DIVISIONS OF REPTILIA. 



265 



breast- bone ; nor, as a rule, are there any traces of hind-limbs. In 

 a few foi-ms, however, the hind-limbs are present in an extremely- 

 rudimentary form, never exhibiting any outward evidence of their 

 existence, beyond the occasional presence of short horny claws or 

 spurs. In the entire absence, then, or rudimentary condition of the 

 limbs, the snakes progress by means of the ribs, which are always 

 excessively numerous, and, in the absence of a breast-bone, are also 

 extremely movable. Their free ends, in fact, are simply attached 

 by muscular fibres to the scales or " scutes," which cover the lower 

 or abdominal surface of the animal. The number of ribs varies 

 gi'eatly, and by means of the above arrangement the snakes are able 

 to progress rapidly, walking, as it were, upon the ends of the ribs. 



Fig. 190. — A, Diagrammatic section of the eye of a Viper (after Cloquet) : a Eyeball ; 

 h Optic nerve : c Chamber into which the tears are poured ; d Epidermic layer 

 covering the eye. B, Head of tlie common Viper (after Bell), showing the poison- 

 fangs. 



Their movements are also much assisted by the extreme flexibility 

 of the whole spine, caused by the cup-and-ball articulation of the 

 bodies of the vertebrae, each of which is concave in front, and 

 convex behind {proecelous). 



Of the other characters of the snakes, a few words may be said as 

 to the tongue, the eye, and the teeth — all important structures in 

 this order. The tongue, in serpents, is probably more an organ of 

 touch than of taste, and consists of two muscular cylinders, which 

 are united towards their bases. The forked organ thus formed can be 

 protruded and retracted at will, being in constant vibration when pro- 

 truded, and being in great part concealed by a sheath when retracted. 

 The eye of serpents (fig. 190, A) is not protected by any eyelids, and 

 hence the peculiar stony and unwinking stare for which these rep- 

 tiles are celebrated. In place of eyelids, the outer layer of the skin 

 is prolonged over the eye as a continuous and transparent film, 

 behind which is a chamber formed by the mucous covering of the 

 eye, into which the tears are discharged. The outer membrane is 



