THE SEMIMEMBRANOSUS MUSCLE. 251 



from its lower border to the fascia of the leg. There the tendon is on the same 

 plane with, but below that of the gracilis, both being under cover of the sartorius, 

 and a bursa separates the three tendons from the internal lateral ligament of the 

 knee-joint. The belly of the muscle is traversed about its middle by a thin oblique 

 tendinous intersection. 



The semimembranosus muscle (fig. 249, 6 ; 254,11) arises from the tuberosity of 

 the ischium, above and to the outside of the origin of the biceps and semitendinosus, 

 by a strong flattened tendon which is grooved on its posterior surface for the reception 

 of the common tendon of those two muscles. The tendon of originals "prolonged 

 downwards on the outer side of the muscle for three-fourths of the length of the 

 thigh ; from it spring numerous short fleshy fasciculi which are directed obliquely 

 downwards and inwards, forming a thick fusiform belly, and terminate upon the 

 tendon of insertion on the opposite border of the muscle. The lower tendon makes 

 its appearance about the middle of the thigh, becomes free from muscular fibres at 

 the level of the knee, and, turning somewhat forwards, is inserted mainly into the 

 lower part of the horizontal groove on the back of the inner tuberosity of the tibia. 

 One considerable expansion is sent upwards and outwards to the posterior ligament 

 of the knee-joint (see p. 184), another is continued downwards to the fascia 

 covering the popliteus muscle, and a few fibres join the internal lateral ligament 

 of the joint. 



Relations. At its upper part the semimembranosus crosses obliquely from without in- 

 wards beneath the conjoined biceps and semitendinosus. and the latter muscle lies upon it in 

 its whole length. Its deep surface rests upon the adductor magnus, and the great sciatic 

 nerve lies along its outer border. Between the lower tendon and the inner head of the 

 gastrocnemius is a large bursa which often communicates with the cavity of the knee-joint, 

 and a second smaller one separates the main portion of the tendon from the prominent upper 

 margin of the groove on the tibia. 



The hamstring muscles descend for the greatest part of their length in contact with one 

 another, being bound down by the fascia lata ; but inferiorly they diverge, the biceps passing 

 to the outer side, and the semimembranosus and semitendinosus to the inner side of the knee, 

 forming the superior borders of a diamond-shaped hollow at the back of the knee the 

 popliteal space, the inferior margins of which are formed by the heads of the gastrocnemius 

 muscle. 



Varieties. Great reduction of the semimembranosus muscle, and complete absence have 

 been observed in several instances. It has also been found double, arising mainly from the 

 great sacro-sciatic ligament, and giving a slip to the femur, or to join the adductor magnus. 



The name of tensor fascice suralis or ischio-aponeuroticus has been given to a muscular 

 slip passing from one or other of the hamstring muscles to the fascia of the back of 

 the leg. 



Nerves. These muscles are all supplied by the great sciatic nerve, the branches being 

 derived from its internal popliteal division, with the exception of that to the short head of the 

 biceps, which is given off by the external popliteal division. 



Actions. The hamstring muscles flex the knee, and when that joint is bent they can 

 rotate the tibia the biceps outwards, the semitendinosus and, to a less extent, the semi- 

 membranosus inwards. They are farther powerful extensors of the hip, and by their position 

 they set a limit to flexion of that joint so long as the knee is extended. 



THE ANTEKIOR MUSCLES OP THE THIGH. 



The sartorius muscle is very long, narrow, and ribbon-shaped, and has the longest 

 fascicles of all the muscles of the body. It arises by a short tendon from the anterior 

 superior spine of the ilium, and from a small part of the anterior margin of that 

 bone immediately below ; passing downwards and inwards across the front of the 

 thigh, it is inserted by a thin flattened tendon into a slight roughness on the inner 

 side of the tibia near the tubercle, sending off one aponeurotic expansion from its 

 upper border to the capsule of the knee-joint, and another from its lower border to 

 the fascia of the leg. 



