~5. Animals Die without Air.~--If you should shut up a mouse or any other small animal in a fruit-jar, its life would go out just as the light of the candle went out. The little animal would die in a short time. A child shut up in a close place would die from the same cause in a very little time. In fact, many children are dying every day for want of a sufficient supply of pure air.
~6. Oxygen.~--The reason why animals need air, and why the fire will not burn without it, is that the air contains _oxygen_, and it is the oxygen of the air which burns the wood or coal and produces heat. So it is the oxygen that burns in our bodies and keeps us warm.
~7.~ When wood and coal are burned, heat is produced; but some parts of the fuel are not made into heat. While the fire burns, smoke escapes through the pipe or chimney; but a part of the fuel remains in the stove in the form of ashes. Smoke and ashes are the waste parts of the fuel.
~8. Poison in the Breath.~--The burning which takes place in our bodies produces something similar to the smoke and ashes produced by the fire in a stove. The smoke is called _carbonic-acid gas_,[A] an invisible vapor, and escapes through the lungs. The ashes are various waste and poisonous matters which are formed in all parts of the body. These waste matters are carried out of the body through the skin, the kidneys, the liver, and other organs.
~9. Another Experiment.~--We cannot see the gas escape from our lungs, but we can make an experiment which will show us that it really does pa.s.s out. Get two drinking-gla.s.ses and a tube. A gla.s.s tube is best, but a straw will do very well. Put a little pure water into one gla.s.s and the same quant.i.ty of lime-water into the other gla.s.s. Now put one end of the tube into the mouth and place the other end in the pure water.
Breathe through the tube a few times. Look at the water in the gla.s.s and see that no change has taken place. Now breathe through the lime-water in the same way. After breathing two or three times, you will notice that the lime-water begins to look milky. In a short time it becomes almost as white as milk. This is because the lime-water catches the carbonic-acid gas which escapes from our lungs with each breath, while the pure water does not.
~10. Why we Breathe.~--By this experiment we learn another reason why we breathe. We must breathe to get rid of the carbonic-acid gas, which is brought to the lungs by the blood to be exchanged for oxygen. There are two reasons then why we breathe: (_a_) to obtain oxygen; (_b_) to get rid of carbonic-acid gas.
~11. How a Frog Breathes.~--Did you ever see a frog breathe? If not, improve the first opportunity to do so. You will see that the frog has a very curious way of breathing. He comes to the top of the water, puts his nose out a little, and then drinks the air. You can watch his throat and see him swallowing the air, a mouthful at a time, just as you would drink water.
~12.~ If you had a chance to see the inside of a frog you would find there a queer-shaped bag. This is his air-bag. This bag has a tube running up to the throat. When the frog comes to the surface of the water he fills this bag with air. Then he can dive down into the mud out of sight until he has used up the supply of air. When the air has been changed to carbonic-acid gas, he must come to the surface to empty his air-bag and drink it full again.
~13. The Lungs.~--We do not drink air as the frog does, but like the frog we have an air-bag in our bodies. Our air-bag has to be emptied and filled so often that we cannot live under water long at a time, as a frog does. We call this air-bag the lungs. We have learned before that the lungs are in the chest. We need so much air and have to change the air in our lungs so often that we would not have time to swallow it as a frog does. So nature has made for us a breathing apparatus of such a kind that we can work it like a pair of bellows. Let us now study our breathing-bellows and learn how they do their work.
~14. The Windpipe and Air-tubes.~--A large tube called the _windpipe_ extends from the root of the tongue down the middle of the chest. The windpipe divides into two main branches, which subdivide again and again, until the finest branches are not larger than a sewing-needle.
The branches are called _bronchial tubes_. At the end of each tube is a cl.u.s.ter of small cavities called _air-cells_. The air-tubes and air-cells are well shown on the following page.
~15. The Voice-box.~--If you will place the ends of your fingers upon your throat just above the breast-bone, you will feel the windpipe, and may notice the ridges upon it. These are rings of cartilage, a hard substance commonly called gristle. The purpose of these rings is to keep the windpipe open. Close under the chin you can find something which feels like a lump, and which moves up and down when you swallow.
[Ill.u.s.tration: AIR-TUBES AND AIR-CELLS.]
This is a little box made of cartilage, called the voice-box, because by means of this curious little apparatus we are able to talk and sing. Two little white bands are stretched across the inside of the voice-box.
When we speak, these bands vibrate just as do the strings of the piano.
These bands are called the _vocal cords_.
~16. The Epiglottis.~--At the top of the voice-box is placed a curious trap-door which can be shut down so as to close the entrance to the air-pa.s.sages of the lungs. This little door has a name rather hard to remember. It is called the _epiglottis_ (ep-i-glot"-tis). The cover of the voice-box closes whenever we swallow anything. This keeps food or liquids from entering the air pa.s.sages. If we eat or drink too fast the voice-box will not have time to close its little door and prevent our being choked. Persons have been choked to death by trying to swallow their food too fast. Do you not think this is a very wonderful door that can open and shut just when it should do so without our thinking anything about it?
~17. The Nostrils and the Soft Palate.~--The air finds its way to the lungs through the mouth or through the two openings in the nose called the _nostrils_. From each nostril, three small pa.s.sages lead backward through the nose. At the back part of the nasal cavity the pa.s.sages of the two sides of the nose come together in an open s.p.a.ce, just behind the soft curtain which hangs down at the back part of the mouth. This curtain is called the _soft palate_. Through the opening behind this curtain the air pa.s.ses down into the voice-box and then into the lungs.
