13.  The White of the Eye.—­The white of the eye is a tough, firm membrane which encloses the eyeball and keeps it in a round shape.

14.  The Lens.—­Do you know what a lens is?  Perhaps you do not know it by this name, but you are familiar with the spectacles which people sometimes wear to help their eyes.  The glasses in the spectacle frames are called lenses.  Well, there is something in the eye almost exactly like one of these lenses, only smaller.  It is also called a lens.  If some one will get the eye of an ox for you, you can cut it open and find this part.  The lens is placed in the eyeball just behind the pupil. (See picture.)

[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 tube which runs down into the nose from the inner corner of the eye.  When the tears are formed so fast that they cannot all get away through this tube, they pass over the edge of the lower eyelid and flow down the cheek.

19.  Muscles of the Eyes.—­By means of little muscles which are fastened to the eyeball, we are able to turn the eye in almost every direction.