mple, the cells
of the pancreas be examined after a period of rest, they are found to
contain small granular bodies. On the other hand, if they are examined
after a period of activity, the granules have disappeared and the cells
themselves have become smaller (Fig. 86). The granules have no doubt been
used up in forming the secretion. These and other facts have led to the
conclusion that secretion is, in part, the separation of materials without
change from the blood, and, in part, a process by which special substances
are prepared and added to the secretion. According to this view the gland
plays the double role of a _filtering apparatus_ and of a _manufacturing
organ_.

[Fig. 86]

Fig. 86--*Secreting cells from the pancreas* (after Langley). _A._ After a
period of rest. _B._ After a short period of activity. C. After a period
of prolonged activity. In _A_ and _B_ the nuclei are concealed by the
granules that accumulate during the resting period.

*Kinds of Secretion.*--In a general way all the liquids produced by glands
may be considered as belonging to one or the other of two classes, known
as the _useful_ and the _useless_ secretions. To the first class belong
all the secretions that serve some purpose in the body, while the second
includes all those liquids that are separated as waste from the blood. The
first are usually called _true secretions_, or secretions proper, while
the second are called _excretions_. The most important glands producing
liquids of the first class are those of digestion (Chapter X).

*Excretory Work of Glands.*--The process of removing wastes from the body
is called _excretion_. While in theory excretion may be regarded as a
distinct physiological act, it is, in fact, leaving out the work of the
lungs, but a phase of the work of glands. From the cells where they are
formed, the waste materials pass into the lymph and from the lymph they
find their way into the blood. They are removed from the blood by glands
and then passed to the exterior of the body.

*The Necessity for Excretion* is found in the results attending oxidation
and other chemical changes at the cells (page 107). Through these changes
large quantities of materials are produced that can no longer take any
part in the vital processes. They correspond to the ashes and gases of
ordinary combustion and form wastes that must be removed. The most
important of these substances, as