THE INNOMINATE BONE 43 



Like the other two surfaces, this is widest superiorly and becomes much 

 narrower towards its inferior end. On the upper part of this surface 

 there is presented the auricular facet which, by coming into apposition 

 with a like facet on the superior aspect of the sacrum, forms the sacro- 

 iliac joint. The area above the facet is roughened for the attachment 

 of ligaments of this joint. Below the facet the surface is almost entirely 

 smooth, and this portion presents the obturator groove so named since 

 along it run the obturator nerve and vessels. The groove runs in a 

 longitudinal manner along this surface towards the obturator foramen, 

 so that it takes a direction, therefore, which is downwards and slightly 

 backwards. This is a point of considerable surgical import, and will be 

 referred to again in dealing with obturator paralysis. 



The remaining surface of the ilium is the iliac, and this surface looks 

 forwards and slightly downwards. It is so named because it is clothed 

 by the iliacus muscle throughout almost the whole of its extent. The 

 nutrient foramen of the bone, which is of considerable size, will be 

 found on this surface near the line of division between the middle and 

 lower thirds. 



On the dried bone two sets of faint grooves or depressions will be 

 observed. The inferior grooves are placed near the nutrient foramen and 

 take a downward direction. They are the impressions caused by the 

 iliaco-femoral vessels. The other set will be found in the upper third 

 of this surface. These grooves are not usually so well marked as are 

 those of the lower set. They take a forward direction and indicate the 

 course taken by the ilio-lumbar vessels. 



These three surfaces are separated from one another by three more 

 or less well-defined edges. The edge which runs inferiorly to the 

 cotyloid cavity is the cotyloid edge. It is curved in its length, the 

 concavity of the curve being directed outwards. Towards its superior 

 end this edge is very narrow, but becomes much thicker as we descend 

 to the acetabulum. 



This edge separates the gluteal and iliac surfaces. Separating the 



