1388 THE STORY OF THE UNIVERSE 



rior extremities, possessed by most vertebrated ani- 

 mals, and characterizing the type of their osseous 

 fabric. In the ordinary structure, the head of each 

 rib has a convex surface, that plays either on the 

 body of a single vertebra with which it is connected, 

 or upon the two bodies of adjacent vertebrae: but in 

 serpents the extremity of the head of the rib has two 

 slightly concave articular surfaces, which play on a 

 convex protuberance of the vertebra. This struc- 

 ture is attended with the advantage of preventing the 

 ribs from interfering with the motions of the ver- 

 tebra? upon one another. At their lower ends the 

 ribs of one side have no connection 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 (Anguis 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 connected with the ends of the ribs, 

 and which are moved by means of short muscles, may 

 be compared 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 



