THE LOWER EXTREMITY, POSTERIOR. 609 



Semitendinosus. Fig. 126. 



Origin. By a tendon common to it and the long head 

 of the biceps from the lower (anterior) and internal facet 

 upon the tuberosity of the ischium. 



Insertion. By a long slender tendon into the inner sur- 

 face of the tibia opposite its tubercle, where it lies behind 

 the sartorius and below the gracilis. Also, into the deep 

 fascia of the leg. 



A bursa intervenes between the three tendons and the 

 internal lateral ligament of the knee. 



Nerve Supply. Great sciatic. See biceps, above. 



Action. (i) Upon the leg, to flex the leg and in a flexed 

 position to rotate it inward (pronation). (2) Upon the 

 thigh, it is an extensor, and used for the lesser muscular 

 efforts. (3) Upon the pelvis it acts as an extensor, as in 

 raising the body from a stooping position. 



Semimembranosus. Fig. 126. 



Origin. From the external and upper (posterior) facet 

 upon the tuberosity of the ischium. To get to this posi- 

 tion the tendon of the semimembranosus passes forward of, 

 then external to, the common tendon of the long head of the 

 biceps, and the semitendinosus muscles. 



Insertion. Into the transverse groove upon the back of 

 the internal tuberosity of the tibia ; by a strong recurrent 

 bundle of fibres, which crosses the knee joint obliquely, 

 into the outer posterior surface of the external condyle of 

 the femur ; and by a continuation of the direct fibres into 

 the fascia covering the popliteus muscle, and so into the 

 oblique line upon the posterior surface of the tibia. 



Nerve Supply. The great sciatic, the filaments coming 

 from the first, second, and third sacral nerves. 



Action. (i) Upon the leg, the muscle acts as a flexor, 

 39 



