42 



surface in the front of tlie next. Thus the utmost 

 freedom of motion is allowed, and the numerous 

 powerful muscles make of the spine a rod of al- 

 most perfect elasticity, and capable of all the nec- 

 essary adjustments. 



The rihs, by which the snake must evidently 

 take its final appui for all motion, are set by pairs — 

 one rib on either side of every vertebra, so that 

 the courses of their articulations form parallel 

 lines, from head to tail, on each side of the spine. 

 These articulations are formed each by a socket of 

 two slight concavities on the upper end of the rib, 

 moving on a protuberance from the vertebra which 

 carries corresponding convexities."^ 



Thus set on, the ribs support the spine like so 

 many curved springs bowing outward. 



At their ground ends each one of a pair of ribs 

 is connected with its fellow by a ligamentous hand^ 

 and these bands offer the medium by which, in 

 transverse continuation of the lower ends of the 

 ribs, the animal takes hold of the ground. 



§ 45. If we call the position of the rihs in their 

 sockets, as the snake lies extended, their normal 

 'position, and assume, for the moment, that the rib 



* This diflers from the analogous articulalions in the higher ani- 

 mals, where the rib carries the ball, and the sockets are between 

 two vertebrae. 



