nverted by digestion.

CAROLINE.

One set of the absorbent vessels, then, is employed in bringing away the
old materials that are no longer fit for use; whilst the other set is
busy in conveying into the blood the new materials that are to replace
them.

EMILY.

What a great variety of ingredients must enter into the composition of
the blood?

MRS. B.

You must observe that there is also a great variety of substances to be
secreted from it. We may compare the blood to a general receptacle or
storehouse for all kinds of commodities, which are afterwards fashioned,
arranged, and disposed of as circumstances require.

There is another set of absorbent vessels in females which is destined
to secrete milk for the nourishment of the young.

EMILY.

Pray is not milk very analogous in its composition to blood; for, since
the nursling derives its nourishment from that source only, it must
contain every principle which the animal system requires?

MRS. B.

Very true. Milk is found, by its analysis, to contain the principal
materials of animal matter, albumen, oil, and phosphat of lime; so that
the suckling has but little trouble to digest and assimilate this
nourishment. But we shall examine the composition of milk more fully
afterwards.

In many parts of the body numbers of small vessels are collected
together in little bundles called _glands_, from a Latin word meaning
acorn, on account of the resemblance which some of them bear in shape to
that fruit. The function of the glands is to _secrete_, or separate
certain matters from the blood.

The secretions are not only mechanical, but chemical separations from
the blood; for the substances thus formed, though contained in the
blood, are not ready combined in that fluid. The secretions are of two
kinds, those which form peculiar animal fluids, as bile, tears, saliva,
&c.; and those which produce the general materials of the animal system,
for the purpose of recruiting and nourishing the several organs of the
body; such as albumen, gelatine, and fibrine; the latter may be
distinguished by the name of _nutritive secretions_.

CAROLINE.

I am quite astonished to hear that all the secretions should be derived
from the blood.

EMILY.

I thought that the bile was produced by the liver?

MRS. B.

So it is; but the liver is nothing more than a very large gland, which
secretes the bile from the blood.

The last of the animal organs which we have mentioned are