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 ; ^ COIUDOUnd 
a, optic ganglion ; b, secondary nerves; c, retina; */ P 
d, pigment layer; e, proper optic nerves ; B, group gve IS not a mosaic I 
of ocelli; f, bulb of optic nerve; g, layer of pig- ’ 
ment; h, vitreous humor; i, cornea. Magnified, 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 front (open, however, to the sea- 
Fig. 157.—Sect ion of Human Eye: a and 0, 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 rolling the 
eye up or down; h, sclerotic; t, transparent cor¬ 
nea ; j , choroid; k, l, ciliary muscle for adjusting 
the eye for distance ; m, iris and pupil; n, canal; 
o, retina; s, vitreous humor; t, crystalline lens; v , 
anterior chamber ; x, posterior chamber. 
