PROGRESSIVE MOTIOlff IN SERPENTS. 315 



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 sur- 

 faces, which play on a convex protuberance of the vertebra. 

 This structure is attended with the advantage of preventing 

 the ribs from interfering with the motions of the vertcbrse 

 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 

 Ophiosaiirus 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 a 

 plane, it alters its shape from a circular or oval form, to one 

 that approaches to a triangle, of which the surface applied 

 to the ground forms the base. Five sets of muscles are pro- 

 vided for the purpose of giving to the ribs the motions back- 

 wards and forwards, by which, as levers, they effect this 

 species of progression. These muscles are disposed in regular 

 layers ; some passing over one or two ribs to be attached to 

 the succeeding rib. In all snakes the ribs are continued 

 backwards much beyond the region occupied by the lungs; 

 and although the anterior set are subservient to respiration, 

 as well as to progressive motion, it is evident, that all those 

 posterior to the lungs must be employed solely for the latter 

 of these purposes. 



