REPTILIA. 579 



anterior margin : these are obviously the rudiments of that os- 

 seous zone which in the class of Birds, as we shall find, performs 

 a very important office. The ciliary processes of the choroid 

 are generally very feebly developed. The pupil is frequently 

 round, but it is sometimes of a rhomboidal figure, as for example 

 in the Gecko ; and in the Crocodile and some serpents the pupil- 

 lary aperture is a vertical fissure like that of a Cat. 



The optic nerve enters the eye in the same way as in qua- 

 drupeds, and, having passed the choroid, it terminates in a round 

 papilla, from the margin of which the retina spreads out : as to 

 the rest, the eye of a Reptile differs so little in any essential cir- 

 cumstance from that of Man as to render any more elaborate 

 description superfluous. 



The eye-ball is moved by six muscles, disposed as in Fishes ; the 

 four recti arising from the margin of the optic foramen, while the 

 two obliqui are derived from its anterior margin. 



In Fishes, from the circumstances under which they live, there 

 is no occasion for the presence of any lacrymal apparatus, or for 

 eyelids adapted to defend and moisten the surface of the cornea ; 

 but in the class before us, especially in the more elevated tribes, 

 these appendages to the eye make their appearance, and gradually 

 assume a complexity of structure even greater than that which they 

 present in the human subject. 



In Serpents, and in some of those Lizards which are most 

 nearly allied to the Ophidians, there are still no eyelids ; and con- 

 sequently in such genera there can be neither any lacrymal appa- 

 ratus, nor a conjunctiva, properly so called : the skin of the head 

 merely passes like a delicate film over the transparent cornea, 

 offering no fold worthy of the name of an eyelid. 



In ordinary Lizards* the skin forms a kind of veil stretched 

 over the orbit, and pierced by a horizontal fissure, which is closed 

 by a sphincter muscle. The lower eyelid is the most moveable, 

 and encloses a small cartilaginous plate; and there is besides ge- 

 nerally a fold of the conjunctiva at the inner canthus of the 

 eye, which is the first appearance of a third eyelid or membrana 

 nictitans. 



In the Chelonian Reptiles, and in the Crocodiles, the upper 

 and lower eyelids are sufficiently perfect accurately to close the 

 eye ; but there are no eyelashes as yet present. Moreover, these 



* Cuv. Le9<>ns d'Anat. Comp. vol. ii. p. 433. 



