SZ MORPHOLOGY OF THE ORGANS OF rERTEBRATES. 



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 w-ithout any well-devel- 

 oped external accessories for protection ; ^ but in the amniotes 



a n sh 



Fig. SS. Horizontal section through human eye, from Hertwig after Arlt. i-', 

 arteria centralis: a:, anterior chamber of eye; t, cornea; ch, choroid: cj, conjunc- 

 tiva; i/, ciliary process; i, iris ; /, lens; m, macula lutea, point of distinct vision: 

 K, optic ner\'e ; ff, ora serrata : /, papilla of optic nerve ; /('. posterior chamber of 

 eve; r, retina: .■, sclerotic; sh, sheath of optic nerve; f, vitreous body ; _, zonula. 

 Zinnii. 



movable lids, which can close o\er 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. 



