_17_, Position of tuber ischii; _o′_, superficial gluteus; _q_, _q′_,
_q″_, biceps femoris; _r_, semitendinosus; _t_, sacro-coccygeus
lateralis; _v_, semimembranosus; _w_, gracilis; _f_, gastrocnemius.
(After Ellenberger-Baum, Anat. f. Künstler.)]
]
_Structure._—The muscle has two heads of origin:[64] 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. They unite, and a short tendon is detached from the
deep face of the muscle, to be inserted into the posterior surface of
the femur near the third trochanter (Fig. 455), a =bursa= being
interposed between the tendon and the bone. The muscle then divides into
three portions, which terminate on a strong aponeurosis over the
junction of the thigh and leg. The anterior branch is directed toward
the patella, the middle toward the tibial crest, while the 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
described. 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
superficial 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 vertebræ 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
vertebræ, 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 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, branches 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=[65] (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
boundary of the pelvic outlet. It is related posteriorly and externally
to the skin and fascia and the semitendinosus; internally, to the anus
and its muscles, the vulva in the female, and the internal pudic artery
and nerve (Figs. 452, 453). Below the pelvis the chief relations are:
externally, the semitendinosus, biceps, and gastrocnemius, branches of
the obturator, femoral, and femoro-popliteal arteries, and the great
sciatic nerve and its chief branches; internally, the crus penis and
ischio-cavernosus muscle (in the male), and the gracilis; in front, the
adductor and the femoral vessels; behind, the skin and fascia.
_Blood-supply._—Obturator and femoral arteries.
_Nerve-supply._—Great sciatic nerve.