the brain and cord contain some 3,000 millions of them.

COMPLEXITY OF THE BRAIN.--Something of the complexity of the brain
structure can best be understood by an illustration. Professor Stratton
estimates that if we were to make a model of the human brain, using for
the neurone fibers wires so small as to be barely visible to the eye, in
order to find room for all the wires the model would need to be the size
of a city block on the base and correspondingly high. Imagine a
telephone system of this complexity operating from one switch-board!

"GRAY" AND "WHITE" MATTER.--The "gray matter" of the brain and cord is
made up of nerve cells and their dendrites, and the terminations of
axons, which enter from the adjoining white matter. A part of the mass
of gray matter also consists of the neuroglia which surrounds the nerve
cells and fibers, and a network of blood vessels. The "white matter" of
the central system consists chiefly of axons with their enveloping or
medullary, sheath and neuroglia. The white matter contains no nerve
cells or dendrites. The difference in color of the gray and the white
matter is caused chiefly by the fact that in the gray masses the
medullary sheath, which is white, is lacking, thus revealing the ashen
gray of the nerve threads. In the white masses the medullary sheath is
present.

4. GROSS STRUCTURE OF THE NERVOUS SYSTEM

DIVISIONS OF THE NERVOUS SYSTEM.--The nervous system may be considered
in two divisions: (1) The _central_ system, which consists of the brain
and spinal cord, and (2) the _peripheral_ system, which comprises the
sensory and motor neurones connecting the periphery and the internal
organs with the central system and the specialized end-organs of the
senses. The _sympathetic_ system, which is found as a double chain of
nerve connections joining the roots of sensory and motor nerves just
outside the spinal column, does not seem to be directly related to
consciousness and so will not be discussed here. A brief description of
the nervous system will help us better to understand how its parts all
work together in so wonderful a way to accomplish their great result.

THE CENTRAL SYSTEM.--In the brain we easily distinguish three major
divisions--the _cerebrum_, the _cerebellum_ and the _medulla oblongata_.
The medulla is but the enlarged upper part of the cord where it connects
with the brain. It is about an inch and a quarter long, and is composed
of both medullated and unmedull