528 



COMPARATIVE PHYSIOLOGY. 



the whole eye exists for the retina, and that the entire mechan- 

 ism besides is subordinated to the formation of images on this 



Fio. 377. — Certain parts of eye. 1 x 10. (After Sappey.) 1, 1, crystalline lens; 3, 

 . hyaloid membrane; 3, zonule of Zinn; 4, iris; 5j a ciliary process; 6. radiating 

 ' fibers of ciliary muscle; 7, section of circular portion of ciliary muscle; 8, venous 

 plexus of ciliary muscle; 9, 10, sclerotic coat; 11, 18, cornea; 13, epithelial layer of 

 cornea; 14, Descemet's membrane; 16, pectinate ligament of iris; 16, epithelium 

 of membrane of Descemet; 17, union or sclerotic coat with cornea; 18, section of 

 canal of Schlemm. 



nervous expansion. The eye of the mammal may be regarded 

 as an arrangement of refracting media, protected by coverings, 

 with a window for the admission of light, a curtain regulating 

 the quantity admitted ; a sensitive screen on which the images 

 are thrown ; surfaces for the absorption of superfluous light ; 

 apparatus for the protection of the eye as a whole, and for pre- 

 serving exposed parts moist and clean. 



EmbryologicaL — We have already learned that the first indi- 

 cation of the eye is the formation of the optic vesicle, an out- 

 growth from the first cerebral vesicle. This optic vesicle be- 



