180 COMPARATIVE ZOOLOGY. 
however, to the sea-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 
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Fia. 154.—Section of the Human Retina, X 400: 1, in- 
ternal limiting membrane; 2, optic-nerve fibres; 
3, ganglion cells; 4, internal molecular layer; 5, 
internal granules; 6, external molecular layer; 7, 
external granules; 8, external limiting membrane; 
9, layer of rods and cones; 10, pigment layer. 
to make a sensitive 
layer. It consists of 
a white spherical case 
(sclerotic) made of 
tough tissue, with a 
transparentfront,call- 
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 humor oc- 
eupying the larger 
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 (¢ris), with a 
hole in the centre 
(pupil); and lining 
the sclerotic coat is 
the choroid mem- 
brane, covered with 
dark pigment. The 
optic nerve, entering at the back of the eye through the 
sclerotic and choroid coats, expands into the transparent 
