head 
arises chiefly from the dorsal and lateral sacro-iliac ligaments, the coccygeal fascia, 
and the intermuscular septum. There is often a large bursa between this head and 
the trochanter major. The short or ischiatic head arises by a strong tendon from 
the ventral spine on the tuber ischii, which also furnishes origin to part of the semi- 
tendinosus. They unite, and a short tendon, detached from the deep face of the 
muscle, is inserted into the posterior surface of the femur near the third trochanter; 
here a bursa is interposed between the tendon and the bone. The muscle then 
divides into three parts, which terminate on a strong aponeurosis over the 
junction of the thigh and leg. The anterior part is directed toward the patella, 
the middle one toward the tibial crest, while the posterior one assists in the forma- 
tion of the posterior contour of the limb. The aponeurosis blends with the deep 
layer of the fascia cruris, as already described. A synovial bursa occurs under the 
patellar insertion, and in some cases there is also one between the muscle and the 
third trochanter. 
Relations.—Superficially, the skin and fascia; deeply, the sacro-iliac and sacro- 
sciatic ligaments, the coccygeal fascia, the femur, the gluteus medius, obturator, 
gemellus, quadratus femoris, adductor, semimembranosus, vastus lateralis, 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 superficial and middle glutei; behind and medially, the semi- 
tendinosus. 
Blood-supply.—Gluteal, obturator, and posterior femoral arteries. 
Nerve-supply.—Posterior gluteal and great sciatic nerves. 
6. Semitendinosus.—This is a long muscle which extends from the first two 
coceygeal vertebr to the proximal third of the medial 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. 268, 288, 580). It has two heads of 
origin. 
Origin.—(1) The transverse processes of the first and second coccygeal ver- 
tebrie, 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 fascia and the tuber calcis. 
Action—To extend the hip and hock joints, acting with the biceps and semi- 
| membranosus in propulsion of the trunk, rearing, etc.; also to flex the stifle and ro- 
tate the leg inward.' 
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 unites with 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 medial 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. 
583). A bursa may occur under the long head where it passes over the tuber ischii. 
‘It should be remembered, however, that the stifle can be flexed only when the hock is also 
flexed and vice versa. 
