130 



GENERAL PRINCIPLES OF ZOOLOGY. 



considerable thickness of the vertebrate retina is due to the fact 

 that it inch^des the optic ganglion. The joarts (fig. 82) called 

 reticular layers, inner granular layer, ganglion cells, and nerve- 

 fibre layer, constitute the optic ganglion ; the layer of visual cells 

 consists only of the outer granular layer and the connected rods 

 and cones. The outer granules are the nuclei of the visual cells 

 to which rods and cones belong. 



Accessory Structures. — The ej^e may be further complicated 

 by special refractive bodies (cornea, lens, vitreous body) which 



c NO ro 



FiQ. 83— Horizontal section through the human eye. (After Arlt, from Hatschek.) 

 7*J, epithelium of the cornea (conjunctiva) ; C, cornea ; r-4, anterior chamber of 

 the eye; J, iris; hA, posterior chamber of the eye; Z, zonula Zinnii: Os, ora ser- 

 rata ; Nc, sclerotic coat ; (7(1, choroidea ; B, retina ; ji, papilla of optic nerve ; iii, 

 macula lutca, area of most distinct vision; I'd, sheath of the optic nerve; A'O, 

 optic nerve; C, arteria centralis; Cc, corpus ciliare; _L, lens; Cr, vitreous body. 



concentrate the light in order to cast an image upon the retina; 

 and an iris to regulate the amount of light. Then, too, means for 

 nutrition (the choroid coat) and for j^rotection (sclerotic coat) 

 must be provided. If all these parts be present, a structure results 

 Kucli as is found in the s(piid and in the vertebrates (fig. 83). 



