706 EEPTILIA. 



figure, as for example in the Gecko ; and in the Crocodile and some 

 Serpents the pupillary aperture is a vertical fissure like that of a Cat. 



(1985.) 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 circumstance from that 

 of Man as to render any more elaborate description superfluous. 



(1986.) The eyeball 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 the anterior margin of the orbit. 



(1987.) 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. 



(1988.) In Serpents, and in some of those Lizards which are most 

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

 quently in such genera there can be neither any lacrymal apparatus 

 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. 



(1989.) 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, generally a fold of the 

 conjunctiva at the inner canthus of the eye, which is the first appear- 

 ance of a third eyelid or membrana nictitans. 



(1990.) 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 animals 

 possess an additional eyelid or nictitating membrane, similar to that of 

 Birds, which can be drawn at pleasure over the front of the eye, so 

 as entirely to conceal it. This is effected by a special muscle provided 

 for the purpose, which arises from the posterior part of the globe of the 

 eye, and, after winding round the optic nerve, passes beneath the eye- 

 ball, to be inserted into the free margin of the membrana nictitans. 

 In Frogs and Toads the upper and lower eyelids are nearly motionless ; 

 but the third is largely developed, and moved in the same way as that 

 of the Crocodile. 



(1991.) In the higher Reptilia a distinct lacrymal gland and puncta 

 lacrymalia are met with, occupying the same positions as those of the 

 human subject. 



* Cuvicr, Lc9ons d'Anat. Comp. torn. ii. p. 433. 



