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. 96 Vision 



Fig. 156.— Eye of a Beetle (Melolontha) : A, section ; such a Compound 



a, optic ganglion ; b, secondary nerves ; c, retina ; J * 

 d, pigment layer ; e, proper optic nerves ; B, group eye is not a mosaic ; 

 of ocelli; /, bulb of optic nerve; g, layer of pig- 



ment; h, vitreous humor ; i, cornea. but each OCelluS gives 



a complete image, although a different perspective 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 the 

 largest and the most 

 perfect among Inver- 

 tebrates. They re- 

 semble the eyes of 

 higher animals in hav- 

 ing a crystalline lens 

 with a chamber in front (open, however, to the sea- 



Fig. 157. — Section of Human Eye : a and 6, 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; i, transparent cor- 

 nea ; j, choroid ; 7c, I, ciliary muscle for adjusting 

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

 o, retina ; s, vitreous humor ; t, crystalline ; u, an- 

 terior chamber; as, posterior chamber. 



