THE NERVOUS SYSTEM 



393 



but each ocellus gives 

 a complete image, 

 although a different 

 perspective from its 

 neighbor. The mul- 

 tiplied images are re- 

 duced to one mental 

 stereoscopic picture, 

 on the principle of 

 single vision in our- 

 selves. 



The eyes of the 

 cuttlefish are the 

 largest and the most 



perfect among mver- FlG 353 ._EyeofaBeetle(A/*&/o*A): A,-section; 

 TheV re- a ' P^ C ganglion ; b, secondary nerves ; c, retina; 

 d, pigment layer; e, proper optic nerves; B, group 



the eyeS Of of ocelli (magnified); f, bulb of optic nerve; g, layer 

 t , i of pigment; k, vitreous humor; /, cornea. 



higher animals in 



having a crystalline lens with a chamber in front (open, 

 however, to the sea water), and a chamber behind it 



filled with " vitre- 

 ous humor." 



The eye of ver- 

 tebrates is formed 

 by the infolding of 

 the skin to create 

 a lens, and an out- 

 growth of the brain 

 to make a sensitive 

 layer ; both inclosed 



FIG. 354. Section of Human Eye : a. and b, upper and . , . 



lower lid; c, conjunctiva, or mucous membrane, lining in a WJllte 

 the inner surface ; d, external membrane ; e, sheath of 

 optic nerve ; f, g, muscles for rolling the eye up or 

 down; h t sclerotic; i, transparent cornea; j, choroid; of tOUffh tiSSUC with 

 k, I, ciliary muscle for adjusting the eye for distance; 



m, iris and pupil; , canal; 0,retina; s, vitreous humor; a transparent front, 

 f, crystalline lens ; v, anterior chamber ; x. posterior , , , , 



chan ; b e r . called the cornea. 



