454 THE MECHANICAL FUNCTIONS. 



convex protuberance of the vertebra. This 

 structure is attended with the advantage of 

 preventing the ribs from interfering with the 

 motions of the vertebrae upon one another. At 

 their lower ends the ribs of one side have no 

 connexion with those of the other, nor are they 

 joined to any bone analogous to a sternum : 

 for, except in the Ophiosaurus and the Blind- 

 worm (Auguis fragilis), there is no vestige 

 either of a sternum or scapula, in any animal 

 of this class. Each rib terminates in a slender 

 cartilage, tapering to a point, which rests, for its 

 whole length, upon the upper surface of one 

 of the scuta, or broad scales on the lower side of 

 the body. These scuta, which are thus con- 

 nected with the ends of the ribs, and which are 

 moved by means of short muscles, may be com- 

 pared to hoofs, while the ribs themselves may 

 be considered as performing the office of legs. 

 The ribs move in pairs ; and the scutum under 

 each pair, being carried along with it in all its 

 motions, and laying hold of the ground by its 

 projecting edge, becomes a fixed point for the 

 advance of the body. This motion. Sir E. 

 Home observes, is beautifully seen when a 

 snake is climbing over an angle to get upon a 

 flat surface. When the animal is moving on a 

 plane, it alters its shape from a circular or oval 

 form, to one that approaches to a triangle, of 

 which the surface aj^plied to the ground forms 



