498 THE SENSORIAL FUNCTIONS. 



well as in water, have eyes accommodated 

 to these variable circumstances. By the pro- 

 trusion 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 perfonned by 

 the cornea. Rudiments of eye-lids are met 

 with in the Salamander, but they are not of suf- 

 ficient extent to cover the whole surface of the 

 eyes. In some serpents, the integuments pass 

 over the globe of the eye, forming a transparent 

 conjunctiva, or external cornea, behind which 

 the eye-ball has free motion. This membrane 

 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 sepa- 

 ration effected, the serpent is rendered blind. 

 Serpents have no proper eyelids ; but the cor- 

 nea is covered by a transparent integument, 

 which does not adhere to it.* Lizards have 



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

 are unprovided with any lacrymal apparatus; but a small la- 

 crymal passage has been recently discovered by Cloquet, leading 

 from the space in the inner corner of the eye, between the trans- 

 parent 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. 



