d unite
into large branches, till they at length collect the chyle into one
vessel, which pours its contents into the great vein near the heart, by
which means the food, thus prepared, enters into the circulation.

CAROLINE.

But I do not yet clearly understand how the blood, thus formed,
nourishes the body and supplies all the secretions?

MRS. B.

Before this can be explained to you, you must first allow me to complete
the formation of the blood. The chyle may, indeed, be considered as
forming the chief ingredient of blood; but this fluid is not perfect
until it has passed through the lungs, and undergone (together with the
blood that has already circulated) certain necessary changes that are
effected by RESPIRATION.

CAROLINE.

I am very glad that you are going to explain the nature of respiration:
I have often longed to understand it, for though we talk incessantly of
_breathing_, I never knew precisely what purpose it answered.

MRS. B.

It is indeed one of the most interesting processes imaginable; but, in
order to understand this function well, it will be necessary to enter
into some previous explanations. Tell me, Emily, --what do you
understand by respiration?

EMILY.

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