veyed by the
blood plasma and the lymph to the various tissues to provide them with
nourishment.

We have learned also that oxygen, taken up in the air cells of the lungs,
is being continually carried to the tissues, and that the blood is
purified by being deprived in the lungs of its excess of carbon dioxid.
From this tissue activity, which is mainly oxidation, are formed certain
waste products which, as we have seen, are absorbed by the capillaries and
lymphatics and carried into the venous circulation.

In their passage through the blood and tissues, the albumens, sugars,
starches, and fats are converted into carbon dioxid, water, and urea, or
some closely allied body. Certain articles of food also contain small
amounts of sulphur and phosphorus, which undergo oxidation into sulphates
and phosphates. We speak, then, of carbon dioxid, salts, and water as
waste products of the animal economy. These leave the body by one of
the three main channels,--the lungs, the skin, or the kidneys.

The elimination of these products is brought about by a special apparatus
called organs of excretion. The worn-out substances themselves
are called excretions, as opposed to secretions, which are
elaborated for use in the body. (See note, p. 121.) As already shown, the
lungs are the main channels for the elimination of carbon dioxid, and
of a portion of water as vapor. By the skin the body gets rid of a
small portion of salts, a little carbon dioxid, and a large
amount of water in the form of perspiration. From the kidneys
are eliminated nearly all the urea and allied bodies, the main
portion of the salts, and a large amount of water. In fact,
practically all the nitrogenous waste leaves the body by the kidneys.

[Illustration: Fig. 96.--Diagrammatic Scheme to illustrate in a very
General Way Absorption and Excretion.

A, represents the alimentary canal;
L, the pulmonary surface;
K, the surface of the renal epithelium;
S, the skin;
o, oxygen;
h, hydrogen,;
n, nitrogen.
]

233. The Skin. The skin is an important and unique organ of the
body. It is a blood-purifying organ as truly as are the lungs and the
kidneys, while it also performs other and complex duties. It is not merely
a protective covering for the surface of the body. This is indeed the most
apparent, but in some respectes, the lest important, of its functions.
This protective duty is necessary and efficient, as is proved by the
familiar experience of