316 THE MECHANICAL FUNCTIONS. 



It is easy to understand how the serpent can slowly ad- 

 vance, 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 alteration of these 

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

 is enabled to glide onwards with considerable rapidity, and 

 without attracting observation. But where greater expedi- 

 tion 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 sepa- 

 rate from each other, the body being propelled by bringing 

 it from a curved to a straight line. 



There is yet a third kind of motion, which serpents oc- 

 casionally resort to, when springing upon their prey, or 

 when desirous of making a sudden escape from danger. 

 They coil themselves into a spiral, by contracting all the 

 muscles on one side of the body, and then, suddenly throw- 

 ing into violent action all the muscles on the opposite side, 

 the whole body is propelled, as if by the release and un- 

 winding of a powerful spring, with an impulse which raises 

 it to some height from the ground, and projects it to a con- 

 siderable distance. 



Thus these animals, to which nature has denied all exter- 

 nal members, are yet capable, by the substitution of a differ- 

 ent kind of mechanism, still constructed from the elements 

 belonging to the primitive type of vertebrated animals, of 

 silently gliding along the surface of the earth, of creeping 

 ^up trees, of striding rapidly across the plain, and of exe- 

 cuting leaps w^ith a vigour and agility which astonish the 

 beholder, and which, in ages of ignorance and superstition, 

 were easily ascribed to supernatural agency. 



