EMILY.
I always supposed that the elevation and depression of the ribs were the consequence, not the cause of breathing.
MRS. B.
It is exactly the reverse. The muscular action of the diaphragm, together with that of the ribs, are the causes of the contraction and expansion of the chest; and the air rushing into, and being expelled from the lungs, are only consequences of those actions.
CAROLINE.
I confess that I thought the act of breathing began by opening the mouth for the air to rush in, and that it was the air alone, which, by alternately rushing in and out, occasioned the dilatations and contractions of the lungs and chest.
MRS. B.
Try the experiment of merely opening your mouth; the air will not rush in, till by an interior muscular action you produce a vacuum—yes, just so, your diaphragm is now dilated, and the ribs expanded. But you will not be able to keep them long in that state. Your lungs and chest are already resuming their former state, and expelling the air with which they had just been filled. This mechanism goes on more or less rapidly, but, in general, a person at rest and in health will breathe between fifteen and twenty-five times in a minute.
We may now proceed to the chemical effects of respiration; but, for this purpose, it is necessary that you should previously have some notion of the circulation of the blood. Tell me, Caroline, what do you understand by the circulation of the blood?
CAROLINE.