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.