This consists chiefly of water, but contains a small per cent of
salts and of urea. The excretory work of these glands seems not to be so
great as was formerly supposed, but they supplement in a practical way the
work of the kidneys and, during diseases of these organs, show an increase
in excretory function to a marked degree. The perspiration also aids in
the regulation of the temperature of the body (Chapter XVI).

*Excretory Work of the Lungs.*--While the lungs cannot be regarded as
glands, they do a work in the removal of waste from the body which must be
considered in the general process of excretion. They are especially
adapted to the removal of gaseous substances from the blood, and it is
through them that most of the carbon dioxide leaves the body. The lungs
remove also a considerable quantity of water. This is of course in the
gaseous form, being known as water vapor.

*Ductless Glands and Internal Secretion.*--Midway in function between the
glands that secrete useful liquids and those that remove waste materials
from the blood is a class of bodies, found at various places, known as the
_ductless glands._ They are so named from their having the general form of
glands and from the fact that they have no external openings or ducts.
They prepare special materials which are passed into the blood and which
are supposed to exert some beneficial effect either upon the blood or upon
the tissues through which the blood circulates. The most important of the
ductless glands are the thyroid gland, located in the neck; the suprarenal
bodies, situated one just over each kidney; and the thymus gland, a
temporary gland in the upper part of the chest. The spleen and the
lymphatic glands (page 68) are also classed with the ductless glands. The
liver, the pancreas, and (according to some authorities) the kidneys, in
addition to their external secretions, produce materials that pass into
the blood. They perform in this way a function like that of the ductless
glands. The work of glands in preparing substances that enter the blood is
known as _internal secretion._

*Quantity of Excretory Products.*--If the weight of the normal body be
taken at intervals, after growth has been attained, there will be found to
be practically no gain or loss from time to time. This shows that
materials are leaving the body as fast as they enter and that the tissues
are being torn down as fast as they are built up. It also shows that
substances do no