Primer of Hygiene and Sanitation ... by John Woodside Ritchie Joseph Stuart Caldwell

Author:John Woodside Ritchie , Joseph Stuart Caldwell
Language: eng
Format: epub, pdf
Publisher: World book co.
Published: 1910-03-25T05:00:00+00:00

The teacher should also look to the proper lighting of the schoolroom, paying spedal attention to whether parts of it are too dark and whether the children are seated facing the light.

CHAPTER TWENTY-FIVE

THE EARS AND THEIB CASE

Fig, 84. The ear. The ear is composed of aa outer, a middle, and an inner part.

When you throw a stone into water, the stone causes waves to run out in the water. When you ring a bell, the bell causes waves to run out in the air. When you shout, when a whistle blows, or when a bird sings, waves are made to run through the air. When these waves strike the ear, you hear the beU, the shouting, the whistle, or the singing of the bird. If the air waves are large, the sound will be loud. If the air waves are small, the sound will be faint in your ears.

The fimctioa of the ear. The ear collects the soimd waves and makes them strike on the ends of the nerves of hearing. This causes the nerves of

hearing to carry messages to the brain, and when these messages arrive in the brain we hear the sound. Certainly nothing in the world is more wonderful than the himian ear, for it changes the air waves that come from the strings of a violin or piano into the sweetest music, and by collecting the waves that are caused by the voices of our friends, it brings to us the thoughts that they wish to express to us.

The structure of the ear. The ear has three divisions: the outer, the middle, and the inner ear. The outer ear is made up of the part that we see and a canal that runs down into the head. At the bottom of this canal is a little skin-iike membrane called the tympanic membrane. This separates the outer and the middle ear.

The middle ear is a little cavity in the bone of the skull. It is filled with air, and from it a little tube nms to the throat. In the middle ear are three smaU bones which stretch across from the tympanic membrane to the inner ear. The inner ear is filled with liquid, and in this liquid lie the endings of the nerve of hearing.

How we hear a sound. The outer ear collects the soxmd waves and turns them down the canal to strike against the tympanic membrane. This sets the tympanic membrane to swinging, and the membrane puts the chain of little bones in motion. The motion of the bones disturbs the liquid in the

inner ear and causes waves in it. These waves wash over the ends of the nerve of hearing and start messages to the brain, and when these messages reach the brain we hear the sound.

The care of the ear. Practically all the serious troubles of the ear come from germs that work up the tube from the throat into the middle ear. In Figure 26 you can see that the openings

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