THE INTEENAL SKELETON. 



19s 



The ilium may be considered as answering to the scapula of 

 the thoracic girdle, in spite of the extreme difference of its shape 

 from that of the blade-bone of the shoulder *. It is of enormous 

 size compared to that of Mammals, being greatly extended both 

 in front of and behind the acetabulum. It is the bone which 

 anchyloses on either side with the many vertebrse which go to 

 form the " sacrum " as already described f. 



The ilium of one side of the body may so anchylose with that 

 of the other side and with the sacrum that the dorsal hinder 

 part of the skeleton of a bird's trunk presents an expanded 

 bony shield like that presented by the sternum on the ventral 

 anterior part of the trunk skeleton. Moreover, the appearance 



Fig. 157. 



P!/- 



Pelvis of a Fowl (after Parker). 



il, il, Ilium ; is, ischium ; pb, pubes ; dl, doi'so-lumbar vertebrae ; cd, caudal 

 vertebrae, at the distal end of which is ,{p^) the pygostyle J ; am, ace- 

 tabulum. 



of a median ridge is more or less produced by lateral depressions, 

 though there is never anything really like the keel of the ster- 

 num, and for a very good reason, as we shall see when the 

 muscular structure of a bird comes to be described. 



The ilium forms the upper margin of the acetabulum, and at 

 the hinder part of that margin develops a strongly marked, 

 somewhat flattened process bearing an articular surface. This 

 is called the antitroehanteric process. 



* See ante, p. 189. 

 t See ante, p. 173. J See ante, p. 175. 



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