VISION. 351 



dwell in the muddy banks of rivers, as the Csecilia, and 

 Mursena casca, or blind eel, the eyes are quite rudimental, 

 and often nearly imperceptible; and in the Gastrobranchus, 

 De Blainville states that it is impossible, even by the most 

 careful dissection, to discover the least trace of eyes. 



Reptiles, being destined to reside in air as well as in wa- 

 ter, have eyes accommodated to these variable circumstances. 

 By the protrusion of the cornea, and the addition of an aque- 

 ous humour, they approach nearer to the spherical form than 

 the eyes of fishes; and the lens has a smaller refractive pow- 

 er, 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 suffi- 

 cient 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, be- 

 hind 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 pre- 

 paring 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 eye- 

 lids; but the cornea is covered by a transparent integument, 

 which does not adhere to it.* Lizards have usually a sin- 

 gle perforated eye-lid, which, when closed by its orbicular 

 muscle, exhibits merely a horizontal slit. There is also a 

 small internal fold, forming the rudiment of a third eye-lid. 

 The Chameleon has remarkably projecting eyes, to which 

 the light is admitted through a very minute perforation in 



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

 vided 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 la- 

 crymal canal opens into the nasal cavity in venomous snakes, and into the 

 mouth in those that are not venomous. 



