THE NERVOUS SYSTEM. 
183 
water), and a chamber behind it filled with “vitreous 
humor. 55 
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- 8 
ed the cornea. This 
case is kept in shape 
by two fluids—the thin 
aqueous humor filling 
the cavity just behind 
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¬ 
brane, covered with Fi g- 158 *—Section of the Human Retina, X 400: 1 , 
' internal limiting membrane; 2, optic-nervefibres; 
dark pigment. The 3 > ganglion cells; 4, internal molecular layer; 5, 
, , internal granules; G, external molecular layer; 7, 
OptlC nerve, entering externalgranules;8,externallimitingmembrane; 
, ,1 t i _£» , -i 9. layer of rods and cones; 10, pigment layer. 
at the back of the eye 
through the sclerotic and choroid coats, expands into the 
transparent retina , which consists of several layers — 
