312 



THE MECHANICAL FUNCTIONS. 



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 ab- 

 sence of all external instruments of prehension and of pro- 

 gressive 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 number of small pieces; secondly, by the great free- 

 dom of their articulations; and thirdly, by the peculiar mo- 

 bility and connexions of the ribs. 



Numerous as are the vertebrae of the eel, the spine of 

 which consists of above a hundred, that of serpents is in 

 general formed of a still greater number. In the rattle- 

 snake [Crotahcs horridus) there are about two hundred; 



and above three hundred have 

 been counted in the spine of 

 the Coluber natrix. These 

 vertebrae are all united by ball 

 and socket joints, as in the 

 adult batrachia; the posterior 

 rounded eminence of each ver- 

 tebra being received into the 

 anterior surface of the next. 

 Fig. 202 is a view of this por- 

 tion of the skeleton in the Boa 

 constrictor, showing the arti- 

 culation of the ribs with the 

 vertebrae. 



While provision has thus been made for extent of mo- 

 tion, extraordinary care has at the same time been bestou'ed 

 upon the security of the joints. Thus, we find them efiectu- 



* 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. 