~18. The Pleura.~--In the chest the air tubes and lung of each side are enclosed in a very thin covering, called the _pleura_. The cavity of the chest in which the lungs are suspended is also lined by the pleura. A limpid fluid exudes from the pleura which keeps it moist, so that when the two surfaces rub together, as the lungs move, they do not become chafed and irritated.
~19. Walls of the Chest.~--The ribs form a part of the framework of the chest. The ribs are elastic. The s.p.a.ces between them are filled up with muscles, some of which draw the ribs together, while others draw them apart. Can you tell any reason why the walls of the chest are elastic?
The lower wall or floor of the chest cavity is formed by a muscle called the _diaphragm_, which divides the trunk into two cavities, the chest and the abdomen.
~20. How we Use the Lungs.~--Now let us notice how we use the lungs and what takes place in them. When we use a pair of bellows, we take hold of the handles and draw them apart. The sides of the bellows are drawn apart so that there is more room between the sides. The air then rushes in to fill the s.p.a.ce. When the bellows are full, we press the handles together and the air is forced out.
~21.~ It is in just this way that we breathe. When we are about to take a long breath, the muscles pull upon the sides of the chest in such a way as to draw them apart. At the same time the diaphragm draws itself downward. By these means, the cavity of the chest is made larger and air rushes in through the nose or mouth to fill the s.p.a.ce. When the muscles stop pulling, the walls of the chest fall back again to their usual position and the diaphragm rises. The cavity of the chest then becomes smaller and the air is forced out through the nose or mouth. This process is repeated every time we breathe.
~22.~ We breathe once for each four heart-beats. Small children breathe more rapidly than grown persons. We usually breathe about eighteen or twenty times in a minute.
~23. How Much the Lungs Hold.~--Every time we breathe, we take into our lungs about two thirds of a pint of air and breathe out the same quant.i.ty. Our lungs hold, however, very much more than this amount. A man, after he has taken a full breath, can breathe out a gallon of air, or more than ten times the usual amount. After he has breathed out all he can, there is still almost half a gallon of air in his lungs which he cannot breathe out. So you see the lungs hold almost a gallon and a half of air.
~24.~ Do you think you can tell why Nature has given us so much more room in the lungs than we ordinarily use in breathing? If you will run up and down stairs three or four times you will see why we need this extra lung-room. It is because when we exercise vigorously the heart works very much faster and beats harder, and we must breathe much faster and fuller to enable the lungs to purify the blood as fast as the heart pumps it into them.
~25. The Two Breaths.~--We have learned that the air which we breathe out contains something which is not found in the air which we breathe in. This is carbonic-acid gas. How many of you remember how we found this out? We can also tell this in another way. If we put a candle down in a wide jar it will burn for some time. If we breathe into the jar first, however, the candle will go out as soon as we put it into the jar. This shows that the air which we breathe out contains something which will put a candle out. This is carbonic-acid gas, which is a poison and will destroy life.
~26. Other Poisons.~--The air which we breathe out also contains other invisible poisons which are very much worse than the carbonic-acid gas.
These poisons make the air of a crowded or unventilated room smell very unpleasant to one who has just come in from the fresh air. Such air is unfit to breathe.
~27. The Lungs Purify the Blood.~--We have learned that the blood becomes dark in its journey through the body. This is because it loses its oxygen and receives carbonic-acid gas. While pa.s.sing through the capillaries of the lungs, the blood gives out the carbonic-acid gas which it has gathered up in the tissues, and takes up a new supply of oxygen, which restores its scarlet hue.
~28. How the Air is Purified.~--Perhaps it occurs to you that with so many people and animals breathing all the while, the air would after a time become so filled with carbonic-acid gas that it would be unfit to breathe. This is prevented by a wonderful arrangement of Nature. The carbonic-acid gas which is so poisonous to us is one of the most necessary foods for plants. Plants take in carbonic-acid gas through their leaves, and send the oxygen back into the air ready for us to use again.
~29.~ We have already learned that the oxygen taken in by the lungs is carried to the various parts of the body by the little blood corpuscles.
The effect of strong liquors is to injure these corpuscles so that they cannot carry so much oxygen as they ought to do. For this reason, the blood of a drunkard is darker in color than that of a temperate person, and contains more carbonic-acid gas. The drunkard"s lungs may supply all the air he needs, but his blood has been so damaged that he cannot use it. Excessive smoking has a similar effect.
SUMMARY.
1. Our bodies need air, just as a candle or a fire does.
2. A small animal shut up in a close jar soon dies for want of air. We need the oxygen which the air contains.
3. Oxygen causes a sort of burning in our bodies.
4. The burning in our bodies keeps us warm, and destroys some of the waste matters.
5. The breathing organs are the windpipe and bronchial tubes, the voice-box, the epiglottis, the nostrils, the soft palate, the lungs, the air-cells, the pleura, the diaphragm, and the chest walls.
6. When we breathe we use our lungs like a pair of bellows.
7. A man"s lungs hold nearly one and a half gallons of air.
8. In ordinary breathing we use less than a pint of air, but when necessary we can use much more.
9. The air we breathe out contains carbonic-acid gas and another invisible poison.
10. A candle will not burn in air which has been breathed, and animals die when confined in such air.
11. The lungs purify the blood. While pa.s.sing through the lungs, the color of the blood changes from purple to bright red.
12. Plants purify the air by removing the carbonic-acid gas.
13. Alcohol and tobacco injure the blood corpuscles so that they cannot take up the oxygen from the air which the lungs receive.
CHAPTER XIV.
HOW TO KEEP THE LUNGS HEALTHY.
~1. Pure Air Necessary.~--A person may go without eating for a month, or without drinking for several days, and still live; but a strong man will die in a few moments if deprived of air. It is very important that we breathe plenty of pure air. There are many ways in which the air becomes impure.