The Care of the Eyes. The most dangerous diseases of the eye are caused by infectious germs, which get into them either from the outside, as in dust, or by touching them with dirty fingers; or through the blood, as in measles, smallpox, tuberculosis, and rheumatism. The more completely we can prevent these diseases, the less blindness we shall have in the nation. About one-sixth of all cases of blindness in our asylums is caused by a germ that gets into babies' eyes at birth, but can be done away with by proper washing and cleansing of the eyes. THE EAR Structure of the Ear. Next after sight, hearing is our most important sense; without it, speaking, and consequently reading and writing, would be impossible. Man learned to speak by hearing the sounds made by other people and things, and then by listening to his own voice and practicing until he could imitate them. Children who are unfortunate enough to be born _deaf_ also become _dumb_, not because there is anything the matter with their voice organs, but simply because, as they cannot hear the sounds they make, they do not form them by practice into words and sentences. By proper training, deaf mutes can now be taught to speak, though their voices sound flat and "tinny," like a phonograph. As in the nose and the eye, the important part of the ear is the nerve spot that can "feel" the air waves that we call sound, just as the retina "feels" light. It is from this sensitive spot that the _auditory nerve_ carries the sound to the brain. This spot has grown into quite an elaborate structure, buried, for safety, deeply in the bones of the skull, close to the base of the brain. It is made up of a long row of tiny little nerve rods, laid side by side like the keys of a piano, only there are about three thousand of them. Each one of these is supposed to respond, or vibrate, to a particular tone, or sound. This keyboard, from the fact that, to save space, it is coiled upon itself like a sea-shell, instead of running straight, is called the _cochlea_ (Greek for "snail-shell"); it is also called, because it is the deepest, or innermost, part of the hearing apparatus, the _internal ear_. Just as the retina has a lens and a vitreous humor in front of it to act upon the light, so the internal ear has an apparatus in front of it to act upon the sound waves. This is called the _drum_ (_tympanum_). It consists of a fold of thin, delicate skin stretched tightly across the bottom of the outer ear canal, as parchment is stretched across the head of a drum. If you should take a hand-mirror--best a hollow, or concave, one--and throw a bright ray of light deep into some one's ear, you would be able, after a little trying, to see this drum-skin stretched across the bottom of it and about an inch and a quarter in from the surface of the head. [Illustration: THE APPARATUS OF HEARING A cross-section diagram from the outer ear to the lobes of the brain.]