402 THE MECHANICAL FUNCTIONS. 



dered necessary by the habit of the serpent of 

 gorging its prey entire. 



The mode in which the Boa exerts a powerful 

 pressure on the bodies of the animals it has seized, 

 and which it has encircled within its folds, required 

 the ribs to be moveable laterally, as well as back- 

 wards, in order to yield to the force thus exerted. 

 The broad convex surfaces on which they play give 

 them, in this respect, an advantage, which the ordi- 

 nary mode of articulation would not have afforded. 

 The spinous processes, in this tribe of serpents, are 

 short and widely separated, so as to allow of flexion 

 in every direction. In the Rattle-snake, on the 

 other hand, their length and oblique position are 

 such as to hmit the upward bending of the spinal 

 column, although, in other respects, its motion is 

 not restricted. The vertebrae at the end of the tail 

 are furnished with broad transverse processes for 

 the attachment of the first joints of the rattle. 



But of whatever variety of flexions we may 

 suppose the lengthened body of a serpent to be ca- 

 pable, it will, at first view, be difficult to conceive 

 how these simple actions can be rendered subser- 

 vient to the purposes of progression on land ; and 

 yet experience teaches us that few animals advance 

 with more celerity on the surface of the ground, or 

 dart upon their prey with greater promptitude and 

 precision. They raise themselves without difficulty 

 to the tops of the highest trees, and escape to their 

 hiding places with a quickness which eludes obser- 

 vation, and baffles the efforts of their pursuers. 



The solution of this enigma is to be sought for 

 partly in the structure of the skin, which, in almost 

 every species, is covered with numerous scales ; and 



