34 ORGANIZATIONOFREPTILES. 



the bony Scales surrounding the cornea, which is here still more convex; the choroid 

 coat is thinner, but covered with an abundant pigment; and the ciliary processes, 

 only slightly developed in the Chelonia, are very evident and distinct in this class 

 of animals. In Serpents the ball of the eye is nearly spherical, the sclerotica is 

 unsupported by scales, and the crystalline lens more convex than in the Saurian 

 Reptiles. The eye appears at first to be fixed, and without lids, or any lachrymal 

 apparatus; Cloquet* has, however, demonstrated the existence of both these 

 appendages to the eye of the Serpent; the lids pass over the globe of the eye, and 

 although extremely thin and transparent, are composed of three layers, the outer 

 of which is continuous with the external organs of integumation, and falls off" with 

 the cuticle when the animal sheds its skin. The lachrymal canals are minute 

 tubes, beginning between the transparent integuments and the cornea at the inner 

 canthus, and opening into the nasal cavities in venomous Snakes, and into the 

 cavity of the mouth in those that are innocuous. The Batrachia approach the 

 Fishes in their organ of vision: in the Frog, the eye is large and prominent, but 

 can be drawn at will into the cavity of the mouth; the globe is spherical, the scle- 

 rotica hard, but without scales in front; the cornea is convex and prominent, the 

 choroid dark on the posterior surface, and the ciliary processes are but partially 

 developed; a small tubercular mass that occupies nearly their relative position has 

 been regarded by Altenaf as a modification of their structure. The iris varies a 

 good deal in the Batrachia; but the tints are beautiful in all the Frogs and Toads. 

 The lens is large in this tribe of Reptiles, and spherical in form, or very nearly so. 

 Frogs have but two eyehds; the part described by some Naturalists^: as a mem- 

 brana nictitans is evidently the inferior lid, which is thin and movable, and when 

 depressed makes a fold, which fold has been considered as the lower eyehd itself. 

 To cover the anterior part of the ball of the eye, this fold must necessarily ascend 

 perpendicularly; whereas a third eyelid, wherever it exists, moves horizontally from 



* Mem. du Museum, torn. vii. p. 65. 



t Quoted by T. Bell, Esq., in an excellent article on the Amphibia in the Cyclopedia of 

 Anatomy and Physiology, Part I. p. 101. 



t Cuvier, Legons d'Anat. Comp., torn. ii. p. 433. 



