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Body Talk

Amazing instruments of vision

You must be aware by now that we have sense organs to see, smell, taste and so on. Of these, the eyes are our most highly developed sense organs. By transmitting light wave images to the brain, they feed in most of the unending stream of information from around us - more than any of the other four senses. They also communicate to others, they can reveal our feelings and they reflect our well-being. It has often been said that the 'eyes are the mirrors of our souls'.

The eyes are self-focusing, self-lubricating, self-cleansing and well protected. They can adapt for bright or dim light. They can focus far off or very near. They are so sensitive that they can help the brain to distinguish between images only one ten thousandth of an inch apart.

Because light travels in straight lines, the eyes can present extremely accurate perceptions of the brightness, colour and shape of the outside world. We can see various colours, big things, little things, planes high up in the air or a tiny ant on the ground, because of our eyes!

How the eye is constructed and works; The healthy eye is round, about two thirds the size of a standard ping-pong ball. It is protected both by the strong bones of the eye sockets, and by pads of fat surrounding the eye, which also help it turn.

Essentially, the eye focuses light to form images, which it converts into nerve impulses for the brain to interpret. But it is rather more complex than that. The eye contains many vital, exquisitely designed structures. Among the most important are:

* The cornea: This is the eye's transparent, crystalline 'domed window'. It's the main lens, that does most of the focusing, bending the light to form images on the back of the eye. It constantly changes its thickness and curvature, depending on how close or distant the objects are. The cornea has more nerve fibres than any other part of the body (and so has a very low pain threshold).

* The iris: This circular, coloured, sheet of muscles contracts or expands the pupil, the hole in the centre of the iris which opens wide in dim light, but can narrow to 17 times smaller in bright light.

* The lens: This is located behind the iris. It also changes shape to bring light waves at different distances into finer focus and transmit them to the retina.

* The ciliary body: This is a circular muscular ring around the lens that contracts or relaxes to alter its shape and adjust to focus, especially on close objects. Six other muscles around the eye ceaselessly direct its movements, even in sleep, and a seventh moves the upper eyelid.

* The retina: At the back of the eye is the retina, a light sensitive layer containing millions of nerve cells shaped like rods, which act as light detectors, plus several more millions of nerve cells shaped like cones that react to colour and discriminate fine detail - the retina can detect more than seven million subtle differences of hue. Its responses are transmitted via the million fibres of the optic nerve, an extension of the brain, to the brain itself which interprets the pin-sized images as visually rich moving pictures.

* The sclera: Is the tough, opaque, protective jacket of the eyeball - the white of the eye. It also serves as an attachment for the muscles, which move the eye. Around the sclera and lining of the eye is a transparent membrane, the conjunctiva, which helps keep the eye lubricated.

* The choroid layer: The dark and fibrous choroid layer is behind and below the retina. It contains pigment and blood vessels which nourish it with essential oxygen and glucose. It also prevents light reflections in the eye.

* The interior: The interior of the eye accounts for most of its size and is filled with vitreous humour, a clear jelly which helps the eye to keep its shape.

* The lachrimal system: Tears that flow down your eyes when you cry are produced and drained by the lachrimal system at the junction of eye and nose. They keep the cornea and conjunctiva moist. Spread around by blinking, tears maintain corneal transparency, lubricate the eye's surface and aid its movement. Containing lypozyme, a natural antiseptic, tears also wash away dust and other small foreign bodies. And, of course, they express emotion.

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How to maintain healthy eyes

Eyes need the best possible care, attention and protection, particularly against strong sunlight and in hazardous occupations or sports.

Tired, sore eyes usually result from working the eye muscles too hard. The usual culprits are poor or ill-directed lighting, or glare, as from the sun, snow, TV and computer screens. When working with computers, give the eyes regular breaks by looking across the room or further away, every so often.

Do not hunch over your desk or keyboard and keep at least an arm's length from the monitor. A glare-free screen can help when you are working with computers. Most eye infections are contagious. A person with infected eyes should use separate towels and face flannels. Above all, respect the amazing instruments that your eyes are. Make sure you have an annual vision test.

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