82 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



We are now in position to describe the eye of the adult ver- 

 tebrate. The eye proper is approximately spherical, although, 

 as in fishes, it may be flattened, or, as in birds, somewhat coni- 

 cal in front. In the ichthyopsida it is without any well-devel- 

 oped external accessories for protection ; l but in the amniotes 



n sh 



FIG. 88. Horizontal section through human eye, from Hertwig after Arlt. a, 

 arteria centralis ; ac, anterior chamber of eye; r, cornea; ch, choroid; cj, conjunc- 

 tiva; cp, ciliary process; z, iris; /, lens; m, macula lutea, point of distinct vision; 

 n, optic nerve ; a, ora serrata ; /, papilla of optic nerve ; pc^ posterior chamber of 

 eye; r, retina; 5, sclerotic; sh t sheath of optic nerve; v, vitreous body ; z, zonula 

 Zinnii. 



movable lids, which can close over the organ, occur. There 

 are typically three of these folds of the skin, an upper and a 

 lower lid, moving in a vertical plane, and inside of these a third 

 transparent lid, the nictitating membrane, which is attached at 

 the anterior or inner angle, and which closes horizontally. In 



1 Some salamanders have feebly developed eyelids. 



