498 THE SENSORIAL FUNCTIONS. 



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 and 

 aqueous humour. Rudiments of eye-lids are 

 met with in the Salamander, but they are not of 

 sufficient extent to cover the whole surface of 

 the eyes. In some serpents, the integuments 

 pass over the globe of the eye, forming a transpa- 

 rent 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 prej^aring 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. 



