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.