182 



COMPARATIVE ZOOLOGY. 



and either convex or flat on the other, so that it produces 



a focus like a lens. Be- 

 hind the cornea, or 

 lens, is the pigment, 

 having a minute aper- 

 ture or " pupil." Next 

 is a conical tube — one 

 for each facet — with 

 sides and bottom lined 

 with pigment. These 

 tubes converge to the 

 optic ganglion, the 

 fibres of which pass 

 through the tubes to 

 the cornea." 'Yision 



Fig. 166.— Bye of a Beetle (*/e!oto««Ao) : A, sectioD ; Uy gu(3}i g, COmOOUud 

 a, optic gaugliou ; b, secondary heives ; c, retiuH ; ^ " 



(i, pigment layer ;«, proper optic neivesjB, group eye is not a mOSaic; 



of ocelli ;/, bulb of optic nerve i 9, layer of pig- 



ment; A, vitreous humor ; t, cornea. Magnifled. but Oach OCellUS glVCS 



a complete image, although a diiferent pei-spective from 

 its neighbor. The 

 multiplied images are 

 reduced to one men- 

 tal stereoscopic pict- 

 ure, on the principle 

 of single vision in 

 ourselves. 



The eyes of the 

 Cuttle-fish are tlie 

 largest and the most 



FiQ. 167.— Section of Human Eye : a and !>, upper and 

 lower lid ; c, conjunctiva, or mucous membrane, 

 lining the inner surface ; d, external membrane ; e, 

 sheath of optic nerve ; /, g, muscles for rolling the 

 eye up or down; h, sclerotic; t, transparent cor- 

 nea ; j, choroid ; k, I, ciliary muscle for adjusting 

 the eye for distance ; m, iris and pupil ; n, canal ; 

 0, retina • a, vitreous humor ; t, crystalline lens ; », 

 ■ anterior chamber ; », posterior chamber. 



perfect among Inver- 

 tebrates. They re- 

 semble the eyes of 

 higher animals in hav- 

 ing a crystalline lens 

 .^ith a chamber in front (open, however, to the sea- 



