ORGANIZATIONOFREPTILES. 33 



in its external covering, size of the lens, imperfect developement of the ciliary 

 processes, and limited mobility of the iris. 



The ball of the eye is generally spherical in shape, and as in the Mammalia, 

 consists of several different structm'es: there are external membranous coverings 

 for retaining all its parts in their proper relative position, vascular portions to 

 nourish the whole, transparent media to refract the rays of light and concentrate 

 them to one point, and a pulpy substance on which they fall and form an image 

 of the object from which they are reflected. Many other parts are always found 

 subservient to the globe of the eye; glands for secreting the tears, to wash its 

 anterior surface; canals through which the tears afterwards pass to the nasal 

 cavities, or to the mouth; together with several curious muscles to move the eye 

 in various directions. 



In the Chelonian Reptiles the eye is most perfect, though it even varies here 

 according to the habits of the animal. The sclerotic coat is thin, but hard and 

 resistant, its anterior part being strengthened by bony scales which surround and 

 support the oval cornea; the choroid is thick, with its ciliary processes but slightly 

 developed: the retina too, is thick, and spread out around the entrance of the optic 

 nerve. The crystalline lens, convex in all the Chelonia, is most so in those species 

 that are aquatic in their habits, and is more convex on the anterior than on the 

 posterior surface.* The iris varies exceedingly in Reptiles, not only in colour, 

 but in the extent and rapidity of its motions; these are most remarkable in the 

 Green Turtle, when exposed to a strong light. The eyehds in some of the Chelonia 

 are thick and covered with scaly plates; they are smooth in many others; the 

 inferior lid, in all, is the larger and more movable. There is a third eyelid or 

 membrana nictitans extended from the internal canthus over the ball of the eye, 

 as in Birds, but less developed, less complete, and less movable. In the Sauria, 

 the form of the eye and general disposition of its individual parts, are very nearly 

 similar to those of the Chelonia; but the sclerotica is thinner, and in many, wants 



* Albers. Denkschrift der Milnch. Akad., 1808, § 84. 

 Vol. I. — 5 



