front to the breastbone. It looks somewhat like a cage, doesn't it?
Put your hands on the sides of your chest and you can feel your own
ribs. Do they slant upward or downward?

This chest-cage is our breathing-machine. Before I tell you how it
pumps, I want you to get a pair of bellows and see how they work. When
you lift up the handle of the bellows, you make the bag of the bellows
larger so that it sucks in air; and when you press the handle down
again, the air puffs out through the nozzle.

Our air machine, though it is somewhat different from the bellows in
shape, works in exactly the same way. You remember that you found that
the ribs slant down and can be moved on hinges. Suppose, now, you
place your hands against your ribs and feel the ribs lift as you draw
in a long breath. The air will be sucked into your nose just as it was
into the bellows when you raised the handle. By lifting your ribs, you
have made the chest-cage larger; and the air has rushed into your
nose, down your windpipe, and filled your lungs. If you breathe very
deeply, you will find that your stomach, too, swells out. This shows
that the muscular bottom of the cage, called the _diaphragm_, has been
pulled down, making the cage larger still.

In this chest-cage are millions of tiny air bags that make up the
lungs; and every time you take a breath, the air bags are puffed out
with the fresh air that comes rushing in. By the time you let your
ribs sink again, the air has given its oxygen to the blood, and the
blood has poured its carbon-dioxid smoke into the air bags for you to
breathe out. Nature, with the same bellows, pumps in the oxygen and
pumps out the "smoke."