THE NERVOUS SYSTEM. 
183 
water), and a chamber behind it filled with “vitreous 
humor.” 
The eye of Vertebrates is formed by the infolding of 
the skin to create a lens, and an outgrowth of the brain 
to make a sensitive 
layer; both enclosed in 
a white spherical case 
(sclerotic ) made of 
tough tissue, with a 
transparent front, call¬ 
ed the cornea . This 
case is kept in shape 
by two fluids—the thin 
aqueous humor filling 
the cavity just behind 6 
the cornea, and the 
jelly-like vitreous hu¬ 
mor occupying the lar¬ 
ger posterior chamber. 
Between the two hu¬ 
mors lies the double- 
convex crystalline lens. 
On the front face of 
the lens is a contractile 
circular curtain (iris), 
with a hole in the cen¬ 
tre (pupil); and lin¬ 
ing the sclerotic coat 
is the choroid mem- 
U ranp pnvprprl with Fig. 158.—Section of the Human Retina, X 400: 1, 
uitiiie, vvitu interoal iimiting membrane; 2, op tic-nerve fibres; 
dark pigment. The 3, ganglion cells; 4, internal molecular layer; 5, 
r ® . interna] granules; 6, external molecular layer; 7, 
optic nerve, entering externalgranules; 8, external limitingmembrane; 
9, layer of rods and cones; 10, pigment layer. 
at the back of the eye J 
through the sclerotic and choroid coats, expands into the 
transparent retina, which consists of several layers — 
