The Biceps femoris (figs. 645, 647, 649) is situated on the posterolateral surface of the thigh. It has two heads of origin : one, the long head, arises from the lower and medial impression on the upper part of the ischial tuberosity (fig. 446) by a tendon common to it and the Semitendinosus, and from the lower part of the sacrotuberous ligament; the other, the short head, from the lateral lip of the linea aspera of the femur, between the Adductor magnus and Vastus lateralis, extending up almost as high as the insertion of the Gluteus maximus; from the lateral supracondylar line to within 5 cm. of the lateral condyle; and from the lateral intermuscular septum. The fibers of the long head form a fusiform belly, which passes downwards and laterally across the sciatic nerve to end in an aponeurosis; this aponeurosis covers the posterior surface of the muscle, receives on its deep surface the fibers of the short head, and gradually contracts into a tendon, which is inserted into the lateral side, of the head of the fibula, and by a small slip into the lateral condyle of the tibia. This tendon forms the lateral hamstring and divides into two portions, which embrace the lateral ligament of the knee-joint; from its posterior border a thin expansion is given off to the fascia of the leg. The lateral popliteal (common peroneal) nerve descends along the medial border of the tendon.
Nerve-supply.-The Biceps femoris is supplied by the sciatic nerve; the long head through the medial popliteal (tibial) nerve (L. 5 and S. 1), the short head through the lateral popliteal (common peroneal) nerve (S. 2 and S. 3).
Actions.–The Biceps femoris, acting from above, flexes the leg on the thigh, and when the knee is semiflexed rotates the leg slightly laterally. Acting from below, it serves to support the pelvis on the head of the femur and draws the trunk backwards as in raising it from the stooping position.
The Semitendinosus (figs. 647, 649), remarkable for the great length of its tendon of insertion, is situated at the posteromedial surface of the thigh. It arises from the lower and medial impression on the upper part of the tuberosity of the ischium, by a tendon common to it and the long head of the Biceps femoris; it also arises from an aponeurosis connecting the adjacent surfaces of the two muscles to the extent of about 7.5 cm. from their origin. The muscle is fusiform and ends a little below the middle of the thigh in a long, round tendon which lies along the medial side of the popliteal fossa; the tendon curves around the medial condyle of the tibia,, passes over the medial ligament of the knee-joint, from which it is separated by a bursa, and is inserted into the upper part of the medial surface of the shaft of the tibia behind the insertion of the Sartorius and below that of the Gracilis. At its insertion it is united with the tendon of the Gracilis and gives off a prolongation to the deep fascia of the leg. A tendinous intersection is usually observed about the middle of the muscle.
Actions.-Acting from above, it flexes the knee-joint and, when the joint is semilexed, rotates the leg slightly medially. When its fixed point is below, its action is similar to that of the Biceps femoris.
The Semimembranosus (figs. 647, 649, 653), so called from its membranous tendon of origin, is situated at the back and medial side of the thigh. It arises by a thick tendon from the upper and lateral impression on the tuberosity of the ischium (fig. 446), above and lateral to the Biceps femoris and Semitendinosus, and is inserted into the groove on the back of the medial condyle of the tibia. The tendon of origin expands into an aponeurosis which passes downwards under cover of the Semitendinosus and long head of the Biceps femoris; from this aponeurosis muscular fibers arise, and converge to another aponeurosis which covers the lower part of the posterior surface of the muscle and contracts into the tendon of insertion. The tendon of insertion gives off certain fibrous expansions : one, of considerable size, passes upwards and laterally to be inserted into the intercondylar line and lateral condyle of the femur, forming the oblique posterior (oblique popliteal) ligament of the knee-joint; a second is continued downwards to the fascia which covers the Popliteus muscle; while a few fibers join the medial ligament of the knee-joint and the fascia of the leg. The muscle overlaps the upper part of the popliteal vessels and is itself overlapped and partly bidden by the Semitendinosus muscle throughout its whole extent (fig. 649).
The tendons of insertion of the Semitendinosus and Semimembranosus form the medial hamstrings.
Nerve-supply.-The Semimembranosus is supplied by the sciatic nerve through the medial popliteal (tibial) portion (L. 4 and 5 and S. 1).
Actions.—The actions of the Semimembranosus are similar to those of the Semitendinosus.
Applied Anatomy-In disease of the knee-joint, contraction of the hamstring tendons is a frequent complication; this causes flexion of the leg, and a partial dislocation of the tibia backwards, with a slight degree of rotation outwards, probably due to the action of the Biceps femoris. The hamstring tendons occasionally require subcutaneous division. The relation of the lateral popliteal nerve, which lies in close apposition to the medial border of the tendon of the Biceps femoris, must always be borne in mind in dividing this tendon, and a free incision with exposure of the tendon, before division, is the safer proceeding.
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