g expelled from the right ventricle, enters the lungs to
go through what we may call the lesser circulation; the large trunk or
vessel that conveys it branches out, at its entrance into the lungs,
into an infinite number of very fine ramifications. The windpipe, which
conveys the air from the mouth into the lungs, likewise spreads out into
a corresponding number of air vessels, which follow the same course as
the blood vessels, forming millions of very minute air-cells. These two
sets of vessels are so interwoven as to form a sort of net-work,
connected into a kind of spongy mass, in which every particle of blood
must necessarily come in contact with a particle of air.

CAROLINE.

But since the blood and the air are contained in different vessels, how
can they come into contact?

MRS. B.

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.

EMILY.

There seems to be a singular analogy between the blood of animals and
the sap of vegetables; for each of these fluids contains the several
materials destined for the nutrition of the numerous class of bodies to
which they respectively belong.