Retina
The retina is a multi-layered sensory tissue
that lines the back of the eye. It
contains millions of photoreceptors that capture light rays and convert
them into electrical impulses. These impulses travel along the optic
nerve to the brain where they are turned into images.
There
are two types of photoreceptors in the retina:
rods and cones. The retina
contains approximately 6 million cones. The
cones are contained in the macula, the portion of the retina responsible for
central vision. They are most
densely packed within the fovea, the very center portion of the macula. Cones
function best in bright light and allow us to appreciate color.
 There
are approximately 125 million rods. They
are spread throughout the peripheral retina and function best in dim lighting.
The rods are responsible for peripheral and night vision.
This
photograph shows a normal retina with blood vessels that branch from the optic
nerve, cascading toward the macula.
Illustration by Mark Erickson
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