864 A MANUAL OF ANATOMY 



Action. — To flex the coccyx. 



The coccygeus is a thin, flat, triangular muscle, the fleshy fibres 

 having a large admixture of tendinous fibres. The internal or 

 pelvic surface is related to the visceral pelvic fascia, sacro-coccygeal 

 plexus and rectum, and the external surface to the small sacro-sciatic 

 ligament. The superior border is adjacent to the pyriformis, with 

 the intervention of the structures which leave the pelvis below that 

 muscle. The inferior border adjoins the posterior border of the 

 levator ani. 



The coccygeus muscle is to be regarded as a detached portion 

 of the levator ani. 



The levatores ani and coccygei muscles form the pelvic diaphragm, 

 which presents a superior concave, and an inferior convex, surface. 

 The greater part of this diaphragm is formed by the levatores ani, 

 and this part of it gives passage in the middle line to the rectum. 



Pyriformis — Origin. — (i) By three fleshy slips from the anterior 

 surfaces of the second, third, and fourth sacral vertebrae, which 

 are interposed between, and lie external to, the adjacent anterior 

 sacral foramina ; (2) the deep surface of the great sacro-sciatic 

 ligament ; and (3) the posterior border of the ilium immediately 

 below the posterior inferior spine. 



Insertion. — ^The upper border of the great trochanter of the femur 

 near its centre. 



Nerve-supply. — ^Two branches from the sacral plexus, more 

 particularly from the dorsal divisions of the first and second sacral 

 nerves. 



Action. — External rotator of the thigh. 



The intrapelvic portion of the muscle is covered by a prolonga- 

 tion of the parietal pelvic fascia, called the fascia of the pyriformis, 

 and it supports the nerves of the sacral plexus, branches of the 

 anterior division of the internal iliac artery, and pelvic colon. For 

 the extrapelvic portion, see Gluteal Region. 



Obturator Internus — Origin. — (i) The internal surface of the 

 obturator membrane ; (2) the posterior surface of the body and 

 descending ramus of the os pubis, and ramus of the ischium ; (3) the 

 inclined plane of the ischium, extending as far back as the great 

 sacro-sciatic foramen, and nearly as high as the iliac portion of the 

 ilio-pectineal line ; and (4) the parietal pelvic fascia covering the 

 musile. 



Insertion. — The inner surface of the great trochanter above 

 and in front of the digital fossa. 



Nerve-supply. — The nerve to the obturator internus, from the 

 sacral plexus. 



Action. — External rotator of the thigh. 



Relations of Intrapelvic Part — Internal. — As low as the i&hite 

 line the internal surface of the muscle is covered by the parietal 

 pelvic fascia and peritoneum with the subperitoneal areolar tissue, 

 and it is directed towards the pelvic cavity. The obturator vessels 

 and nerve here lie between the subperitoneal areolar tissue and the 

 parietal pelvic fascia. Below the level of the white line the internal 



