ORGANS OP THE SENSES. 153 



Sauria, as Lacerta, Iguana, Monitor. The cornea is more convex 

 than in Fishes, and the anterior chambers of the eye of various 

 dimensions. The choroid coat is very thick, frequently covered ex- 

 ternally and internally with a dense layer of black pigment ; the 

 ciliary body usually gives off some projecting folds, or ciliary pro- 

 cesses. The iris in the Batrachia is invested in front, with a 

 gold-colored pigment. The pupil is capable of a slight degree of 

 expansion ; for the most part of a circular form, as in Pipa ; it fre- 

 quently, however, as in the Frog and Salamander, presents the 

 figure of a transversely directed nearly rhombic oval ; in the Croc- 

 odile, and many Ophidia, as the Viper, and Rattle-snake, but 

 without bearing any reference to the poisonous qualities of the 

 Serpent, the pupil forms a perpendicular slit. The optic nerve per- 

 forates the tunics of the eye in the direction externally and in- 

 feriorly, but usually enters the eye of a rounded shape. The 

 papillary layer of the retina is very greatly developed. The 

 vitreous humor is mostly small in proportion to the lens, which 

 last is in those Reptiles that live in water, very spherical, or else 

 compressed, and frequently, as in Emys, elliptical. In several 

 Sauria, as in Lacerta, Iguana, Chameleo, Monitor, a feebly-de- 

 veloped process from the choroid, slightly plicated, and invested 

 by a layer of pigment, is prolonged into the interior of the eye, 

 from the point of entrance of the optic nerve, and appears to be 

 analogous to the pecten in the eye of the Bird, though it never has 

 so many folds, there being occasionally only two. The globe of the 

 eye is usually moved by six muscles, four recti, and two oblique or 

 rotator muscles. To those is superadded in the Frog an infundi- 

 buniform muscle (suspensorius oculi) divided into several fasciculi, 

 which arises, in a manner perfectly similar to that described in the 

 Mammalia, from the bottom of the orbitar cavity, and is attached to 

 the posterior part of the eyeball. 



The Protective and Glandular appendages of the eye exhibit 

 very diversified degrees of development in the Reptilia. In the 

 Ichthyodea, the external integument is partly continued over 

 the eye, as a transparent lamella. The Squamigerous Reptiles 

 have for the most part a superior, and larger inferior eyelid pro- 

 vided with a cartilaginous plate, and usually very moveable, and 

 in addition to these, a third internal eyelid, or nictitating mem- 

 brane. Numerous varieties however occur ; thus in Scincus, and 

 Gecko, both eyelids are small and immoveable ; the Salamanders 

 possess only two short eyelids, but the Frog the additional third 



