456 THE MECHANICAL FUNCTIONS. 



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 propelled by bringing it from a 

 curved to a straight line. 



There is yet a third kind of motion, which 

 serpents occasionally resort to, when springing 

 upon their prey, or when desirous of making a 

 sudden escape from danger. They coil them- 

 selves into a spiral, by contracting all the 

 muscles on one side of the body, and then, 

 suddenly throwing into violent action all the 

 muscles on the opposite side, the whole body is 

 propelled, as if by the release and unwinding of 

 a powerful spring, with an impulse which raises 

 it to some height from the ground, and projects 

 it to a considerable distance. 



Thus these animals, to which Nature has 

 denied all external members, are yet capable, 

 by the substitution of a different kind of me- 

 chanism, 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 ra- 

 pidly across the plain, and of executing leaps 

 with a vigour and agility which astonish the 

 beholder, and which, in ages of ignorance and 

 superstition, were easily ascribed to supernatural 

 agency. 



