The cavity of the chest then becomes smaller and the air is forced out through the nose or mouth.

This process is repeated every time we breathe.
~22.~ We breathe once for each four heart-beats.

Small children breathe more rapidly than grown persons.

We usually breathe about eighteen or twenty times in a minute.
~23.

How Much the Lungs Hold.~--Every time we breathe, we take into our lungs about two thirds of a pint of air and breathe out the same quantity.