400 THE MECHANICAL FUNCTIONS. 



they greatly resemble by the length and flexibility 

 of the spine. These peculiarities of conformation 

 may be traced in a great measure to the mode of 

 life for which they are destined. The food assigned 

 to them is living prey, which they must attack and 

 vanquish before they can convert it into nourisli- 

 ment. 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. Veno- 

 mous serpents, on the other hand, coil themselves 

 into the smallest possible space, and suddenly dart- 

 ing 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 prehen- 

 sion and of progressive motion, it is necessary 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 num- 

 ber of small pieces ; secondly, by the great freedom 

 of their articulations ; and thirdly, by the peculiar 

 mobility and connexions of the ribs. 



Numerous as are the vertebra3 of the Eel, the 

 spine of which consists of above a hundred pieces, 

 that of serpents is in general formed of a still 

 greater number. In the rattle-snake {Crotalus 

 horridus) there are about two hundred vertebrae ; 

 and above three hundred have been counted in 



