CH. XVII.] 



GENERAL CHARACTERS. 



313 



Like the shoulder girdle it is divided, by its mode of 

 ossification, into an upper (dorsal) and a lower (ventral) 

 segment, and the point of union between these is near the 

 middle of the articular cavity. 



Unlike the shoulder girdle of most Mammals, the lower 

 segment is always largely developed, and ossifies from two 

 separate centres, which form an anterior and a posterior 



ti 



FIG. 113. External surface of right innominate bone of a young Lamb (Ovh aries}. 

 . // ilium (gluteal surface) ; si supra-iliac border ; #acetabular border ; z'^ischial 

 border; fs ischium ; s/> spine ; ti tuberosity of ischium; P pubis ; ^ symphysis ; 

 thf thyroid or obturator foramen : a acetabulum. 



bar, in contact above and below, but leaving a space 

 between them in the middle, filled only by membrane, and 

 called the thyroid or obturator foramen (thf). 



The upper segment is named the ilium (//), the anterior 

 bar of the lower segment the pubis (P), and the posterior 

 bar the ischium (Is). In the process of growth these three 



