parated by the secreting
cells of the kidneys.

*Work of the Liver.*--The liver, already described as an organ of digestion
(page 152), assists in the work of excretion both by changing waste
nitrogenous compounds into urea and by removing from the blood the wastes
found in the bile. While the chief work of the liver is perhaps not that
of excretion, its functions may here be summarized. The liver is, first of
all, a _manufacturing organ_, producing, as we have seen, three distinct
products--bile, glycogen, and urea. On account of the nature of the urea
and the bile, the liver is properly classed as an _excretory organ_; but
in the formation of the glycogen it plays the part of a _storage organ_.
Then, on account of the use made of the bile after it is passed into the
food canal, the liver is also classed as a _digestive organ_. These
different functions make of the liver an organ of the first importance.

*Excretory Work of the Food Canal.*--The glands connected with the food
canal, other than the liver, while secreting liquids that aid in
digestion, also separate waste materials from the blood. These are passed
into the canal, whence they leave the body with the undigested portions of
the food and the waste from the liver. Though the nature and quantity of
the materials removed by these glands have not been fully determined,
recent investigations have tended to enhance the importance attached to
this mode of excretion.

*The Perspiratory Glands.*--The perspiratory, or sweat, glands are located
in the skin. They belong to the type of simple tubular glands and are very
numerous over the entire surface of the body. A typical sweat gland
consists of a tube which, starting at the surface of the cuticle,
penetrates to the under portion of the true skin and there forms a
ball-shaped coil. The coiled extremity, which forms the secreting portion,
is lined with secreting cells and surrounded by a network of capillaries.
The portion of the tube passing from the coil to the surface serves as a
duct (Figs. 91 and 121).

[Fig. 91]

Fig. 91--*Diagram of section through a sweat gland.* _a._ Outer layer of
skin or cuticle. _b._ Dermis or true skin. _d, e._ Sections of the tube
forming the coiled portion of the gland. _c._ Duct passing to the surface.
The other structures of the skin not shown.

The sweat glands secrete a thin, colorless fluid, called _perspiration_,
or sweat.