The first two branches into which it divides form the right and left lung tubes, known as _bronchi_.

The third, fourth, fifth, sixth, etc., divisions, and so on, form what are known as the _bronchial tubes_.

These keep on splitting into tinier and tinier twigs, until they end, like the bush, in little leaves, which in the lung, of course, are hollow and are called the air cells (_alveoli_).

This budding off of the lungs from the gullet is the reason why the air we breathe and the food we swallow go down the same passage.

Every mouthful of our food slides right across the opening of the windpipe, which has to be protected by a special flap, or trap-door of gristle, called the _epiglottis_.