406 THE MECHANICAL FUNCTIONS. 



propelled by bringing it from a curved to a straight 

 line. 



There is yet a third kind of motion, which ser- 

 pents occasionally 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 throwing into vio- 

 lent 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 sub- 

 stitution of a different 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 

 executing leaps with a vigour and agility which 

 astonish the beholder, and which, in ages of igno- 

 rance and superstition, were easily ascribed to super- 

 natural agency. 



§ 4. Sauria. 



The instruments of progressive motion are more 

 efficient in the class of Saurian reptiles, which in- 

 cludes all the Lizard tribes, than in the Batrachia. 

 Several links of connexion with the preceding class 

 may still be noticed, marking the progress of de- 

 velopement, as we follow the ascending series of 

 animals. Rudiments of the bones of the extre- 



