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nerve are active processors and analyzers of visual information, it is not inappropriate to think of
these structures as an extension of the brain itself.
Figure 4.9 The Retina With Its Specialized Cells
When light falls on the retina, it creates a photochemical reaction in the rods and cones at the back of the retina. The
reactions then continue to the bipolar cells, the ganglion cells, and eventually to the optic nerve.
Rods are visual neurons that specialize in detecting black, white, and gray colors. There are
about 120 million rods in each eye. The rods do not provide a lot of detail about the images we
see, but because they are highly sensitive to shorter-waved (darker) and weak light, they help us
see in dim light, for instance, at night. Because the rods are located primarily around the edges of
the retina, they are particularly active in peripheral vision (when you need to see something at
night, try looking away from what you want to see). Cones are visual neurons that are