PROGRESSIVE MOTION IN SERPENTS. 455 



the base. Five sets of muscles are provided for 

 the purpose of giving to the ribs the motions 

 backwards 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 ante- 

 rior 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. 



It is easy to understand how the serpent can 

 slowly advance, by this creeping, or vermicular 

 motion, consisting in reality of a succession of 

 very short steps. But its progress is accelerated 

 by the curvatures into which it throws its body ; 

 the fore part being fixed, and the hind part 

 brought near to it ; then, by a reverse process, 

 the hind part is fixed, and the head projected 

 forwards. By an alternation of these move- 

 ments, assisted by the actions of the ribs, the 

 serpent is enabled to glide onwards with con- 

 siderable rapidity, and without attracting obser- 

 vation. But where greater expedition is neces- 

 sary, they employ a more hurried kind of pace, 

 although one which exposes them more to im- 

 mediate view. The body, instead of being bent 

 from side to side, is raised in one great arch, 





