Respiration, I conceive, consists simply in alternately inspiring air into the lungs, and expiring it from them.
MRS. B.
Your answer will do very well as a general definition. But, in order to form a tolerably clear notion of the various phenomena of respiration, there are many circumstances to be taken into consideration.
In the first place, there are two things to be distinguished in respiration, the mechanical and the chemical part of the process.
The mechanism of breathing depends on the alternate expansions and contractions of the chest, in which the lungs are contained. When the chest dilates, the cavity is enlarged, and the air rushes in at the mouth, to fill up the vacuum formed by this dilatation; when it contracts, the cavity is diminished, and the air forced out again.
CAROLINE.
I thought that it was the lungs that contracted and expanded in breathing?
MRS. B.
They do likewise; but their action is only the consequence of that of the chest. The lungs, together with the heart and largest blood vessels, in a manner fill up the cavity of the chest; they could not, therefore, dilate if the chest did not previously expand; and, on the other hand, when the chest contracts, it compresses the lungs and forces the air out of them.
CAROLINE.