COMPARATIVE ZOOLOGY. 
182 
a 
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 
J ig. 156.—Eye of ci Beetle (il 'IcloloTithct) i Aj section \ T \\7 miell POITHIOU11 (1 
a, optic ganglion; b, secondary nerves; c> retina ; •/ ^ 
d, pigment layer; e, proper optic nerves; B, group gye is not a mosaic ; 
of ocelli; /, bulb of optic nerve; g, layer of pig- ■ 
ment; h, vitreous humor ; i, cornea. blit each OCellllS 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 
Fig. 157. —Section of Human Ej r e: a and 6, upper and 
lower lid; c, conjunctiva, or mucous membrane, 
lining the inner surface; d, external membrane; e, 
sheath of optic nerve; f, g, muscles for rolliug the 
eye up or down; h , sclerotic; i, transparent cor- 
nea;y, choroid; k, l, ciliary muscle for adjusting 
the eye for distance ; m, iris and pupil; n, canal; 
o, retina; s, vitreous humor; t, crystalline; r, an¬ 
terior chamber; x , posterior chamber. 
front (open, however, to the sea- 
