182 
COMPARATIVE ZOOLOGY. 
and either convex or fiat 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 ; COmpOUlld 
a , optic ganglion ; b , secondary nerves; c, retina; J I 
a, pigment layer ; e, proper optic nerves ; B, group eye is not a mosaic I 
of ocelli; /, bulb of optic nerve; g, layer of pig- *' 
ment; h, vitreous humor; i, cornea. Magnified, blit each OCCllUS 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. 15T. —Section of Human Eye: a and b, 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; £, transparent cor¬ 
nea; j, choroid; 7c, 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. 
