176 SNAKES. 



single head with a facet on the side of each vertebra, in the same manner as in 

 lizards. Only certain groups of lizards have the vertebrae with the additional 

 articular facets on the front and back surfaces known as zygantra and zygo- 

 sphenes, but in snakes (as shown in the figure below) these are invariably 

 present ; and it is owing to this complicated system of articulation that a snake 

 is able to make the wonderful foldings and contortions characteristic of its kind 

 without fear of dislocating its spine. It may be added that no snake has any 

 trace of a breast-bone, nor any vestige of a pectoral arch, there being no rudiments 

 of either blade-bone, coracoid, or collar-bone. When progressing on a firm surface, 

 an ordinary snake, in common with the limbless lizards, walks entirely by the aid 

 of its ribs, which are but very loosely articulated to the vertebrae, and thus readily 

 admit of a large amount of motion. In describing their mode of progression, Dr. 

 Giinther remarks that " although the motions of snakes are in general very quick, 

 'and may be adapted to every variation of ground over which they move, yet all 



SKELETON OF SNAKE. 



the varieties of their locomotion are founded on the following simple process. 

 When a part of their body has found some projection of the ground which affords 

 it a point of support, the ribs, alternately of one and the other side, are drawn 

 more closely together, thereby producing alternate bends of the body on the 

 corresponding side. The hinder portion of the body being drawn after, some part 

 of it finds another support on the rough ground or a projection, and the anterior 

 bends being stretched in a straight line the front part of the body is propelled in 

 consequence. During this peculiar kind of locomotion, the numerous broad shields 

 of the belly are of great advantage, as, by means of the free edges of those shields, 

 they are enabled to catch the smallest projections on the ground, which may be 

 used as points of support. Snakes are not able to move over a perfectly smooth 

 surface." It may be added that a snake is only able to move by lateral undulations 

 of its body in a horizontal plane ; and that the pictures often seen in which these 

 reptiles are depicted as advancing with the folds of the body placed in a vertical 

 plane are altogether erroneous. In conformity with their elongated bodies, the 



