2537 



riorly the ribs, as shown in the figure on p. 2463, articulate by a 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 zygosphenes, but in snakes (as shown in 

 the figure below) these are invanably present; and it is owing to this complicated 

 system of articulation that a snake is able to make the wonderful foldings and con- 

 tortions characteristic of its kind without fear of dislocating its spine. It may be 

 added that no snake has any trace of a breastbone, 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 limb- 

 less lizards, walks entirely by the aid of its ribs, which are but very loosely articu- 

 lated 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 mo- 

 tions of snakes are in general very quick, and may be adapted to every variation of 



SKELETON OF SNAKE. 



ground over which they move, yet all the varieties of their locomotion are founded 

 on the following simple process: When a part of their body has found some pro- 

 jection 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 projec- 

 tion, 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 



