A cross section of the retina, 
about midway between the 
fovea and far periphery, where 
rods are more numerous than 
cones. From top to bottom is 
about one-quarter millimeter.

    Because the rods and cones 
are at the back of the retina, 
the incoming light has to go 
through the other two layers in 
order to stimulate them. We do 
not fully understand why the 
retina develops in this curious 
backward fashion. One possible 
reason is the location behind 
the receptors of a row of cells 
containing a black pigment, 
melanin (also found in skin). 
Melanin mops up the light that 
has passed through the retina, 
keeping it from being reflected 
back and scattering around 
inside the eye; it has the same 
function as the black paint 
inside a camera. The melanin-
containing cells also help 
chemically restore the light-
sensitive visual pigment in the 
receptors after it has been 
bleached by light (see Chapter 
8). For both functions, the 
melanin pigment must be close 
to the receptors. If the 
receptors were at the front of 
the retina, the pigment cells 
would have to be between them 
and the next layer of nerve 
cells, in a region already 
packed with axons, dendrites, 
and synapses.

    As it is, the layers in front of 
the receptors are fairly 
transparent and probably do not 
blur the image much. In the 
central one millimeter, 
however, where our vision is 
most acute, the consequences 
of even slight blurring would be 
disastrous, and evolution seems 
to have gone to some pains to 
alleviate it by having the other 
layers displaced to the side to 
form a ring of thicker retina, 
exposing the central cones so 
that they lie at the very front. 
The resulting shallow pit 
constitutes the fovea.

    Moving from back to front, 
we come to the middle layer of 
the retina, between the rods 
and cones and the retinal 
ganglion cells. This layer 
contains three types of nerve 
cells: bipolar cells, horizontal 
cells, and amacrine cells. 
Bipolar cells receive input from 
the receptors, as the diagram 
to the left shows, and many of 
them feed directly into the 
retinal ganglion cells. 
Horizontal cells link receptors 
and bipolar cells by relatively 
long connections that run 
parallel to the retinal layers; 
similarly, amacrine cells link 
bipolar cells and retinal 
ganglion cells.

    The layer of cells at the front 
of the retina contains the 
retinal ganglion cells, whose 
axons pass across the surface of 
the retina, collect in a bundle 
at the optic disc, and leave the 
eye to form the optic nerve. 
Each eye contains about 125 
million rods and cones but only 
1 million ganglion cells. In the 
face of this discrepancy, we 
need to ask how detailed visual 
information can be preserved.