DIVISIONS OF EEPTILIA. 



265 



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

 a few forms, 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 jjrogress by means of the ribs, whicii ai'e 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 oi ribs varies 

 greatl}^ 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, Diagraimiiatic ser'tion of the eye of a Viper (after Cloqiiet) ; a Eycliall : 

 b Optie nerve : c Clianilier into wliich the tears are poured ; d Epideniiic layer 

 co^-ering the eye. B, Head of tlie common Viper (after Bell), showing the poisuu- 

 fangs. 



Their movements are also much assisted by the extreme flexibility 

 of the whole sj)ine, caused by the cup-and-ball articulation of the 

 bodies of the vertebra^, each of ■which is concave in front, and 

 convex behind (proco^/jius). 



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 tlius formed can be 

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

 truded, and being in great part cmicealed l\v 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 tliese rep- 

 tiles are celebrated. In i)lace of eyelids, tlie outer layer of the skin 

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

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

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



