regular teeth be straightened, but the crowded teeth of a young person can be spread apart, and the bones of the jaw be made to grow until the teeth have room. In this way a weak-looking chin can be made to grow into one that is square and strong.

FIGS. 23 and 24. A boy whose teeth need straightening, and the same boy several years after the straightening was done. Not only were his teeth made more useful, but the lower jaw grew until the appearance of the lower part of the face was changed. Notice how the weak chin developed into one of strength and firmness. (After photographs in The Popular Science Monthly for July, 1909.)

The advantage of having good teeth. Good teeth are important from the standpoint of health, but there are still other good reasons why you should keep your teeth white and clean. See how many of these reasons you can give.

Questions: 1. Mention some facts that show how important the teeth are. 2. Tell two ways in which bad teeth injure the health. 3. Give two ways by which bad teeth

cause germ diseases. 4. What causes decay in teeth? 5. How can decay be prevented? 6. How often ought the teeth to be cleaned? 7. Why is it important to remove particles of food from between the teeth? 8. What effect has a decaying tooth on the other teeth? 9. Tell why it is best to visit a dentist occasionally and have the teeth given the care that they need. 10. What is enamel? II. Mention some ways by which the enamel may be injured. 12. What often happens if the enamel on a tooth is broken? 13. Why should the first set of teeth be cared for by a dentist? 14. What should be done with crowded and uneven teeth?

Suggestions and topics for development: Why a tooth aches. (Illustrate structure by decayed teeth, which may be secured from a dentist.) How to distinguish the first permanent molar from a temporary tooth. What happens to meat or other food matter if it is left in a warm place like the mouth. How the teeth can be kept clean by a child who has no toothbrush. What it would cost to buy toothbrushes for a person for twenty years, and what it costs to have a badly decayed set of teeth repaired. The first permanent molars, which come in about the sixth or seventh year, are often mistaken for temporary teeth and are allowed to decay. Count the double teeth; when there are three double teeth on one side of the jaw, the back one is a permanent tooth.

One of the most important medical discoveries of recent years is that many (probably most) cases of chronic rheumatism, heart disease, kidney disease, catarrh, and other serious ailments are due to infections of the tonsils, at the roots of the teeth, or in the cavities of the bones of the face. From these foci of infection the germs spread to other parts of the body, and often the only successful treatment for these chronic ailments is to break up the permanent breeding places of the germs that cause them. The care of the teeth is far more important than was understood until very recently, and the teacher should give this topic all possible emphasis.

THE AIR WE BREATHE

CATO, a Roman philosopher, once said that he could kill himself at any time by holding his breath. Cato

probably knew more about philosophy than he knew about physiology; for if he had taken the trouble to try the experiment he would have found that he could hold his breath for hardly a minute. Spoiled food and unclean water we can refuse, but the air that comes to us we must breathe, whether it be clear or smoky, pure or dust-laden. Nearly a thousand times an hour we take a fresh supply into the lungs. It is clear that no dwelling, schoolhouse, or factory should be built without providing some way of giving the people who must live or work in it a supply of fresh, life-giving air.

FIG. 25. Experiment showing that a flame cannot burn without oxygen.

Why the body must have air. About one fifth of the air is oxygen. Oxygen is constantly used in the body, and without it we cannot live for even five minutes. Set a glass vessel over a burning candle so that no air can get in, and you will see the flame slowly die out for lack of oxygen. So the heat and strength and life of your body will die out if its

supply of oxygen is cut off. The first reason why the body needs air is to get oxygen.

All the time we are breathing out from the lungs a

gas called carbon dioxid. In too large quantities this gas is poisonous. We must therefore keep breathing the air into the lungs in order that, as it passes out again, it may carry the carbon dioxid out of the body. The second reason why

we must have air is to get rid of carbon dioxid. Heat is constantly being produced in the

body, and to keep the body temperature from rising too high this heat must be given off. It is lost chiefly through the air that comes in contact with the body and by the evaporation of the sweat from the skin. The third reason why

opening of tube from the ear

off the body heat.

head and throat.

Under ordi

Why ventilation is necessary. nary conditions we have plenty of oxygen and we do not suffer because of too much carbon dioxid. Ventilation is necessary, therefore, for the proper

regulation of body heat. The important points in ventilation are the temperature and motion of the air, and the amount of moisture in it.

Moisture, temperature, and motion important in ventilation. In crowded rooms the air is often laden with moisture, causing the people to suffer from overheating and headache. In such rooms the temperature should be kept down to 65 degrees, as much fresh air as possible should be admitted, and motion should be set up in the air by opening doors and windows or by electric fans.

In rooms heated by hot air, the air is frequently very dry and evaporates the sweat so rapidly that persons in the room feel chilly even with the temperature as high as 75 degrees. Where heating systems of this kind are used, there should be some arrangement for moistening the air until the rooms will be comfortable at 68 degrees.

How to obtain fresh air. Every school building or other building where many people gather together ought to have some system of forcing in fresh air and drawing off the air that has been used. Where this has not been provided for, we must get as much fresh air as possible in some other way. By a little experimenting, it will often be found that certain windows in a room can be opened without causing harmful draughts on any one. Opening several windows a little is usually a good way to ventilate a room. A common method is to set a