PROGRESSIVE MOTION IN SERPENTS. 405 



one that approaches to a triangle, of which the sur- 

 face applied to the ground forms the base. Five 

 sets of muscles aie provided for the purpose of 

 giving to the ribs the motions backwards and for- 

 wards, by which, as levers, they effect this species 

 of progression. These muscles are disposed in re- 

 gular 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. 



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 being 

 fixed, and the head projected forwards. By a suc- 

 cession of these alternate movements, assisted by 

 the actions of the ribs, serpents are enabled to glide 

 onwards with considerable rapidity, and without 

 attracting observation. But where greater expe- 

 dition 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 

 points of support, are made successively to approach 

 and to separate from each other ; the body being 



