THE SENSE ORGANS
The human eye, which presents us with a world of space and
depth and a continual variety of sights, is packs into a single cubic inch of
space more than 150 million light receptors. The eye is often compared to a
camera. The outermost layer of the eyeball forms the white of the eye and at
the front becomes the completely transparent cornea.
The next layer is the
iris, which has an adjustable aperture, the pupil, which becomes larger or
smaller depending upon the amount of light entering. Just behind the iris and
its pupillary opening is the oval-shaped and elastic lens, which bulges out
when its muscles contract, and flattens when they relax, thus adjusting to
properly bend and focus light rays on the retina. (The lens muscles are the
fastest and best-coordinated in the body.)
The retina, which lines the eye and serves as the
"film," contains the light receptors that react to incoming images.
The responses of the receptors are transmitted along a million nerve fibers
which form one outgoing cable, the optic nerve, at the back of each eye. The
exit of the optic nerve leaves a "blind" spot-an area without receptors.
It has been estimated that 85 percent of everything we learn comes to us
through our eyes.
It's the brain that does the actual seeing; the eyes are
light-transmit- ting machines. This is the process: Light rays strike an object
and are reflected to the eyes. The rays pass through the cornea, the clear
front window, the aqueous humor (a watery liquid behind the cornea), the pupil,
and the lens.
The lens bends and focuses the rays on the retina. As the
rays impinge on light-sensitive pigments in the retina, chemical reactions take
place that send impulses through the optic nerve to the brain. Actually, images
are received upside down because the lens inverts them, but the brain has
learned to interpret them in accord with reality.
Central vision-what you use
when you look straight at an object-is sharpest. But you also have side, or
peripheral, vision. And while peripheral vision is not very acute, it is
important; without it, you would bump into things and be unaware of objects
approaching from the side. You can demonstrate side vision with a simple
experiment: With both eyes open, hold your right thumbnail 16 inches in front
of your face.
Have someone hold a wrist watch at arm's length to your left
and gradually move it toward your thumb. Without moving your eyes, you will be
able to identify the watch as a watch probably when it is about 15 inches away
from your thumb. Chances are, though, that you will not be able to tell time
until the watch is about two inches away.
Interestingly enough, each of us has
his own individual view of the world. That's because, for one thing, the eyes
can transmit millions of impulses per second but the brain chooses details on
the basis of individual past experience, mood, and interests at the moment. How
we see things also can be affected by their meaning for us.
