forces the air out
of them.

CAROLINE.

The lungs, then, are like bellows, and the chest is the power that works
them.

MRS. B.

Precisely so. Here is a curious little figure (PLATE XV. Fig. 5.), that
will assist me in explaining the mechanism of breathing.

CAROLINE.

What a droll figure! a little head fixed upon a glass bell, with a
bladder tied over the bottom of it!

MRS. B.

You must observe that there is another bladder within the glass, the
neck of which communicates with the mouth of the figure--this represents
the lungs contained within the chest; the other bladder, which you see
is tied loose, represents a muscular membrane, called the _diaphragm_,
which separates the chest from the lower part of the body. By the chest,
therefore, I mean that large cavity in the upper part of the body
contained within the ribs, the neck, and the diaphragm; this membrane is
muscular, and capable of contraction and dilatation. The contraction may
be imitated by drawing the bladder tight over the bottom of the
receiver, when the air in the bladder, which represents the lungs, will
be forced out through the mouth of the figure--

EMILY.

See, Caroline, how it blows the flame of the candle in breathing!

MRS. B.

By letting the bladder loose again, we imitate the dilatation of the
diaphragm, and the cavity of the chest being enlarged, the lungs expand,
and the air rushes in to fill them.

EMILY.

This figure, I think, gives a very clear idea of the process of
breathing.

MRS. B.

It illustrates tolerably well the action of the lungs and diaphragm; but
those are not the only powers that are concerned in enlarging or
diminishing the cavity of the chest; the ribs are also possessed of a
muscular motion for the same purpose; they are alternately drawn in,
edgeways, to assist the contraction, and stretched out, like the hoops
of a barrel, to contribute to the dilatation of the chest.

EMILY.

I always supposed that the elevation and depression of the ribs were the
consequence, not the cause of breathing.

MRS. B.

It is exactly the reverse. The muscular action of the diaphragm,
together with that of the ribs, are the _causes_ of the contraction and
expansion of the chest; and the air rushing into, and being expelled
from the lungs, are only _consequences_ of those actions.

CAROLINE.

I confess that I thought the act of breathing began by opening the mouth
for the air to rush in, and that it was the air