SERPENTS. 449 



The food assigned to them is living prey, which 

 they must attack and vanquish before they can 

 convert it into nourishment. The usual mode 

 in which the boa seizes and destroys its victims 

 is by coiling the hinder part of its body round 

 the trunk or branch of a tree, keeping the head 

 and anterior half of the body disengaged ; and 

 then, by a sudden spring, fastening upon the 

 defenceless object of its attack, and twining round 

 its body, so as to compress its chest, and put a 

 stop to its respiration. Venomous serpents, on 

 the other hand, coil themselves into the smallest 

 possible space, and suddenly darting upon the 

 unsuspecting or fascinated straggler, inflict the 

 quickly fatal wound.* 



It is evident, from these considerations, that, 

 in the absence of all external instruments of 

 prehension and of progressive motion, it is neces- 

 sary that the spine should be rendered extremely 

 flexible, so as to adapt itself to a great variety of 

 movements. This extraordinary flexibility is 

 given, first, by the subdivision of the spinal 

 column into a great number of small pieces ; se- 

 condly, by the great freedom of their articu- 

 lations ; and thirdly, by the peculiar mobility 

 and connexions of the ribs. 



* Their prey is swallowed entire ; and therefore, as we shall 

 afterwards find, the bones of the jaws and face are formed to 

 admit of great expansion, and of great freedom of motion upon 

 one another. 



VOL. I. G G 



