CLASS REPTILIA 



393 



418. Snakes. — Snakes differ from lizards and chameleons in the 

 structure of the lower jaws; in the absence of both free Hmbs and 



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Fig. 281. — Skeleton of a bull snake, Pituophis sayi Schlegel, showing all the ribs reaching 

 the same level. The broad ventral scutes manipulated by muscles in connection with the 

 ribs enable the snake to move about. No sternum is present. (Prepared, photographed, 

 and contributed by George E. Hudson.) 



girdles, though in a few forms a trace of the pelvic girdle is present; 

 and in the absence of a urinary bladder. 



Owing to the fact that the lower jaws 

 on the two sides are but very loosely con- 

 nected in the median line and that they 

 are also loosely attached to the quadrate 

 bone, which is in turn loosely attached 

 to the skull, the mouth may be greatly 

 expanded. Since there is no sternum and 

 the ribs are not attached ventrally, the 

 throat and body are capable of great dis- 

 tension. Thus snakes, which always swal- 

 low their food whole, are able to ingest 

 objects actually much greater in diameter 

 than the head or body of the snake itself. 

 During the process of swallowing, the teeth, 

 which point backward, are used to hold the 

 prey and those of the two sides are brought 

 into use alternately. While the teeth of the 

 jaws on one side are holding the prey, those 

 of the jaws on the other side are loosened 

 and that side of the mouth is carried forward 

 over the object being swallowed, after which 

 those teeth are again set in. Then the 

 teeth on the first side are loosened and that side of the mouth is carried 

 still farther ahead and those teeth in turn set in. Thus by working the 



Fig. 282. — A constricting snake, 

 Boa sp., clinging to a post and 

 maintaining its position for more 

 than an hour with about a third of 

 its body extended. 



