The term _Secretion_, in its broadest sense, is applied to that process
by which substances are separated from the blood, either for the
reparation of the tissues or for excretion. In the animal kingdom this
process is less complicated than in vegetables. In the former it is
really a _separation_ of nutritive material from the blood. The process,
when effected for the removal of effete matter, is, in a measure,
chemical, and accordingly the change is greater.

Three elementary constituents are observed in secretory organs: the
cells, a basement membrane, and the blood-vessels. Obviously, the most
_essential_ part is the _cell_.

The physical condition necessary for the healthy action of the secretory
organs is a copious supply of blood, in which the nutritive materials
are abundant. The nervous system also influences the process of
secretion to a great extent. Intense emotion will produce tears, and the
sight of some favorite fruit will generally increase the flow of saliva.

The process of secretion depends upon the anatomical and chemical
constitution of the cell-tissues. The principal secretions are (1),
Perspiration; (2), Tears; (3), Sebaceous matter; (4), Mucus; (5),
Saliva; (6), Gastric juice; (7), Intestinal juice; (8), Pancreatic
juice; (9), Bile; (10), Milk.

PERSPIRATION is a watery fluid secreted in minute glands, which are
situated in every part of the skin, but are more numerous on the
anterior surfaces of the body. Long thread-like tubes, only 1/100th of
an inch in diameter, lined with epithelium, penetrate the skin, and