[Illustration: THE INSIDE OF THE EYE.]

~15. The Nerves of Sight.~--But a person might have an eyeball with all
the parts we have learned about and yet not be able to see. Can you tell
what more is needed? There must be a nerve. This nerve comes from some
little nerve cells in the brain and enters the eyeball at the back of
the eye; there it is spread out on the inside of the black lining of the
white of the eye.

~16. The Eyelids.~--Now we know all that it is necessary for us to learn
about the eyeball, so let us notice some other parts about the eye.
First there are the eyelids. They are little folds of skin fringed with
hairs, which we can shut up so as to cover the eyeball and keep out the
light when we want to sleep or when we are in danger of getting dust or
smoke into the eye. The hairs placed along the edge of the lids help to
keep the dust out when the eyes are open.

~17. The Eyebrows.~--The row of hairs placed above the eye is called the
eyebrow. Like the eyelids, the eyebrows catch some substances which
might fall into the eye, and they also serve to turn off the
perspiration and keep it out of the eyes.

~18. The Tear Gland.~--Do you know where the tears come from? There is a
little gland snugly placed away in the socket of the eye just above the
eyeball, which makes tears in the same way that the salivary glands make
saliva. It is called the _tear gland_. The gland usually makes just
enough tears to keep the eye moist. There are times when it makes more
than enough, as when something gets into the eye, or when we suffer pain
or feel unhappy. Then the tears are carried off by means of a little