120 



COMPARATIVE ANATOMY 



(Carinatas). The preacetabular portion of the ilium extends for- 

 ward for a considerable distance, and a number of vertebrae 

 belonging to other than the true sacral region become secondarily 

 connected with the ilium (see p. 48). The acetabulum is per- 

 forated, and the pars acetabularis (p. 119) forms a spinous process. 

 The elements of the pelvis usually become anchylosed together. 

 The pubis meets its fellow in the middle line only in Struthio, 

 and the ischium only in Rhea. 



Mammals. The elements of the pelvis here remain separated 

 for a long time by cartilage, but later they become fused together. 

 The pubis always takes less part in the formation of the aceta- 

 bulum than do the other two bones, and may be more or less 

 entirely shut out from it by an ossification of the pars acetabularis, 

 which subsequently unites with either the ilium, ischium, or 

 pubis (Figs. 98 and 99). This acetdbular lone is especially well 

 developed in the Mole, in which it shuts the ilium, as well as the 

 pubis, out of the acetabulum : the latter is perforated in Mono- 

 tremes. The angle between the axes of the ilium and sacrum is 

 large in Orriithorhynchus, and more acute in other Mammals. 



The original type with both pubic and ischiatic symphyses is 

 seen in Monotremes, Marsupials (Fig. 100), many Rodents, In- 

 sectivores and Ungulates. In many other Insectivores, in Carnivores, 

 and more particularly in the Primates, the ischia no longer meet 

 below. The greatest amount of variety in the form of the pelvis 



FIG. 98. EXTERNAL VIEW OF THE 

 RIGHT HALF OF THE HUMAN 

 PELVIS. (From the outer side.) 



The three bones ilium (//), ischium 

 (Is}, and pubis (P) are shown dis- 

 tinct from one another in the 

 acetabulum. Fo, obturator for- 

 amen. 



FIG. 99. DIAGRAM SHOWING THE 

 RELATIONS OF THE PARS ACETA- 

 BULARIS (in Viverra civetta). 



J, ilium ; Js, ischium ; P, pubis ; 

 A, acetabular bone ; Ac, aceta- 

 bulum. 



in any one order is seen in Insectivores, in some of which (e.g., 

 Mole), as well as in most Bats, there is no symphysis pubis. The 

 obturator foramen is always surrounded by bone. 



