place,
during its passage through the arteries, the blood undergoes a
considerable alteration, some of its constituent parts being gradually
separated from it for the purpose of nourishing the body, and of
supplying the various secretions. The consequence of this is, that the
florid arterial colour of the blood changes by degrees to a deep purple,
which is its constant colour in the veins. On the other hand, the blood
is recruited during its return through the veins by the fresh chyle, or
imperfect blood, which has been produced by food; and it receives also
lymph from the absorbent vessels, as we have before mentioned. In
consequence of these several changes, the blood returns to the heart in
a state very different from that in which it left it. It is loaded with
a greater proportion of hydrogen and carbon, and is no longer fit for
the nourishment of the body, or other purposes of circulation.

EMILY.

And in this state does it mix in the heart with the pure florid blood
that runs into the arteries?

MRS. B.

No. The heart is divided into two cavities or compartitions, called the
_right_ and _left ventricles_. The left ventricle is the receptacle for
the pure arterial blood previous to its circulation; whilst the venous,
or impure blood, which returns to the heart after having circulated, is
received into the right ventricle, previous to its purification, which I
shall presently explain.

CAROLINE.

For my part, I always thought that the same blood circulated again and
again through the body, without undergoing any change.

MRS. B.

Yet you must have supposed that the blood circulated for some purpose?

CAROLINE.

I knew that it was indispensable to life; but had no idea of its real
functions.

MRS. B.

But now that you understand that the blood conveys nourishment to every
part of the body, and supplies the various secretions, you must be
sensible that it cannot constantly answer these objects without being
proportionally renovated and purified.

CAROLINE.

But does not the chyle answer this purpose?

MRS. B.

Only in part. It renovates the nutritive principles of the blood, but
does not relieve it from the superabundance of water and carbon with
which it is encumbered.

EMILY.

How, then, is this effected?

MRS. B.

By RESPIRATION. This is one of the grand mysteries which modern
chemistry has disclosed. When the venous blood enters the right
ventricle of the heart, it contracts by its m