120 



GENERAL PRINCIPLES OF ZOOLOGY 



eye is therefore developed as a camera obscura, the space between retina 

 and lens being filled by the vitreous body (transparent cells or jelly fig. 84). 

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

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

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

 by a richly vascular coat, the chorioidea, and for protection there is a firm 

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

 combined in the most diverse ways in the different classes of animals ;- 

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

 cephalopods (figs. 86, 349), have developed along entirely different 

 ontogenetic and phylogenetlc lines. 



CNO VO 



FIG. 86. Horizontal section through the human eye (after Arlt, from Hatschek). 

 E, epithelium of the cornea (conjunctiva); C, cornea; vA, anterior chamber of the 

 eye; /, iris; hA, posterior chamber of the eye; Z, zonula Zinnii; Os, ora serrata; Sc, 

 sclerotic coat; Ch, choroidea; R, retina; />, papilla of optic nerve; m, macula lu'tea' 

 area of most distinct vision; VO, sheath of the optic nerve; NO, optic nerve; C, 

 arteria centralis; Cc, corpus ciliare; L, lens; Cv, vitreous body. 



The Eye of the Vertebrates. The eye of the vertebrates usually is an 

 approximately spherical body. Over the greater part of the circumference 

 there is an opaque, fibrous or cartilaginous sclera, or sderotica, transparent only 

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



