418 THE MUSCULAK SYSTEM. 



Actions. The muscle is an abductor and lateral rotator of the hip. 



M. Obturator Interims. The obturator internus arises on the pelvic aspect 

 of the hip bone, from (1) the whole of the margin of the obturator foramen 

 (except the obturator notch) ; (2) the surface of the obturator membrane ; (3) 

 the whole of the pelvic surface of the hip bone behind and above the obturator 

 foramen ; and (4) the parietal pelvic fascia covering it medially. 



It is a fan-shaped muscle. Its fibres converge to the lesser sciatic foramen, 

 and end in several tendons, united together, which hook round the margin of the 

 foramen (a bursa intervening), and after passing over the posterior surface of the 

 hip-joint, are inserted into a facet on the medial surface of the greater trochanter of 

 the femur above the trochanteric fossa (Figs. 370, p. 416, and 373, p. 417). 



In the pelvis minor the muscle occupies the side wall, covered by the parietal 

 pelvic fascia, which separates it from the pelvic cavity above and the ischio-rectal 

 fossa below. In the buttock the tendon is embraced by the gemelli muscles which 

 are attached to its superior and inferior margins. 



The gemelli muscles form accessory portions of the obturator internus. 



M. Gemellus Superior. The superior gemellus arises from the gluteal surface 

 of the ischial spine (Fig. 369, p. 415). 



It is inserted into the upper margin and superficial surface of the tendon of the 

 obturator internus muscle. 



M. Gemellus Inferior. The gemellus inferior arises from the superior part of 

 the gluteal surface of the ischial tuberosity (Fig. 369, p. 415). 



It is inserted into the inferior margin and superficial aspect of the tendon of 

 the obturator internus. 



Nerve-Supply. The obturator internus and superior gemellus receive branches from a special 

 nerve, the nerve to the obturator internus from the anterior aspect of the sacral plexus (S. 1. 2. 3.). 

 The inferior gemellus is supplied by the nerve to the quadratus femoris, a branch derived also 

 from the anterior aspect of the sacral plexus (L. 4. 5. S. 1.). 



Actions. The obturator internus and gemelli are abductors and lateral rotators of the hip. 



M. Quadratus Femoris. The quadratus femoris arises from the lateral 

 margin of the tuber ischiadicum (Figs. 366, p. 412, and 369, p. 415). It is inserted 

 into the quadrate tubercle and quadrate line of the femur (Fig. 370, p. 416). 



The muscle is concealed by the glutaeus maximus and the hamstring muscles. 

 Its anterior surface is in contact with the obturator externus muscle and the lesser 

 trochanter of the femur, a bursa intervening. The muscle is not infrequently 

 fused with the adductor magnus. 



Nerve-Supply. A special nerve from the sacral plexus (L. 4. 5. S. 1.) which enters its 

 deep (anterior) surface. 



Actions. The muscle is an adductor and lateral rotator of the thigh. 



THE MUSCLES ON THE POSTERIOR ASPECT OF THE THIGH. 

 The Hamstring Muscles. 



The muscles comprised in this series include the biceps, semitendinosus, am 

 semimembranosus. A part of the adductor magnus, already described, also belongs, 

 morphologically, to this group. They lie in the buttock and posterior aspect of 

 the thigh, and diverge at the knee to bound the popliteal fossa. The origins of 

 the muscles are concealed by the glutseus maximus. In the back of the thigh, 

 enveloped by the fascia lata, they are placed behind the adductor magnus th< 

 semitendinosus and semimembranosus medially, the biceps laterally. The forme] 

 two muscles help to form the medial boundary of the popliteal fossa, of which the 

 biceps is a lateral boundary. 



M. Biceps Femoris. The biceps femoris has a double origin. (1) Its loi 

 head arises, by means of a tendon, in common with the semitendinosus, froi 

 the inferior and medial facet upon the sciatic tuberosity (Figs. 366, p. 41! 

 and 369, p. 415) and from the sacro-tuberous ligament. This head, united for 



