170 CAVITY OF THE PELVIS. 



this unequal surface to connect them more firmly. They are 

 connected together at their forepart by the ligamentous cartilage 

 interposed between the two ossa pubis, and therefore have no 

 motion in a natural sjate, except what is common to the trunk 

 of the body, or to the os sacrum. 



Considering the great weight that is supported in our erect 

 posture, by the articulation of the ossa innominati with the os 

 sacrum, there is great reason to think, that, if the conglutinated 

 surfaces of these bones were once separated, (without which 

 the ossa pubis cannot move on each other,) the ligaments 

 would be violently stretched, if not torn. 



Each os innominatum affords a socket (the acetabulum) for 

 the thigh bones to move in ; and the trunk of the body rolls so 

 much on the heads of the thigh bones as to allow here the most 

 conspicuous motions of the trunk, which are commonly thought 

 to be performed by the bones of the spine. 



The form of the cavity of the pelvis, at its upper opening, or 

 brim, is somewhat oval ; as a line drawn from one side to the 

 other, is about an inch longer than a line drawn from the back 

 to the front part of it. 



This margin is well defined by the ridge on the surface of 

 the ossa ilia, and the upper edge of the os pubis ; but the 

 margin of the lower opening is very irregular ; and it ought to 

 be observed, that the dimensions of this opening are made less 

 by the sacro-sciatic ligaments, than they appear upon an 

 examination of the bare bones. 



In consequence of the oblique position of the sacrum, 

 sloping downwards and backwards, the position of the pelvis 

 is very oblique. A line drawn through the centre of this 

 cavity, perpendicular to the plane of the upper orifice, or brim, 

 would not coincide with the vertical diameter of the cavity of 

 the abdomen, but would pass out of that cavity near the 

 umbilicus. 



This cavity, and the bones which form it, are different in the 

 two sexes. 



In women, the brim of the pelvis is wider, and inclines more 

 to the oval form. 



In men this opening is more circular. 



