OS PUBIS. 167 



muscle. To a ridge at the inside of this, the external, or pos- 

 terior sacro-sciatic ligament is so fixed, that between it, the 

 internal ligament, and the sinuosity of the os ischium, a passage 

 is left for the internal obturator muscle. The upper thick 

 smooth part of the Uibcr, called by some its dorsum, has two 

 .oblique impressions on it. The inner one gives origin to the 

 long head of the biceps flexor cruris, and semitendinosus 

 muscles ; and the semimembranosus rises from the exterior one, 

 which reaches higher and nearer the acetabulum than the 

 other. The lower, thinner, more scabrous part of the knob, 

 which bends forwards, is also marked with two flat surfaces ; 

 whereof the internal is what we lean upon in sitting, and the 

 external gives rise to the largest head of the triceps adductor 

 femoris. Between the external margin of the tuberosity, and 

 the great hole of the os innominatum, there is frequently an 

 obtuse ridge extended down from the acetabulum, which 

 gives origin to the quadratus femoris. As the tuber advances 

 forwards, it becomes smaller, and is rough for the origin of the 

 musculus transversalis and erector penis. The small leg of it, 

 which mounts upwards to join the os pubis, is rough and prom- 

 inent at its edge, where the two lower heads of the triceps 

 adductor femoris take their rise. 



The upper and back part of the os ischium is broad and 

 thick ; but its lower and forepart is narrower and thinner. Its 

 substance is of the structure common to broad bones. 



The OS ilium and pubis, of the same sides, are the only 

 bones which are contiguous to the os ischium. 



The Os Pubis, 



The least of the three portions of the os innominatum, is 

 placed at the upper and front part of it. The thick, largest 

 part of this bone is employed in forming the acetabulum ; from 

 which, becoming much smaller, it is stretched inwards to its 

 fellow of the other side, where it again grows larger, and 

 forms a surface to be connected with the cartilage of its sym- 

 physis and then sends a small branch downwards to join the 



