Parts of the Eyeball.—The outside of the eyeball is a tough white coat except in front, where it is as clear as glass. Within the outer coat is a very thin black lining to keep the light from scattering. In front the lining is not against the outer coat, but hangs loose and has in it a round hole called the pupil to let the light pass through. The part around the hole is the iris. It may be blue, black, or brown, and can squeeze up so as to make the pupil very small when the light is strong.
Fig. 91 —A slice from before backward through the eye.
The end of the nerve of sight forms a tender pink covering over most of the inner surface of the eyeball. The cavity within the eyeball is filled with three clear substances. The lens, shaped like a flat door knob, is fixed just behind the pupil. In front of the lens is a watery fluid and behind it is a clear jellylike mass. The use of the lens and also the other substances is to bend the rays of light together so that they will meet at one place.
How the Eyeball is Moved.—Six muscles fixed to the bones of the socket holding the eye have their other ends fastened to the tough coat of the eyeball. One muscle turns the ball upward, another turns it downward, one turns it inward and another turns it outward. If an inner or an outer muscle is too strong, a person may have cross eyes.
Keeping the Eye Strong.—Nearly all young children have perfect eyes. After a year or two in school the eyes of some children become weak. Many children get weak eyes after they are ten or twelve years old. This is because they have not taken care of the eyes.
The eyes are often hurt by reading a book with fine print, reading in a dim light, or by leaning over the book so that the eyes look downward instead of straight forward. As the eyes are very weak after measles and most other diseases, they should not be used much until a week or more after recovery.
In reading the book should be held a little over a foot in front of the chest and you should sit nearly straight and let the light fall on the page from one side. Never read while lying down because it strains the eyes. Stop reading as soon as the eyes smart.
Helping the Eyes to See.—Very few old people can see to read without the help of glasses, because the lens of the eye hardens in old age. To see things near by, the shape of the lens must be changed. In some children, the shape of the eyes has become so changed by straining them to read fine print or see things in a dim light that the eyes hurt after being used for any kind of work, and the head may often ache and make the whole body feel bad. Such eyes need help. You should have them examined by an eye doctor who can fit you with glasses which will help you see clearly without headache.