VERTEBRATA: REPTILIA. 



539 



ball of the eye in front, is periodically shed with the rest of the 

 epidermis, the animal being rendered thereby blind for a few 

 days. The pupil of the eye is round in most Snakes, but forms 

 a vertical slit in the venomous Serpents and in the Boas. 



Fig; 295. The Rattlesnake (C retains horridtis). 



As regards the dental and maxillary apparatus of the 

 Serpents, the following points require notice : Firstly, in 

 consequence of the articulation of the lower jaw with a 

 movable quadrate bone, which is often directed backwards, 



Fig. 296. A, Diagram of the eye of a Serpent (after Cloquet) : a Ball of the eye covered 

 by a conjunctival sac, into which the lachrymal secretion is discharged ; b Optic 

 nerve ; d Antocular membrane, formed by the epidermis ; e e Ring of scales surround- 

 ing the eye. B, Head of the common Viper (Pelias berus) after Bell showing the 

 bifid tongue, and the poison-fangs in the upper jaw. 



in consequence of the quadrate bone being connected with a 

 movable squamosal bone, and in consequence of the rami of 

 the jaw being united in front by ligaments and muscles only, 



