OPHIDIA. 



455 



retracted at will, being in constant vibration when protruded, 

 and being in great part concealed by a sheath when re- 

 tracted. 



As regards the eye of Serpents (fig. 196, A), the chief 

 peculiarity lies in the manner in which it is protected exter- 

 nally. There are no eyelids, and hence the stony unwinking 

 stare of all snakes. In place of eyelids, the eye is surrounded 

 by a circle of scales (e e), to the circumference of which is 

 attached a layer of transparent epidermis, which covers the 

 whole eye (//), and is termed the antocular membrane. This 

 is covered internally by a thin layer of the conjunctiva, which 

 is reflected forwards from the conjunctiva covering the ball of 

 the eye itself. In this way a cavity or chamber is formed 

 between the two layers of conjunctiva, and the lachrymal 

 secretion by which the eye is moistened is received into this. 



Fig. 196. 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 

 surrounding the eye. B, Head of the common Viper (Pelias bents) after Bell 

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



The outer epidermic layer (antocular membrane) covering the 

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

 epidermis, the animal being rendered thereby blind for a few 

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

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



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, 

 in consequence of the quadrate bone being connected with a 

 movable squarnosal bone, and in consequence of the rami of 

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

 the mouth in the Snakes is capable of opening to an enormous 

 width, and the most astonishing feats in the way of swallowing 

 can be performed. Secondly, this structure of the jaws accords 



