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
unmedullated fibers—that is of both "white" and "gray" matter. In the medulla,
the unmedullated neurones which comprise the center of the cord are passing to
the outside, and the medullated to the inside, thus taking the positions they
occupy in the cerebrum. Here also the neurones are crossing, or changing sides,
so that those which pass up the right side of the cord finally connect with the left
side of the brain, and vice versa.
The Cerebellum.—Lying just back of the medulla and at the rear part of the
base of the cerebrum is the cerebellum, or "little brain," approximately as large
as the fist, and composed of a complex arrangement of white and gray matter.
Fibers from the spinal cord enter this mass, and others emerge and pass on into
the cerebrum, while its two halves also are connected with each other by means
of cross fibers.