XIII THE SKELETON 243 



the large, transverse processes of the ninth or sacral verte- 

 bra. Each half of the pelvic girdle is composed of three 

 elements, the ilium^ the ischium, and X\it pubis. The first 

 named is a long bone, attached to the ninth vertebra in 

 front, and meeting its fellow behind ; on the dorsal side is a 

 high, thin crest ; at the anterior end of the ventral side of 

 the junction of the two ilia is a prominence called the ante- 

 rior spine of the pelvis. The ilium forms the anterior part 

 of the cup, or acetabulum, which receives the head of the 

 femur. 



The pubis is a triangular mass of hyaline, or in older frogs 

 calcified, cartilage on the ventral side of the pelvic girdle ; 

 it forms a part of the lower side of the acetabulum. 



The posterior portion of the pelvic girdle is repre- 

 sented by the ischium ; it forms the posterior part of 

 the acetabulum, and extends forward dorsally as far 

 as the top of a prominence, the posterior spine of the 

 pelvis, which is situated above, and a little behind the 

 acetabulum ; here it is fused with the ilium ; the suture 

 between these bones can best be seen in young frogs. 

 The lines of union of the pubic and ischial bones of 

 opposite sides are known respectively as the pubic and 

 ischial symphyses. 



The pelvic girdle of the frog is remarkable on account of 

 the elongation of the ilia, the reduction of the pubis and 

 ischium, and the intimate fusion of the two latter with the 

 posterior expanded part of the ilium to form an almost 

 circular mass. 



The Hind Limbs. — The skeleton of the hind limb of the 

 frog is constructed on essentially the same plan as that of 

 the fore limb. It consists of an upper bone, \.\\e fej?iur, cor- 

 responding to the humerus of the fore limb. Below this is 

 the fibio-fibu/a, corresponding to the radio-ulna ; and follow- 



