already noted (page 110), are carbon
dioxide, water, and urea.(74) A number of mineral salts are also to be
included with the waste materials. Some of these are formed in the body,
while others, like common salt, enter as a part of the food. They are
solids, but, like the urea, leave the body dissolved in water.

Waste products, if left in the body, interfere with its work (some of,
them being poisons), and if allowed to accumulate, cause death. Their
removal, therefore, is as important as the introduction of food and oxygen
into the body. The most important of the excretory glands are

*The Kidneys.*--The kidneys are two bean-shaped glands, situated in the
back and upper portion of the abdominal cavity, one on each side of the
spinal column. They weigh from four to six ounces each, and lie between
the abdominal wall and the peritoneum. Two large arteries from the aorta,
called the _renal arteries_, supply them with blood, and they are
connected with the inferior vena cava by the _renal veins_. They remove
from the blood an exceedingly complex liquid, called the _urine_, the
principal constituents of which are water, salts of different kinds,
coloring matter, and urea. The kidneys pass their secretion by two slender
tubes, the _ureters_, to a reservoir called the _bladder_ (Fig. 87).

[Fig. 87]

Fig. 87--*Relations of the kidneys.* (Back view.) 1. The kidneys. 2.
Ureters. 3. Bladder. 4. Aorta. 5. Inferior vena cava. 6. Renal arteries.
7. Renal veins.

*Structure of the Kidneys.*--Each kidney is a compound tubular gland and is
composed chiefly of the parts concerned in secretion. The ureter serves as
a duct for removing the secretion, while the blood supplies the materials
from which the secretion is formed. On making a longitudinal section of
the kidney, the upper end of the ureter is found to expand into a
basin-like enlargement which is embedded in the concave side of the
kidney. The cavity within this enlargement is called the _pelvis of the
kidney_, and into it project a number of cone-shaped elevations from the
kidney substance, called the _pyramids_ (Fig. 88).

From the summits of the pyramids extend great numbers of very small tubes
which, by branching, penetrate to all parts of the kidneys. These are the
_uriniferous tubules_, and they have their beginnings at the outer margin
of the kidney in many small, rounded bodies called the _Malpighi