GLUTEAL REGION 299 



femoris, and cannot be properly seen from behind until that 

 muscle has been reflected ; they are all inserted into, or in the 

 neighbourhood of, the greater trochanter of the femur, and 

 they are applied to the posterior surface of the capsule of 

 the hip joint. The first five are lateral rotators of the thigh, 

 when the hip joint is extended, but they become abductors 

 when the joint is flexed. The obturator externus is a lateral 

 rotator in both positions. 



M. Piriformis. The piriformis arises within the pelvis 

 from the three middle pieces of the sacrum, and slightly from 

 the upper margin of the great sciatic notch of the hip bone. 

 The sacral origin cannot be seen at present, but the iliac 

 origin should be made out. After it has passed through 

 the great sciatic foramen, the muscle is directed downwards, 

 laterally, and forwards. Its fleshy belly rapidly tapers and it 

 ends in a rounded tendon, which crosses superficial to the 

 common tendon of the obturator internus and gemelli, and 

 is inserted into a small impression on the highest part of the 

 greater trochanter of the femur (Fig. 1 1 7, p. 262). It is closely 

 adherent to the subjacent obturator tendon for some distance. 

 The piriformis is supplied by branches from the first and 

 second sacral nerves. 



Mm. Obturator Internus et Gemelli. These muscles, 

 together, constitute a tricipital muscle with one large intra- 

 pelvic belly (obturator internus), and two small extra-pelvic 

 bellies (gemellus superior and inferior). The common tendon 

 is inserted into an impression on the upper part of the 

 greater trochanter of the femur, immediately posterior to and 

 below the insertion of the piriformis (Fig. 117, p. 262). 



The gemellus superior arises from the spine of the ischium, 

 at the upper margin of the small sciatic notch. Its fibres 

 pass laterally, along the superior border of the tendon of the 

 obturator internus, and are inserted obliquely into that tendon. 



The gemellus inferior arises from the tuberosity of the 

 ischium, at the lower margin of the lesser sciatic notch, and 

 is inserted into the lower border of the obturator tendon, in 

 a similar manner to the gemellus superior. Close to their 

 origins the gemelli meet under cover of the obturator tendon, 

 and form a fleshy bed on which the tendon lies ; near the 

 trochanter the fibres of the gemelli overlap the obturator 

 tendon, and tend to cover its superficial surface. 



The tendon of the obturator internus has already been 



