~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