”I am Your Body”

EYE

    
FOR CONCENTRATED complexities, no other organ in your body can equal me. No larger than a Ping-Pong ball, I have tens of millions of electrical connections and can handle 1.5 million simultaneous messages. I gather 80 percent of all the knowledge you absorb. You think of me as a miniature television camera. I consider the comparison insulting. I'm much more sensitive than the biggest, costliest TV camera ever made. I am responsible for one of the greatest of all miracles-sight.

Today's world is giving me a hard time. I wasn't built for it. For your prehistoric ancestors, the eye's main job was to see things at a distance - dangers to be avoided, game to be killed. Only lately have I been called on for continuous close-up work.

Look at my anatomy and you'll understand why I am having difficulty adjusting to today's demands. First, my front window - my clear dime - size cornea. It starts the seeing process by bending light rays into orderly patterns. Next, my pupil - an adjustable gateway for light. In bright sun it is nearly closed; on a dark night it is wide open. Up to this point there is nothing about seeing that a cheap camera couldn't handle.

My wonders really begin with my lens-a little envelope of fluid the size and shape of an oval vitamin pill. My lens is surrounded by a ring of tiny, superbly strong, unbelievably hard-working muscles. When they tense, my lens fattens for near vision; when they relax, it flattens for distant vision. This was a fine arrangement for your caveman ancestors. Since they were mainly interested in things 20 or more feet away, the muscles were relaxed most of the time. But you now live in a close-up world-lots of reading, deskwork and such. This keeps my ciliary muscles tensed a lot of the time. They grow tired.

In front of and behind my lens I have two fluid-filled chambers. In front the fluid is like water; in back it is about the consistency of egg white. The watery fluid keeps me firmly inflated. Both fluids must be absolutely clear to permit passage of light. Those "specks" you sees when you look at a bright light are cellular remnants left over from the days in the womb when I was under construction. They will float aimlessly in your eye fluid as long as you live.

When you look at some object, the light passes through my lens, which brings it into correct focus on my retina, a kind of onionskin wallpaper, which covers the rear two thirds of my interior. Except in your brain, I don't think that anywhere else in your body is so much packed into so small a space. Covering less than a square inch, my retina contains 137 million light-sensitive receptor cells: 130 million shaped like rods for black-and-white vision, seven million shaped like cones for color vision.

The rods are scattered all over my retina. Let a firefly pass at night and a complex chemistry gets under way. The faint light bleaches rhodopsin, a purplish-red pigment in my rods. The bleaching process generates a tiny wisp of electricity-a few millionths of a volt, far too little to tickle a mosquito. This feeds into my straw-size optic nerve and is transmitted to your brain at about 300 miles per hour. The brain interprets the signals flooding in and hands down its verdict: a firefly. All of this intricate electrochemical activity has been completed in about .002 seconds!

If my rods seem complex, my cones are far more so. They are concentrated in the fovea, a pinhead-size, yellowish depression at the rear of my chamber. This is the center for acute vision-reading, any close work-and for color. A leading theory is that these cones, too, have bleachable pigments, one each for red, green and blue. Like an artist mixing paints on a palette your brain blends these colors to make scores of other hues. If anything should go wrong with this intricate electrochemical process, you would be color blind-as one in eight persons is to some degree. In dim light, activity of my cones diminishes, color sense vanishes and everything becomes gray, as my rods take over.

While you sees with me, you see in your brain. A crushing blow at the back of your head, severe enough to destroy the optical center of the brain, would produce permanent blindness. A lesser blow and you see "stars"-a chaotic electrical disturbance. You get clinching evidence of the brain's role when you dream. You "see" pictures, even with my lids closed in total darkness. Had you been born blind, you would dream in terms of other sensory stimuli: touch, sound, even smell.

You weren't born with the eyes you have today. At birth, you could see only light and shadow. In the first few months you where farsighted, like your caveman ancestors. To study your rattle, you held it as far as possible from your face. At first, your eyes were poorly coordinated; I'd wander in one direction, my partner in another. Our wandering worried your mother. It shouldn't have. A few months after birth we were moving in exact unison. By the time you were six, your vision was excellent. But my peak sight didn't come until age eight.

When you were young, you used to read in dim light. Your mother warned that were "ruining" your eyes. Nonsense. The young see better in dim light than adults, and viewing under even the most adverse circumstances does no harm.

I have a number of other unusual attributes. Tiny though they are, my muscles, milligram for milligram, are among the body's strongest. In an average day, I move about 100,000 times-to bring objects into sharp focus. You would have to walk 50 miles to give your leg muscles similar exercise.

My cleaning equipment is similarly striking. My lacrimal glands produce a steady stream of moisture-tears to flush away dust and debris.

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