THE NERVOUS SYSTEM. 179 
it produces a focus like a lens. Behind the cornea, or 
lens, is the pigment, 
having a minute ap- 
erture, or “ pupil.” 
Next (in place of the 
“vitreous humor” of 
Vertebrates) is a con- 
ical 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 cor- 
nea.’ Vision by sueh Fie. 152.—Eye of a Beetle (Melolontha): A, section 5 
: a, optic ganglion; b, secondary nerves; c¢, retina; 
a compound eye is not d, pigment layer; e, proper optic nerves ; B, group 
_ of ocelli; , bulb of optic nerve; g, layer of pig- 
a mMmosalc 5 but each ment; h, vitreous humor; 7, cornea. 
ocellus gives a complete image, although a different per- 
spective from its 
neighbor. The mul- 
tiplied images are re- 
duced to one mental 
stereoscopic picture, 
on the principle of 
single vision in our- 
selves. 
The eyes of the 
Cuttle-fish are the 
Fra. 153.—Section of Human Eye: aand 6, upper and 
lower lid; ¢, conjunctiva, or mucous membrane, largest and the most 
lining the inner surface; d, external membrane; @, 
sheath of optic nerve; f, 9, muscles for rolling the perfect among Inver- 
eye up or down; A, sclerotic; 7, transparent cor- 
nea; j, choroid; &,J, ciliary muscle for adjusting tebrates. They re- 
the eye for distance; m, iris and pupil; , canal; é 
0, retina; s, vitreous humor; ¢, crystalline; v, an- semble the ey es of 
terior chamber; #, posterior chamber. hi gher animals in 
having a crystalline lens with a chamber in front (open, 
