316 THE PELVIC GIRDLE. [CHAP. 



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. 

 The innominate bone (Fig. 114) is remarkably broad in 

 proportion to its length. The ilium is flattened and ex- 

 panded, and has a greatly extended, almost semicircular 

 supra-iliac border (si}. The sacral surface (ss) is small, and 

 scarcely rises above the vertebral attachment. The iliac 

 surface (is) is very broad and hollowed. Thegluteal surface 

 is likewise much expanded, and, though presenting several 

 curves, is, in the main, convex. The acetabular border (ab) 

 is veiy short, and has a strong, rounded, rough prominence 

 for the attachment of the tendon of the rectns (extensor) 

 muscle of the leg. The pubic border (pb) is slightly 

 marked, constituting the linea arcuata interna, or linea ilio- 

 pectinea of human anatomy. The ischial border (ib) is 

 short and deeply hollowed. The acetabulum (a) is large, 

 circular, with very prominent borders, incomplete only for a 

 small space on the infero-internal aspect. 



1 See Weyhe, " Uebersicht der Saeugethiere nach ihren Becken- 

 formen, "Zeitschr. f. d. gesammten Natunuss. xi. 1875. 



