404 THE MECHANICAL FUNCTIONS. 



sessed by most vertebrated animals, and charac- 

 terising tlie type of their osseous fabric. In the 

 ordinary structure, the head of each rib has a 

 convex surface, which plays either upon 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 

 upon a convex protuberance of the vertebra. This 

 structure is attended with the advantage of pre- 

 venting 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 Ophio- 

 saurus and the Blind-worm ( Anguis fragilis), there 

 is no vestige either of a sternum or scajjula, in any 

 animal of this class. Each rib terminates in a 

 slender cartilage, tapering to a point, which rests, 

 for its whole length, on 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 

 tlie ribs themselves may be consideied as perform- 

 ing 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 



