Pathology of the Eye - Index
eye is a form of camera. It has an adjustable
opening to admit light, a lens which makes the light waves come together
to form an image, and a sensitive film on which the image is recorded.
The eye is connected to the brain by a nerve called the optic nerve which serves as a highway along which a nerve impulses travel from the eye to the brain.
These nerve impulses are caused by the image on the "film" at the back of the eye camera and when they reach the brain we sort them out and "see" a man, a dog or whatever it is that we are looking at.
Except for a bulge where the light enters, the eye is round. Its outer membrane, or skin, called the sclera (of which the "white" of the eye is a part is very tough and strong, it is opaque, or milky, except where it covers the bulge of the eye, where it changes its character and becomes transparent, or clear.
This outer layer of the front of the eye, called the cornea, has very important duties - it helps to bend the light rays as they enter the eye, it must remain clear, so that light will pass through it undimmed and since it guards the opening into the eye it must be very sensitive, so that and dust or dirt that might settle on it may be quickly felt and removed.
Close inside the outer skin and going round the back and sides of the eye is the choroids membrane which supplies the eye with life-giving blood and, like a lining within this, is found the all-important retina.
The retina is the camera film of the eye on which pictures are recorded.
It is made up of ten exceedingly thin layers of cells, the ninth being the real vision layer.
Where the optic nerve joins the retina in the middle of the eye, at the back, there is a blind spot which is useless for vision.
Close to the blind spot is the yellow spot, a point where the cells are so arranged that the vision is sharpest.
The inside of the eye is filled with a substance called vitreous, or glassy, humour, which is really nothing more than a very weak salt jelly.
It is the duty of the vitrous humour to keep the eye round and shapely.
The front of the eye contains the iris and pupil (the circle of the colour and the black dot in its centre) and it is their job to let light into the eye.
The pupil actually lets the light in and the iris controls the amount let in by controlling the size of the pupil.
They can therefore be compared to the opening and shutter of a camera.
The iris contains tiny muscles which close the pupil to a pinhole in bright light when less is needed, and open it in dull light when more is needed.
The pupil is black because it opens into the interior of the eye, which is dark.
Directly behind the iris and the pupil lies the lens, so called because it is transparent and curves outwards just like the lens of a magnifying glass.
It is much more delicate, however, than any artificial lens for it is elastic and its muscles adjust it according to whether an object is near at hand or far away.
It is the lens that bends the rays of light so that they will all be focused on the retina and thus produce an image of the object which the eye is looking at.
The spaces between the lens and iris, and between the iris and the cornea, are filled with a liquid called the aqueous, or watery, humour, which is a salt solution similar to the vitreous humour.
The eyes of all the higher forms of animal life are fairly similar in construction to the human eyes.
The eye is so delicate that it needs the greatest possible protection, it is housed in a soft-cushioned bony socket and protected still further by the eyelids and lashes.
If, in spite of this protection, dust falls on the eyeball, tears are provided to wash it away. Tears come from the lacrimal glands (tear glands), one to each eye, which lie at the upper and outer part of the socket.
Little tubes called lacrimal canals (tear ducts) clears the tears away and are found on the inner side of the socket near the nose - there are two of them for each eye, one on the upper and one on the lower lid.
Thus tears leave the glands and flow across the eye, some of them flowing into the ducts and down into the nose.
Like any other part of the body the eye some times becomes diseased and needs the care of an eye specialist.
Cataract, or milkiness of the lens (which ought to be clear and transparent,is one very common eye disease and it can be removed by an operation.
However, disease is responsible for only a few eye troubles, most of them are due to a fault in the shape of the eye and can usually be corrected by wearing glasses.
If the cornea does not curve evenly, light does not enter the retina in a point, as it should, and round objects look as though they were oval. This condition is known as astigmatism.
If the eyeball is unusually long from back to front, people cannot see very far and are called near-sighted, if it is unusually short, they see distant things better than near ones and are called long-sighted.
Sometimes the muscles that move the eye are too short or tight and then the eyes may be crossed, but this can usually be corrected by operation, glasses or exercise.
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