356 THE MUSCLES 



Structure. — The greater part of the iiiusele arises by three fleshy sHps on the 

 ridges of bone between, and external to, the anterior sacral foramina. From this 

 origin, which receives accessory slips of small size from the great sacro-sciatic 

 ligament and the upper portion of the great sacro-sciatic notch, the fibres converge 

 as they pass transversely outwards and somewhat backwards through the great 

 sacro-sciatic foramen. The tendon is first visil)le upon the deep as})ect of the 

 muscle, and it liecomes free near the posterior border of the greater trochanter. 

 Shortly liefore its insertion it is closely l)lended with the tendon of the obturator 

 internus museU'. 



Nerve-supply. — From the sacral plexus by small l)ranches which pass from 

 the second sacral nerve into the anterior surface of the muscle near its origin. 



Action. — To rotate the thigh outwards. When the thigh is fixed, it will rotate 

 the pelvis so that the face is turned to the opposite side; if the thigh be flexed, the 

 pyriformis will alKluct it. 



Relations. — In front, the sacral plexus and rectum, the back of the hip-joint, 

 and some of the branches of the internal iliac artery; behind, the gluteus maximus; 

 above, the gluteus medius and minimus, with the superior gluteal vessels and nerve; 

 below, the coccygeus, the lesser sacro-sciatic ligament and gemellus superior, the 

 sciatic and pudic vessels, and most of the branches from the sacral plexus. 



Variations. — The pj'riforruismay be absent. It is often divided by a part of the great sciatic 

 nerve into two muscles, and sometimes into three. It maj' be more or less blended with the 

 gluteus medius or minimus above, and the superior gemellus below. 



2, 3, AND 4. OBTURATOR INTERNUS AND GEMELLI 



The obturator internus and gemelli form really a single muscle, the greater 

 part of which arises inside the pelvis; while the gemelli form two accessory slips 

 W'hich join it from the margin of that cavity. 



The obturator internus — named from the fact that it arises from the mem- 

 brane which closes up the obturator foramen, and from its position within the 

 pelvis — is a somewhat triangular sheet, or rather, perhaps, it should be described 

 as the sector of a circle, for its origin Avithin the pelvis is bounded by a curved line 

 like a part of the circumference of a circle. 



Origin. — The wdiole of the interior of that part of the pelvis which is formed by 

 the innominate bone : viz. ( 1 ) The ])ack of the body and descending ramus of the 

 OS pubis, and of the ascending ramus of the ischium; (2) the whole of the inner 

 surface of the ol)turator membrane; (3) the broad surface of bone behind the fora- 

 men, corresponding to the acetabulum on the exterior; (4) the outer surface of the 

 pelvic and ol)turator fascife. These extend from the ilio-pectineal line above to the 

 great sacro-sciatic foramen behind, and to the spine and tulierosity of the ischium 

 below. 



Insertion. — The inner aspect of the upper l)order of the greater trochanter at 

 the jxiint where it unites with the u})per border of the neck of the femur. 



Structure. — The muscle arises fleshy from the Avhole of the interior of the 

 pouch-like cavity formed by the pelvic and obturator fascia? internally and the bony 

 wall of the ])elvis externally, and o})ening backwards and downwards at the lesser 

 sacro-sciatic foramen. From this extensive origin the filirc^s converge downwards and 

 backwards upon a broad tendinous expansion which begins about one inch (2 "5 cm. ) 

 al)ove the lesser sacro-sciatic foramen upon the outer surface of the muscle. This 

 ex])ansion is corrugated into four or five folds, which are separated from the cartila- 

 ginous lining of the lesser sacro-sciatic notch by a bursal cavity Avhich allows of the 

 smooth play of the tendon upon the bone. This cartilage presents corresponding 

 grooves for the folds of the tendon. After passing through this foramen the tendon 

 changes its course, and is directed transversely outwards and forwards to its inser- 

 tion. The fleshy fibres extend upon the inner, which has now become its posterior 

 surface for about half the distance from the notch to the facet upon the greater 

 trochanter. For a short distance before its insertion it is intimately connected with 

 the tendon of the ]>yriformis. 



Nerve-supply. — From the first and second sacral nerves of the sacral jjIcxus 



