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
thro