BEPTILE SKELETONS 259 



is the necessity for a stiff framework against which the legs 

 may push in propelling the body. If the backbone were 

 too flexible it would simply bend when the legs pushed 

 against it, and the animal would not move any more than 

 does an object when pushed with a feather. 



The frog skeleton will be seen to differ from that of the 

 higher forms of animals, though not so much as from that 

 of the fish. For example, the frog has a much shorter back- 

 bone than is found in higher forms, the ribs are wanting, 

 and the skull is made up largely of face bones with a very 

 tiny cranium or brain box. As a machine for support and 

 motion, it is very primitive compared with our skeleton. 

 The legs are so attached that the thigh bones point out- 

 ward from the body instead of being under the part to be 

 supported. The shoulder girdle is rigid and the elbows 

 point outward, which makes it awkward to use the front 

 legs as pushing agents. In fact, in the frog these front 

 legs are of smaU use in locomotion on land; the greatly 

 elongated hind legs being so arranged as to make leaping 

 the natural mode of progress. 



Reptile skeletons. — The reptiles differ from the amphibi- 

 ans in possessing ribs (see Fig. 109), which have been de- 



FiG. 109 — Skeleton of a crocodile. 



veloped to protect the sides of the body and to aid in the 

 operation of the lungs as breathing organs. In the snakes 

 the legs are lacking and there is no breastbone. In all the 



