WHY AND HOW WE BREATHE.
~1. An Experiment.~--Let us perform a little experiment. We must have a
small bit of candle, a fruit jar, or a bottle with a large mouth, and a
piece of wire about a foot long. Let us notice carefully what we are
about to do and what happens.
~2.~ We will fasten the candle to the end of the wire. Now we will light
it, and next we will let it down to the bottom of the jar. Now place the
cover on the top of the jar and wait the results. Soon the candle burns
dimly and in a little time the light goes out altogether.
~3.~ What do you think is the reason that the candle will not burn when
shut up in a bottle? A candle uses air when it burns. If shut up in a
small, tight place, it soon uses up so much air that it can burn no
longer. Try the experiment again, and when the candle begins to burn
dimly, take it out quickly. We see that at once the light burns bright
again.
~4.~ Suppose we shut the stove draught tight, what is the result? The
fire will burn low, and after a time it will probably go out. Why is
this? Evidently the stove needs air to make the wood or coal burn, just
as the candle needs air to make it burn.
~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
pass out. Get two drinking-glasses and a tube. A glass tube is best, but
a straw will do very well. Put a little pure water into one glass and
the same quantity of lime-water into the other glass. 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 glass 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
cluster 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.
[Illustration: 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-passages 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 passages. 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 passages lead backward
through the nose. At the back part of the nasal cavity the passages of
the two sides of the nose come together in an open space, 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 passes 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 spaces 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 space. 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 space. 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
quantity. 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 passing 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 passing 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.