THE PHENOMENON OF EYESIGHT
By David Mallory, M.D.
Eyesight is our richest and most valuable sense, capturing
the shapes and colors of life around us. Seventy-five
percent of all that we perceive comes solely from our eyes.
The eyes function to respond to the light originating from
the sun. When our eyes are open to an illuminated world,
they are able to sense about 10 million gradations of light
and about 7 million shades of color. As an extension of the
brain itself, our eyes gather about a billion pieces of
information every second they are open. All parts of the eye
fulfill specific roles in the amazing process of capturing,
focusing and processing light.
The cornea is the clear front "watch glass" part of the eye.
The difference between the cornea and the rest of the outer
wall of the eyes (sclera), is that the cells in the cornea
are arranged in a perfectly regular pattern, and that the
cornea has absolutely no blood vessels in it. With its
perfect mirror surface and clarity, the cornea takes light
rays and bends them into the eye, providing 60 percent of
the eye's total focusing power.
The iris is the colored part of the eye, and it is located
behind the cornea. The iris is in actuality a circular
curtain-like muscle with an opening in its center called the
pupil. The iris automatically dilates (expands) or
constricts (narrows) to let the right amount of light into
The lens of our eye is suspended directly behind the pupil.
It provides the remaining 40 percent of the focusing power
of the eye. Before the age of 40-45, the lens automatically
changes shape, becoming thicker to increase its focusing
power for near work. The lens also serves to block out most
harmful ultraviolet light. The lens is normally crystal
clear, and in fact it is called the crystalline lens. The
lens may become cloudy and cause blurred vision. This is
called a cataract.
The retina is a thin membrane of incredibly complex tissue
lining the inside of the eye. It is a direct extension of
the brain, and its function is to catch the entering light
rays and change the images into the language of the brain.
The retina contains two kinds of receptors: rod cells for
night vision and cone cells for color and fine, detailed
vision. The retina becomes up to 100,000 times more
sensitive when dark adapted, so it is no wonder that some
people have trouble driving at night when oncoming
headlights hit the eyes. The retina accounts for 40 percent
of all nerve fibers connected to the brain but is only
one-millionth of a person's total body weight.
The optic nerve is the cable that then carries the
electrical impulses from the retina to the processing center
of the brain. Upon receiving this information the brain
interprets the impulses, and we have the miraculous
phenomenon of eyesight.
David Mallory, M.D. is a cataract, intraocular lens and
laser surgery specialist in practice at the Southwest Eye