PELVIC LIMBS 657 



boundary is a line drawn nearly at right angles to its long diameter 

 through the upper fifth of the acetabulum. 



The ischium and pubes surround the oval opening or obtura'tor 1 

 foramen on the lower part of the bone ; they are L-shaped, and so 

 arranged that the end of the horizontal branch of the one joins the 

 end of the vertical branch of the other (Fig. 518). 



The ischium is the stout L which lies dorsal to the obturator 

 foramen. From in front it is directed downward, backward, and out- 

 ward, then bends at a right angle and passes downward, inward, and 

 forward to meet a branch of the pubes. It enters into the formation 

 of about three-fifths of the acetabulum. 



The pubes lies in front and on the median side of the obturator 

 foramen. It does not really reach the acetabulum ; the fourth element, 

 the cot'yloid 2 bone, excludes it. If we regard the cotyloid bone as 

 an epiphysis of the pubes, we find that the pubes then forms about 

 one-fifth of the acetabulum. 



The Hium is a flat, tongue-shaped bone. It is slightly narrower 

 in the middle than at the upper free end, and still wider below where 

 it joins the pubes and the ischium. The greatest breadth is contained 

 two and one-fifth times in its length, and its average thickness is con- 

 tained four times in the greatest width, and therefore about eleven times 

 in the length. It presents an external and an internal surface, limited 

 by anterior and posterior borders and by the upper border, called the 

 crest. 



The external surface (Fig. 517) has the length and breadth about 

 equalling the length and breadth of the entire ilium. It is limited in 

 front by the anterior border, which can be recognized as the one 

 continued below to the pubes. It is limited behind by the posterior 

 border, namely, the one which is more emarginate and is continued 







below on the ischium. The crest of the ilium limits the surface above, 

 and an imaginary straight line drawn at right angles to the long axis 

 through the upper fifth of the acetabulum separates it from the ex- 

 ternal surfaces of the ischium and pubes below. The anterior border 

 begins above at the crest in a rounded angle, known as the anterior 

 superior spine of the ilium, below which for about its upper third it 

 is straight. It then bends backward, at a well-marked anterior inferior 

 spine of the ilium, and, leaving the apparent edge of the bone, runs 



1 From obturo, to stop up. 2 From cotyle, a cup, and eides, like. 



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