ORGANIZATIONOFREPTILES. 35 



the internal towards the external angle of the eye.* The arrangement of the 

 eyelid is the same in the Toads as in the Frogs and Hylee, the upper lid being 

 larger and warty, the lower smaller and less movable. In the Salamander, the 

 structure of the eye is nearly the same as in the Frogs; but the lens has a hard 

 central portion, as in Fishes. 



The optic nerves come from different sides of the brain, decussate or cross each 

 other; and at the place of crossing in some, as in the Chelonia and Sauria, there 

 is a communication of the substance of the two nerves; while in others, as in the 

 Serpents and Batrachia, there is only a simple crossing of the nerves from the 

 right to the left side, without their contracting any union or any intermixture of 

 their substance. The optic nerves perforate directly the sclerotica, and form a 

 small rounded prominence, around which is extended the retina, which we do not 

 regard as an expansion of the optic nerve, but rather as a membrane of peculiar 

 structure on which the nerve terminates. Such being the structure of the eye in 

 Reptiles, it follows that vision is much less perfect in this class of animals than in 

 the Mammaha or Birds, nor is the organ of equal power in all the Reptile tribes. 

 The Emydes have the most acute sight; many of them, as the E. serrata and the 

 E. picta are extremely shy, and retreat suddenly when approached. Many others, 

 as various Serpents, have the eye extremely brilliant and sparkling, yet it is not 

 the brightness of eye accompanying intelligence, as observed in the higher classes, 

 but the glare of animal instinct and passion. 



Hearing. — An organ of hearing exists in all Reptiles, though much less com- 

 plete than in the Mammalia and Birds; many parts essential to the perfection of 

 the organ are wholly wanting, or at most, only slightly developed. We observe 

 a more manifest decrease in the degree of perfection in the ear than in the eye, 

 when compared with the higher classes of animals. 



The structure and arrangement of its parts vary still more in the different tribes 

 *Carus, Vergleich. Zoot. Erst. Theil., p. 395. 



