120 



GKXF.RAL TRIXCirLES OF ZOOLOGY 



eye is therefore developed as a camera ohsciira, the space lietween retina 

 and lens being tilled \>x the I'llrcoiis hodv (transparent cellsor jelh' — tig. 84"). 

 The amount of light is regulated by an iris, a pigmented membrane with 

 circular opening, the pupil, the width of wliich is enlarged or contracted 

 in accordance mth the intensity of the light. Then nutrition is provided 

 by a richly vascular coat, the cliorioiJca, and for protection there is a tirni 

 outer coat, the sclera. These accessory structures are developed and 

 comlnned in the most diverse ways in the diti'erent classes of animals; 

 eyes which are very similar in structure, like those of vertebrates and 

 cephalopiods (.figs. So, 340), ha\-e developed along entirely different 

 ontogenetic and phylogenetic lines. 



C NO VO 

 Fig. So. — Horizontal section through the human eye (after .Vrll, from Ilalschck). 

 E, epithelium of the cornea (conjunctiva); O, cornea; vA, anterior cliaraber of the 

 eye; /, iris; liA, posterior chamber of the eye; Z, zonula Zinnii; Os, ora serrata; ^V 

 sclerotic coat; CIi, choroidea; R. retina; />, papilla of optic nerve; 111, macula lutea' 

 area of most distinct vision; '['O, shealh of the optic nerve; NO, optic nerve; C, 

 arteria centralis; Cc, corpus cillare; L, lens; C';', \itreous bodj-. 



The Eye of the Vertebrates.— The eye of the vertebrates usuallv is an 

 approximately spherical body. Over the greater part of the circumference 

 there is an opaque, fibrous or cartilaginous .vc/itiz, or sclcrolica, transparent only 

 in the most anterior part, where it forms a projecting portion like a watch-glass, 



