THE PELVIC GIRDLE 299 



to it. They are firmly united to the sacrum in front, and with 

 each other in the ventral symphysis behind. 



Each half, or os innonunatum, consists of three bones — 

 ilium, ischium, and pubes — and presents about the middle of 

 its outer surface a deep cup-shaped acetabular cavity for the 

 femur, in the formation of which all the three bones take part, 

 i. The ilium is the anterior and dorsal member 

 of the girdle, corresponding to the scapula in 

 the pectoral girdle. It is expanded and blade- 

 like in front, and narrowed behind. On its 

 inner or sacral surface, a little way in front 

 of the acetabulum, is a roughened crescentic 

 surface for articulation with the sacrum ; 

 while its outer surface is divided by a longi- 

 tudinal ridge into ventral or iliac, and dorsal 

 or gluteal fossae. The ilium forms about 

 half of the acetabulum, the lines of boundary 

 between it and the ischium and pubes being 

 easily seen in the young skeleton, 

 ii. The ischium forms the posterior and dorsal 

 portion of the os innominatum, and is sepa- 

 rated from the pubes by the large oval 

 obturator foramen. It forms about a third 

 of the acetabulum, behind which it bears 

 on its inner surface the sharp, backwardly 

 directed ischial spine. Its posterior part is 

 flattened, the margin being thickened to form 

 the ischial tuberosity, which is raised on its 

 outer surface into a prominent hooked pro- 

 cess. The ischium corresponds to the coracoid 

 in the shoulder-girdle, 

 iii. The pubes is the smallest bone of the three. 

 Its acetabular portion ossifies separately in 

 the rabbit, and remains throughout life as a 

 small cotyloid bone, completing the margin 

 of the acetabulum below. The pubes unites 

 along its ventral border with its fellow of the 

 opposite side to form the anterior part of the 



