How We See: Eye Anatomy And Function

Eyes Your eyes are among the most valuable of all your possessions

. For that reason, they ought to have fairly frequent examinations by competent persons to make certain that they are free from disease and capable of doing an efficient job. A good examination of the eye must detect symptoms and signs of difficulties such as swollen and inflamed lids, pain with or without redness, and any change in vision, such as double vision or dimming of the eyesight. Among the common difficulties with eyes are irritations of the lid such as sties and crusting of the lids. Overwork of the eye may lead to such signs of fatigue as discomfort, dizziness, scowling, rubbing of the eye, frequent blinking, or inability to do close work. Some people quite obviously use only one eye, and turn the head to make that eye more effective. Other people hold the head in an unusual position in order to see better. This is due to the fact that they do not see objects in the normal positions. When there are crossed eyes or eyes that do not focus together properly the difficulty is easily apparent. The inability to see distant objects is another indication that changes have taken place. In fact, when people hold reading material or other fine work unusually near or unusually far from the eyes they should have an immediate examination, probably with the intent to secure glasses that will meet their needs. People who do not see clearly incline to stumble over objects in their path or fail to appreciate the height of steps. They get their fingers caught in machines. They have auto accidents. They fall downstairs. Good eyesight prevents innumerable unnecessary accidents. Development Of The Eyes We depend so much on our eyes that we ought to give ourselves every benefit that we can in relation to their education, their hygiene, and their control. Here are some interesting facts about the development of the eye from birth. The eye of the newborn child is about 70 per cent of the size of the eye of the person fully grown. It is a shorter eye than the eye of an adult; the lens of the eye of the newborn child is a sphere or circular globe. During the first few years of life the eye grows rapidly, and it reaches adult size at about the age of eight or nine years. The lens of the eye continues to grow throughout life. The pupil of the eye is small at birth and remains small until about the end of the first year. During childhood and up to the age of youth the pupil of the eye develops its maximum size. Then it gradually becomes smaller, so that in older people the pupil depends to a large extent on the adaptation of the retina of the eye to light. The retina is the nerve tissue at the back of the eye by which we are able to see. If a great deal of light suddenly pours into the eye the pupil will become smaller by contracting. Gradually the retina will adapt itself to the increased illumination. Then the pupil will again enlarge to approximately its normal size. There are, however, many different factors which may modify the size of the pupil from time to time. The iris of the eye is the colored portion. People of dark races have a darker color in the iris than those of the blond races. Most children are born with a blue iris, the color being due to the appearance of the color layer at the back of the iris. The color changes during the first years of life as the material becomes thicker. Then the eye may gradually become brown or even darker. If there is a lack of pigment the eye has a strange pinkish color such as is seen in albinos. As the child grows the eye becomes longer. The retina is farther back, and the lens becomes flatter. This occurs mostly between the ages of six and sixteen. If the rays of light which enter the eye focus short of the retina the child is nearsighted. If, for example, the eye of the child focuses normally at the time of birth, it is sure to become nearsighted as the eye becomes longer. The lens in children is quite flexible, but as people grow older this flexibility tends to decrease. It is apparent, therefore, that parents should have the eyes of children tested regularly, to make certain that they are getting the best vision possible with the type of eyes they have at the time they are born. Children must also be taught to use their eyes correctly. This involves co-ordination of nerves and muscles and of the brain, which can be improved with proper training. As the child grows older it develops what the specialists call "binocular vision"-that is to say, it uses both eyes in seeing. Sooner or later one eye becomes more important than the other and we tend to rely more on one eye than on the other. As Dr. W. S. Knighton has said, "One of the two assumes the role of the master eye." Mechanism Of Vision We do not see with the eye but with the brain and the nervous system. The chief factors involved in seeing are the optic nerve and the center in the brain for vision. Next comes the retina, a tissue back of the eye, which is a part of the nervous system and which conveys what is seen to the optic nerve. The lens is actually a lens, and serves to focus objects on the retina. The muscles control the size and shape of the lens in its focusing. There are also accessory muscles which move the eyeball. The iris makes up the pupil. By dilating and contracting, the iris controls the amount of light which enters the eye. The eye can adapt itself to various conditions of light, but even this mechanism of adaptation may be exhausted by overuse. It is better to provide suitable lighting than to strain the eye by insufficient light. The eye may also be strained by too much badly distributed light or glare. The effects of glare and of eyestrain result in the fatigue of the eye, with increased danger of accident. Devices for measuring the amount of light in use at any point in the office, the shop, or the home are now available. Shades are made to distribute light suitably and thus prevent glare. Walls are painted and ceilings enameled to reflect a maximum amount of good light where it is most needed. Attention to these factors may mean many more years of good vision for those who otherwise would soon be incapacitated. An eye which is fatigued and unable to work satisfactorily becomes easily irritated. Moreover, it is more likely to be invaded by foreign bodies like cinders and dust, simply because the tissues do not react to get rid of such foreign material. People with bad eyesight frequently have red rims on the eyes, swollen eyelids, and constant watering. The eyelids will be crusted together in the morning.

How We See: Eye Anatomy And Function

By: David Crawford

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