REPTILES 1389 



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 provided for the purpose 

 of giving to the ribs the motions backward and for- 

 ward, 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 at- 

 tached to the succeeding rib. In all snakes the ribs 

 are continued backward much beyond the region oc- 

 cupied by the lungs; and although the anterior set 

 are subservient to respiration, as well as to progres- 

 sive 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, con- 

 sisting 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 forward. By an alternation of these move- 

 ments, assisted by the actions of the ribs, the serpent 

 is enabled to glide onward with considerable rapid- 

 ity, and without attracting observation. But where 

 greater expedition is necessary, they employ a more 

 hurried kind of pace, although one which exposes 

 them more to immediate view. The body, instead 

 of being bent from side to side, is raised in one great 

 arch, of which the two extremities alone touch the 

 ground; and these being alternately employed as 



