AEEANGEMENT OF THE FASCLE OF THE PELVIS. 135 



which extends upwards in front of the coccygeus, the small 

 •sacro-sciatic ligament, and the pyriformis muscle. 



The pubo-coccygeus arises from the back of the pubis along 

 an oblique line extending from near the lowest part of the sym- 

 physis upwards and outwards to the obturator foramen. The 

 most ventral fibres are muscular at their origin, those more 

 ■dorsally situated markedly aponeurotic, like the origin of the 

 ilio-coccygeus. Passing to their insertion, the fibres, now 

 strongly muscular, join in the middle line with those from the 

 opposite side, and in this way complete the muscular sling at its 

 lowest part. A few of the innermost, most ventral fibres of the 

 muscle terminate in the walls of the rectum. 



It is thus seen that in the orang-utan, as in man, the levator 

 ani increases in thickness, and consequently in functional import- 

 ance, as we pass from the dorsal to the ventral part of the 

 muscle, and this is probably explained by the fact that the 

 weight of the superimposed viscera is largely borne by those 

 parts of the muscles springing from the back of the pubes. 



But in the orang-utan the obturator internus muscle, and 

 consequently the obturator fascia, have not the same extensive 

 relation to the lateral pelvic wall as in man. The upper limit 

 of the muscle is defined by a horizontal line drawn on the side 

 wall of the pelvis at the level of the ischial spine, so that about 

 two inches or so of the internal surface of the bone intervene 

 between its upper border and the ilio-pectineal line. In dissect- 

 ing the levator ani from below, it is necessary to separate the 

 muscle from the obturator fascia — the two being only loosely 

 •connected — and the thin aponeurotic portion from the periosteum 

 covering the bone above the internal obturator. And just as 

 there is a tendency for the aponeurotic origin of the ilio-coccy- 

 geus in man to fuse with the upper part of the obturator fascia, 

 so that the muscle appears to rise from it, so in the orang-utan 

 the upper part of the muscle tends to fuse with the periosteum, 

 so that it appears to spring from the side wall of the pelvis 

 much lower down. 



The conclusion arrived at from a consideration of the muscles 

 and fascise on the side wall of the pelvis in man, that the 

 portion of the parietal pelvic fascia above the origin of the 

 pelvic diaphragm is not obturator fascia simply, but joined with 



