SKELETON 



86 1 



acetabulum a thick process of the ilium descends to meet the os 

 pubis, and behind a similar process meets the ischium. Behind 

 the acetabular rim is a considerable thickening of the ilium, which 

 frequently bears a facet on which plays the great trochanter of 

 the femur. The postacetabular ilium is very variable in shape, 

 either broadened out vertically only as in Columha and FJiea, or 

 transversely, in the latter case forming a plain dorsal surface most 



Fovea iliaca 

 anterior 



Pr.pectin. Q' 



\ Pubis, 



obtur 



Pavo cristatus. 



pronounced in Pavo. The ischium originally extends backward, 

 parallel to the postacetabular ilium, and with it encloses the 

 ischiadic notch. Among recent Birds this primitive condition 

 persists only in Ratitx and Cryptwri. In all others the distal por- 



Cp-2 Cp.r 



COLYMBUS ARCTICUS. 



tions of the ilium and ischium meet so that this notch becomes a 

 foramen, through which pass the big stems of the ischiadic nerves 

 (page 625) and most of the chief blood-vessels of the hind limb.^ 

 The OS pubis consists of an anterior and a posterior portion, the latter 

 being long and slender, and running backwards more or less parallel 



^ A unique modification occurs in Rlica : the two ischia fuse in the middle 

 line forming a long ischiadic symphysis which lies above the intestines separating 

 them from the kidneys. In adults the distal ends of the ilia fuse with these 

 united ischia, forming foramina, and herewith is correlated a still more striking 

 feature, namely the gradual resorhtion as maturity approaches of nearly the whole 

 postsacral vertebral column, so that the caudal vertebrae seem to be attached to 

 the united ischiadic and iliac ossification. 



