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 eye.

    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 Clinic.