286 THE PELVIC GIRDLE. [chap. 



girdle, and to incorporate it more completely for mechanical 

 purposes with the vertebral column, there is, in addition to 

 the articulation between the ilium and true sacral vertebrae, 

 a very strong double ligamentous union between the ischium 

 and the side of the anterior caudal or pseudo-sacral vertebrae, 

 constituting the greater and lesser sacro-sciatic ligaments, 

 which are replaced in some Mammals (as most of the Eden- 

 tata) by a complete bony union. 1 



The two innominate bones, together with the sacrum, 

 constitute the pelvis, a complete circle of bone, or rather a 

 short tube. This has two outlets : an anterior (sometimes 

 called inlet or brim) bounded by the inferior surface of the 

 first sacral vertebra above, by the pubic borders of the 

 ilia laterally, and by the anterior borders of the converging 

 pubic bones, meeting at the symphysis below ; and a 

 posterior outlet, bounded by the posterior part of the 

 sacrum above, by the great sacro-sciatic ligaments laterally, 

 and by the converging posterior borders of the ischia below. 

 In consequence of the oblique position of the innominate 

 bones, the plane of the anterior outlet (in the horizontal 

 position of the body) looks downwards and forwards ; 

 that of the posterior outlet upwards and backwards ; but 

 these two planes are not exactly parallel, the long axis of 

 the cavity being usually more or less curved. 



Modifications of the Pelvic Girdle in the different Groups 

 of Mammalia. 



Order Primates. — The pelvis of Man is very consider- 

 ably modified from the usual form met with in Mammals. 



1 Practically, though not morphologically, the pelvis is a part of the 

 trunk or axial skeleton. The functions of the hind limb in propelling 

 and raising the body necessitate that it should be so. 



