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_.