160 COMPARISONS OF STRUCTURE IN ANIMALS. 



of the one succeeding, and so on tlirougliout 

 the chain. Tliese joints are farther strength- 

 ened by capsular ligaments, and besides this, 

 the obhque or articulating processes of each 

 vertebra are locked to those of the next by 

 ligaments; and thus, while almost unlimited 

 freedom is allowed, the security of the joints 

 is amply provided for. The spinous pro- 

 cesses of the vertebrae are short; -and these, 

 with the mode in which the oblique pro- 

 cesses are united, tend, in some degree, to 

 limit the backward flexure of the spine, espe- 

 cially in the rattlesnake, in which the spinous 

 processes are more developed than in most 

 other species. The slender tree-snakes, how- 

 ever, can twine their bodies in any direction. 

 The ribs of snakes are attached to the trans- 

 verse processes of the vertebra, which latter 

 offer a convex, articulating surface, fitting 

 into a concavity at the head of the rib. 

 This articulation permits the ribs to play 

 backward and forward very freely, and they 

 are tmder the agency of five distinct sets 

 of muscles, disposed in regular layers, the 

 contractions of which operate in bringing 

 them forward, or drawing them backward. 

 In these active and often c]angerous reptiles, 



