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Almost all living things are sensitive to light. You know for example, that a clam closes its shell when a shadow falls across it. This action is made possible by a number of special cells lining the edge of the shell. Light falling on these cells excites them. Nervous messages or impulses then pass to the muscles which close the shell; and we say a simple nervous reflex is completed.

But this sensitivity to light is quite a different thing from vision. A clam cannot be said to see in the sense that a man can see. Before an animal can really see it must have two things. First, it must have eyes-true eyes, not the simple type found in a shellfish. Second the eyes must be connected with a well-developed brain. There are many animals with perfectly good eyes-quite as good as our own, in fact, yet which see very little. The explanation of this rather surprising fact is that such animals have well-developed eyes connected with brains that are still primitive. They can use their eyes and simple brains to react to light in a reflex action. But they cannot really see because there are no centers in their brains that control vision.

Thus a fish may appear to prefer one type of bait to another. But it is most unlikely that it can actually see either of them. And certainly, it can not make a conscious choice between them. This is so simply because the fish has no centers for vision. The eyes of all higher animals are fundamentally similar. Each eye consists of a number of structures contained within an almost spherical body-the eyeball. Most of the eyeball is enclosed in a bony cavity at the front of our skull; only a little of it is visible from the outside. The eye is also partly hidden or surrounded by a variety of  other structures –eyelids, tear glands and ducts, muscles, blood vessels and nerves, to mention only a few. Each of these has a special part to play in making the eye an efficient working organ.

The eye is nearly always compared to a camera. Indeed the two are very similar. In both, there is sensitive screen on which the rays of light are focused. Both have a lens to do the focusing. Both have a dark inner coating to prevent reflection and blurring. And finally both the eyes and the camera can be adjusted to give sharp pictures at different distances. In the case of the eye, this adjustment of the focus for objects at various distances is known as accommodation.

 By Rehana khan(111/10)