606 A MANUAL OF ANATOMY. 



second, third, and fourth pieces of the sacrum, and from 

 the ridges of bone between the first, second, third, and 

 fourth sacral foramina. 



From the deep surface of the great sacrosciatic ligament, 

 and the upper margin of the great sacrosciatic notch. 



Insertion. Into the upper border of the great trochanter 

 near the front. The tendon of the pyriformis muscle is 

 closely joined, near the trochanter, to the common tendon 

 of the obturator internus and the gemelli muscles. If the 

 tendons are carefully separated it will be found that the 

 tendon of the latter muscles passes beneath and in front of 

 that of the former muscle ; the posterior muscles have the 

 anterior insertion into the great trochanter. 



Nerve Supply. Muscular branches from the second sacral 

 nerve of the sacral plexus. 



Action. If the thigh is extended the muscles act solely 

 as an external rotator. When the thigh is flexed it acts as 

 an abductor. Acting from a fixed point upon the trochan- 

 ter the pyriformis will carry the other side of the pelvis 

 backward, whether the femur be extended or flexed. 



Obturator Internus and Gemelli. Figs. 114, 126, 127. 



(i) Obturator internus. From the inner surface of the 

 true pelvis (including the obturator membrane), below a 

 line drawn from the upper margin of the obturator foramen 

 to the junction of the iliopectineal line near its ter- 

 minus, and above a line drawn about the base of the spine 

 of the ischium. Behind the muscle reaches to the anterior 

 margin of the great sacrosciatic foramen, and in front, it 

 extends on to the rami of the pubes and ischium. It also 

 arises from the obturator fascia which covers it. See 

 page 398. (2) Gemellus superior. From the external 

 surface of the spine of the ischium. (3) Gemellus inferior. 



