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Wednesday, January 7, 2015

Sense Organs- Eye

THE SENSE ORGANS


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.

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