oluntary control over certain functions of the body. It
is equally obvious that the _sympathetic system is not under the
immediate control of consciousness, is not subject to the will, but is
dominated by mental influences that act without, or even contrary to,
our conscious will and sometimes without our knowledge._

Yet you are not to understand that these two great nerve systems are
entirely distinct in their operations. On the contrary, they are in many
respects closely related.

[Illustration: SEPARATE NERVE CENTERS, PLEXUSES AND GANGLIA, THE "LITTLE
BRAINS" OF THE HUMAN BODY]

Thus, the heart receives nerves from both centers of government, and
besides all this is itself the center of groups of nerve cells. The
power by which it beats arises from a ganglionic center within the heart
itself, so that the heart will continue to beat apart from the body if
it be supplied with fresh blood. But the rapidity of the heart's beating
is regulated by the cerebro-spinal and sympathetic systems, of which the
former tends to retard the beat and the latter tends to accelerate it.

In the same way, your lungs are governed in part by both centers, for
you can breathe slowly or rapidly as you will, but you cannot, by any
power of your conscious will, stop breathing altogether.

Your interest in the brain and nerve system is confined to such facts as
may prove to be of use to you in your study of the mind. These
anatomical divisions interest you only as they are identified with
conscious mental action on the one hand and unconscious mental action on
the other.

It is, therefore, of no use to you to consider the various divisions of
the sympathetic nerve system, since the sympathetic nerve system in its
entirety belongs to the field of unconscious mental action. It operates
without our knowledge and without our will.

[Sidenote: Looking Inside the Skull]

The cerebro-spinal system consists of the spinal cord and the brain. The
brain in turn is made up of two principal subdivisions. First, there is
the greater or upper brain, called the cerebrum; secondly, there is the
lower or smaller brain, called the cerebellum. The cerebrum in turn
consists of three parts: the convoluted _surface_ brain, the _middle_
brain and the _lower_ brain. So that in all we have the _surface_ brain,
the _middle_ brain, the _lower_ brain and the _cerebellum_. All these
parts consist of masses of brain cells with connecting nerve fibers.

[Sidenote: