They act on each other through the membrane which forms the coats of these vessels; for although this membrane prevents the blood and the air from mixing together in the lungs, yet it is no impediment to their chemical action on each other.
EMILY.
Are the lungs composed entirely of blood vessels and air vessels?
MRS. B.
I believe they are, with the addition only of nerves and of a small quantity of the cellular substance before mentioned, which connects the whole into an uniform mass.
EMILY.
Pray, why are the lungs always spoken of in the plural number? Are there more than one?
MRS. B.
Yes; for though they form but one organ, they really consist of two compartments called lobes, which are enclosed in separate membranes or bags, each occupying one side of the chest, and being in close contact with each other, but without communicating together. This is a beautiful provision of nature, in consequence of which, if one of the lobes be wounded, the other performs the whole process of respiration till the first is healed.
The blood, thus completed, by the process of respiration, forms the most complex of all animal compounds, since it contains not only the numerous materials necessary to form the various secretions, as saliva, tears, &c. but likewise all those that are required to nourish the several parts of the body, as the muscles, bones, nerves, glands, &c.