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.

I am delighted that you come to that subject, for it is one that has
long excited my curiosity. But I cannot conceive how it is connected
with respiration. The idea I have of the circulation is, that the blood
runs from the heart through the veins all over the body, and back again
to the heart.

MRS. B.

I could hardly have expected a better definition from you; it is,
however, not quite correct, for you do not distinguish the _arteries_
from the _veins_, which, as we have already observed, are two distinct
sets of vessels, each having its own peculiar functions. The arteries
convey the blood from the heart to the extremities of the body; and the
veins bring it back into the heart.

This sketch will give you an idea of the manner in which some of the
principal veins and arteries of the human body branch out of the heart,
which may be considered as a common centre to both sets of vessels. The
heart is a kind of strong elastic bag, or muscular cavity, which
possesses a power of dilating and contracting itself, for the purposes
of alternately receiving and expelling the blood, in order to carry on
the process of circulation.

EMILY.

Why are the arteries in this drawing painted red, and the veins purple?

MRS. B.

It is to point out the difference of the colour of the blood in these
two sets of vessels.

CAROLINE.

But if it is the same blood that flows from the arteries into the veins,
how can its colour be changed?

MRS. B.

This change arises from various circumstances. In the first