ls, basement membrane, capillary and lymph
vessels, and nerve fibers--form the essential parts of all glands. The
capillaries and the lymph vessels supply the secreting cells with fluid,
and the nerves control their activities.

*Kinds of Glands.*--Glands differ from one another chiefly in the
arrangement of their essential parts.(73) The most common plan is that of
arranging the parts around a central cavity formed by the folding or
pitting of an exposed surface. Many such glands are found in the mucous
membrane, especially that lining the alimentary canal, and are most
numerous in the stomach, where they supply the gastric juice. If these
glands have the general form of tubes, they are called _tubular_ glands;
if sac-like in shape, they are called _saccular_ glands. Both the tubular
and the saccular glands may, by branching, form a great number of similar
divisions which are connected with one another, and which communicate by a
common opening with the place where the secretion is used. This forms a
_compound_ gland which, depending on the structure of the minute parts,
may be either a _compound tubular_ or a _compound saccular_ gland. The
larger of the compound saccular glands are also called _racemose_ glands,
on account of their having the general form of a cluster, or raceme,
similar to that of a bunch of grapes. The general structure of the
different kinds of glands is shown in Fig. 85.

[Fig. 85]

Fig. 85--*Diagram illustrating evolution of glands.* _A._ Simple secreting
surface. 1. Gland cells. 2. Basement membrane. 3. Blood vessel. 4. Nerve.
_B._ Simple tubular gland. _C._ Simple saccular gland. _D._ Compound
tubular gland. _E._ Compound saccular gland. _F._ A compound racemose
gland with duct passing to a free surface. _G._ Relation of food canal to
different forms of glands. The serous coat has a secreting surface.

*Nature of the Secretory Process.*--At one time the gland was regarded
merely as a kind of filter which separated from the blood the ingredients
found in its secretions. Recent study, however, of several facts relating
to secretion has led to important modifications of this view. The
secretions of many glands are known to contain substances that are not
found in the blood, or, if present, are there in exceedingly small
amounts. Then again the cells of certain glands have been found to undergo
marked changes during the process of secretion. If, for exa