he exterior of the body is a
tube, called the _urethra_, through which the urine is voided.

The excretion of the kidneys, termed the _urine_, is an amber-colored or
straw-colored fluid, naturally having a slightly acid reaction, and a
specific gravity ranging from 1,015 to 1,025. Its principal constituents
are _urea_ and _uric acid_, together with various other animal matters
of less importance, and saline substances, held in solution in a
proportionately large amount of water. The composition of the urine and
the quantity excreted vary considerably, being influenced by the
moisture and temperature of the atmosphere, by the character of the food
consumed, and by the empty or replete condition of the alimentary tract.
On an average a healthy man secretes about fifty ounces of urine in the
twenty-four hours. This quantity usually holds in solution about one
ounce of urea, and ten or twelve grains of uric acid. In the amount of
other animal matters, and saline substances, there is great variation,
the quantity of these ranging from a quarter of an ounce to an ounce.
The principal saline substances are common salt, the sulphates and
phosphates of potassium, sodium, calcium, and magnesium. In addition to
the animal and the saline matters, the urine also contains a small
quantity of carbonic acid, oxygen and nitrogen.

* * * * *

CHAPTER XII.

PHYSIOLOGICAL ANATOMY.

THE NERVOUS SYSTEM.

Hitherto, we have only considered the anatomy and functions of the
organs employed in Digestion, Absorption, Circulation, Respiration,
Secretion and Excretion. We have found the vital process of nutrition to
be, in all its essential features, a result of physical and chemical
forces; in each instance we have presupposed the existence and activity
of the nerves. There is not an inch of bodily tissue into which their
delicate filaments do not penetrate, and form a multitude of conductors,
over which are sent the impulses of motion and sensation.

[Illustration: Fig. 54.
The Nervous System.]

Two elements, _nerve-fibers_ and _ganglionic corpuscles_, enter into the
composition of nervous tissue. Ordinary nerve-fibers in the living
subject, or when fresh, are cylindrical-shaped filaments of a clear, but
somewhat oily appearance. But soon after death the matter contained in
the fiber coagulates, and then the fiber is seen to consist of an
extremely delicate, structureless, outer membrane, which form