light—in the ultraviolet, x-ray, and gamma-ray regions of the spec-
trum—interactions with molecular and cellular structures are likely
to be damaging: chemical bonds break and molecules fall apart, free
radicals form, DNA is damaged, cell membranes become leaky, and
other bad stuff happens. (As noted in Chapter 11, some insects, birds,
and other animals do see ultraviolet radiation that is slightly more
energetic than visible light but still within a safely detectable range of
energy.) If the radiation is lower in energy (longer wavelength) than
visible light—in the infrared, microwave, and radio wave regions of
the spectrum—interaction with molecular and cellular structures
might not be energetic enough to generate a reliable neural signal.
(Again, as noted in Chapter 11, rattlesnakes and other pit vipers do
“see” infrared radiation, via receptors that reliably detect electromag-
netic radiation slightly less energetic than visible light.)
The receptor organ for human vision is the eye. It is analogous to a
camera in that in a camera a lens focuses incoming light onto a pho-
tosensitive film or detector, and in the eye, the cornea, lens, and pupil
focus light onto the photosensitive retina at the rear of the eyeball
(Fig. 14.1). The retina is a very complex structure, consisting of a layer
of light-sensitive photoreceptor cells and several layers of intercon-
nected nerve cells. The retina is also heavily crisscrossed with blood
vessels, as the photoreceptor and other neural cells require a robust
supply of biochemical fuel (glucose and oxygen) in order to function.
The word retina derives from the Latin rete, meaning “net,” a reference
to the complex network of cell bodies, nerve fibers, and blood vessels
that compose it.