[Sidenote:  Organs of Consciousness and Subconsciousness]

It is obvious that the cerebro-spinal nerve system is the organ of consciousness, the apparatus through which the mind exercises its conscious and voluntary 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:  Brains Parts and Functions]

And now, as to the functions of these various parts.  Beginning at the lowest one and moving upward, we find first that the spinal cord consists of through lines of nerves running between the brain and the rest of the body.  At the same time it contains within itself certain nerve centers that are sufficient for many simple bodily movements.  These bodily movements are such as are instinctive or habitual and require no distinct act of the will for their performance.  They are mere “reactions,” without conscious, volitional impulse.