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CATARACT SURGERY
A Patient's Guide To Cataract Treatment
by Robert K. Maloney, M.D, Inc. Addicus books Omaha, Nebraska |
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The Human Eye and How Vision Works |
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Anatomy of the Eye
If you could examine an entire human eye,
you'd see a sphere about an inch in diameter-the
size of a large gumball. The eyeball itself is really
three thin layers surrounding a fluid-filled center.
Sclera
The outside layer is the "white of the eye,"
called the sclera. It is the tough, opaque tissue that
serves as the eye's protective outer coat. Six tiny
muscles connect to it around the eye and control the
eye's movements. The optic nerve is attached to the
sclera at the very back of the eye.
Cornea
Curving out from the sclera, the clear cornea is
the "front window" of the eye. The cornea is
amazingly strong and protective against dust and
germs. Packed with nerve fibers, it is also quite
sensitive to pain. This pain sensitivity is one of the
cornea's protective qualities; the pain is a signal that
something is trying to invade the eye.
Iris
Under the sclera is another thin layer, which
consists of the iris-the eye's visible colored ring;
when we say that someone's eyes are blue, brown,
or green, we are talking about the color of the iris.
Besides being an interesting and expressive feature
of the face, the iris is essential to clear vision. It
surrounds the round, black pupil-along with
muscle fibers and blood vessels.
Lens
The muscle fibers hold the lens of the eye in
place and allow it to change its shape so that it can
focus on objects at different distances. Located
behind the iris and the pupil, the lens is about twothirds
water and one-third protein fibers. There are
three distinct layers in the lens, sometimes
compared to the layers of a peach:
The capsule (the peach "skin") is a thin, clear
membrane that forms the outside layer of the lens.
The cortex (comparable to the peach "flesh") is
the soft, clear material just beneath the capsule.
The nucleus (the "pit" of the peach) is the firm
center, or core, of the lens.
Retina
The blood vessels, located toward the back of
the eye, feed essential nutrients to the retina, a
smooth, thin layer of nerve tissue at the back of the
eye. When you are looking at an object, the retina is
where the image comes into focus. Most of the
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Cataract Surgery
retina contains specialized cells that convert the
reflected light (the image) to signals your brain can
interpret.
The macula is the focal point at the center of
the retina. Within the macula are millions of lightsensitive
nerve endings that act as photoreceptors.
The photoreceptors called rods are sensitive to
brightness and allow us to see in dim light.
The photoreceptors called cones respond to the
varying wavelengths of light that produce different
colors.
The fovea centralis, in the center of the macula,
is densely packed with cone cells. It is the fovea that
gives your eye the ability to sharpen an image. The
clearest vision-what we call "20/20 vision"-would
be impossible without the fovea.
Another part of the retina, the retinal pigment
epithelium, consists of dark tissue cells that absorb
excess light and carry nutrients to, and waste
products from, the retina. |
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