Eye

Eye 

In one experiment, when subjects were asked to estimate the size of coins and cardboard disks that were exactly the same size, they guessed, on the average, that the coins were one-fourth larger than the disks-and the poorer off financially a subject was, the more he overestimated coin size. The eyes even serve a purpose beyond seeing. 

They have a marked effect on taste, as shown by studies at a U.S. Air Force medical laboratory where volunteers were fed in a completely darkened room. Unable to see the food, they could detect no difference in taste between white and whole-wheat bread or between various canned foods. Eye Problems and Their Prevention Defects and diseases to which the eyes are susceptible account, in part, for the fact that there are nearly half a million people in the United States who are totally blind and two million more who are partially blind. 

But in part, too, this unpleasant fact is the result of misunderstanding, neglect, and delay in seeking aid, for many potentially blinding disorders can be prevented, or arrested, or even cured with prompt attention. In addition, of course, about 40 percent of the population wears glasses, indicating that the vision of almost one of every two people leaves something to be desired.

Three common eye defects-farsightedness, nearsightedness, and astigmatism-are the result of simple optical aberrations in the eye. If the lens is to focus light rays directly on the retina, it must be at the proper distance. When the eyeball is too short, the lens will be too close;  Prevention: Body conversely, if the eyeball is elongated, the lens will be too far away. Moreover, to accommodate to both near and far objects, the lens must change its curvature to maintain proper focus. At birth, the average baby has foreshortened eyeballs and is far- sighted. 

From about age 6 to age 20, the eyeballs elongate. After about age 45, people tend to become farsighted again because the lens, going through a normal hardening process, loses some of its ability to change curvature to focus on near objects.

Nearsightedness is usually due to a lengthening of the eyeball, an increase in curvature of the cornea, or a change in refraction of the lens. Astigmatism is usually caused by an irregularity of the shape of the cornea. When the cornea does not have a perfect curve, images is distorted. The effect can be similar to that of looking through a dirt- streaked pane of glass. 

Fortunately, nearsightedness, farsightedness, and astigmatism are readily corrected with eyeglasses. And it is important that these conditions be corrected, for both comfort and good eye health. If, for example, one eye is more effective than the other, the good eye may do all the work, which could be bad for it and also for the one that is not being used.

This is why children's eyes should be checked at an early age, even before school, and why you should mind your own eyesight, remembering that changes do occur with time. Have your eyes examined as soon as you find yourself holding things off at a distance to see them, or when you notice that you no longer see as well as you once did in poor light. 

Always have an eye doctor examine your eyes and prescribe any necessary lenses. It is true, of course, that eyeglasses are an expense and possibly a bit of a nuisance. But what a joy it is to see properly and to know you are protecting your precioussight. 

Sense Organs- Eye

THE SENSE ORGANS

THE EYE

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.