434 ZOOLOGY 



of the arm. The \'entral arm of the Y runs down to the sternum, 

 which affords it a firm support — this is the collar-bone. The 

 skeleton of the fore lim1) consists of the same parts as that of 

 man. They permit of a large amount of bending and rotation 

 of the appendages. The pelvic girdle forms a complete ring 

 surrounding tlie viscera of the posterior part of the abdomen. 

 It consists on each side of three pieces, fused into one in the 

 adult, which radiate from a centre as spokes from a hub, where 

 the skeleton of the leg impinges. One of these pieces passes 

 dorso-anteriorly and is fastened to the sacrum ; it is called the 

 ileum ; the other two pass ventrally, each to unite with its fellow 

 of the opposite side in the mid-ventral line. Indeed, all four 

 bones are tied together here. The anterior pair of these bones 

 is called the pubis, the posterior pair the ischium — being the 

 bone which supports us when we sit. The rigidity and size 

 of the ])elvis is necessary because it has to support the whole 

 body when the anmial sits up on its hind legs. The skeleton 

 of the hind limbs of tlie rabl^it differs little from that of man 

 except in having only four toes instead of five. 



Organs and Function of Digestion. — The teeth .show an 

 advance over those of reptiles in reduction of number, in 

 specialization of form, and in Ijeing each lodged in a special 

 socket. The nimrber of dentitions has become reduced to two, 

 the milk teeth and the permanent teeth, in contrast to the croc- 

 odiles, where the dentitions are changed throughout life. In 

 the permanent dentition the front pair of teeth are called cut- 

 ting teeth, or incisors. In rabbits the two middle incisors of the 

 upper jaw continue to grow throughout life and, as the front 

 edge onlj' has a thick layer of hard enamel, the teeth keep a 

 sharp, chisel-shaped edge ver>' useful for cutting. The per- 

 sistent growth of the teeth keeps them from wearing off com- 



