282 I'HILOSOPHY OF ZOOLOGY, 



moved, and in the vertebrae. The body, in general, is of 

 a rounded form, but, when preparing for progressive mo- 

 tion, the ribs are drawn somewhat dorsally, so as to flatten 

 the scales of the belly, and by moving anteriorly or poste- 

 riorly, give to the scales, with which they are connected, a 

 corresponding degree of motion. The ribs in this case act 

 as limbs to the scales, which may be compared to feet. 

 This singular use of the ribs of snakes, in assisting progres- 

 sive motion, was detected by the acute Tyson, in his ad- 

 mirable anatomical examination of the rattlesnake *. The 

 same subject was still farther illustrated by Sir Everard 

 Home, in his dissection of the Boa constrictor +. 



Besides the capabilities of executing progressive motion, 

 many serpents can twist their bodies round the branches of 

 trees, or suffer a considerable portion to hang down. In 

 this attitude, the larger kinds are ready to fall down upon 

 their prey passing beneath, such as deers and antelopes. 

 Such animals are not only retarded by their weight, but 

 incommoded by the foe twisting itself in wreaths round 

 their body, and by contractile efforts crushing it to death. 



This method of seizing their prey is confined to the 

 larger kinds. The smaller sorts are able, by their mouth 

 and teeth, to seize and retain their victims. There is no 

 mastication, the food being swallowed entire. To facilitate 

 the deglutition, the under jaw consists of two bones, as in 

 birds, and, like tliese animals, they are joined to the cra- 

 nium by the intervention of a bone, similar to the os qua- 

 dratum. The upper jaw is also loosely connected with the 

 head, and, in some species, admits of considerable motion 

 at the point of junction. The moutii can thus be opened 

 very wide, and larger animals admitted, than, from the or- 

 dinary size of those devoured, one would be led to suppose. 



* Phil. Trans. No. 144 , copied by Ray into his Synopsis Animalium, 

 p. 291. t -tbid. 1812, p. 1(J3. 



