VISION. 445 



Gaslrohranchiis, De Blainville states that it is im- 

 possible, even by the most careful dissection, to 

 discover the least trace of eyes. 



Reptiles, being destined to reside in air as well 

 as in water, have eyes accommodated to these 

 variable circumstances. By the protrusion of the 

 cornea, and the addition of an aqueous humour, 

 they approach nearer to the spherical form than 

 the eyes of fishes ; and the lens has a smaller 

 refractive power, because the principal refraction 

 is now performed by the cornea. Rudiments of 

 eye-lids are met with in the Salamander, but they 

 are not of sufficient extent to cover the whole sur- 

 face of the eyes. In some serpents, the integu- 

 ments pass over the globe of the eye, forming a 

 transparent conjunctiva, or external cornea, behind 

 which the eye-ball has free motion. This mem- 

 brane is shed, along with the cuticle, every time 

 that the serpent is moulting ; and at these epochs, 

 while the cornea is preparing to detach itself, air 

 insinuates itself underneath the external membrane 

 and renders it opaque ; so that until this operation 

 is completed, and an entire separation effected, 

 the serpent is rendered blind. Serpents have no 

 proper eyelids ; but the cornea is covered by a 

 transparent integument, which does not adhere to 

 it.* Lizards have usually a single perforated eye- 



* It was the general opinion, until very lately, that serpents are 

 unprovided with any lacrymal apparatus ; but a small lacrymal 

 passage has been recently discovered by Cloquet, leading from the 

 space in the inner corner of the eye, between the transparent 

 integument and the cornea. This lacrymal canal opens into the 

 nasal cavity in venomous snakes, and into the mouth in those that 

 are not venomous. 



