166 OS ISCHIUM. 



culation with the os sacrum. From all the circumference of 

 this large unequal surface, ligaments are extended to the os 

 sacrum, to secure more firmly the conjunction of these bones. 



The passages of the medullary vessels are very conspicuous, 

 both in the dorsum and costa of many ossa ilia ; but in others 

 they are inconsiderable. 



The posterior and lower parts of these bones are thick ; but 

 they are generally exceedingly thin and compact at their 

 middle, where they are exposed to the actions of the musculi 

 glutaei and iliacus Internus, and to the pressure of the bowels 

 contained in the belly. The substance of the ossa ilia is cellu- 

 lar, except a thin external plate. 



The Os Ischium, 



Or, hip-bone, is of a middle size, between the two other parts 

 of the OS innominatum, and of a very irregular figure. Its 

 extent might be marked by a horizontal line drawn a little 

 below the middle of the acetabulum ; for the upper bulbous 

 part of this bone forms rather less than the lower half of that 

 great cavity, and the small leg of it rises to much the same 

 height on the other side of the great hole common to this 

 bone and the os pubis. 



From the upper thick part of the os ischium, a sharp process, 

 called by some authors spinous, stands out backwards, from 

 which chiefly the musculus coccygaeus and superior gemellus, 

 and part of the levator ani, rise ; and the anterior, or internal 

 sacro-sciatic ligament is fixed to it. Between the upper part 

 of this ligament and the bones, it was formerly observed, that 

 the pyriform muscle, the posterior crural vessels, and the 

 sciatic nerve, pass out of the pelvis. Immediately below this 

 process, is a depression for the tendon of the obturator internus 

 muscle. In a recent subject, this part of the bone serves as a 

 pulley on which the obturator muscle plays with a ligamentous 

 cartilase. 



Below the depression of the obturator muscle, is the great 

 knob or tuberosity, covered with cartilage or tendon. The 

 upper part of the tuberosity gives rise to the inferior gemellus 



