ass
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.

[Illustration: 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 witho