Another broad division of nerves is into two great nerve systems. There are the cerebrospinal system and the sympathetic system. The first, the cerebrospinal system, includes all the nerves of consciousness and of voluntary action; it includes all nerves running between the brain and spinal cord on the one hand and the voluntary muscles on the other. The second, the sympathetic nerve system, consists of all the nerves of the unconscious or functional life; it therefore includes all nerves running between the brain and sympathetic or involuntary nerve centers on the one hand and the involuntary muscles on the other.

Every bodily movement or function that you can start or stop at will, even to such seemingly unconscious acts as winking, walking, etc., is controlled through the cerebro-spinal system. All other functions of the body, including the great vital processes, such as heart pulsation and digestion, are performed unconsciously, are beyond the direct control of the will, and are governed through the sympathetic nerve system.

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 inflences 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.

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.

Separate Nerve Centers, Plexuses And Ganglia, The Little Brains Of The Human Body

Separate Nerve Centers, Plexuses And Ganglia, The "Little Brains" Of The Human Body

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.

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.

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.

Moving up one step higher, we find that the cerebellum is the organ of equilibrium, and that it as well as the spinal cord operates independently of the conscious will, for no conscious effort of the will is required to make one reel from dizziness.

As to the divisions of the greater brain or cerebrum, we want you to note that the lower brain serves a double purpose. First, it is the channel through which pass through lines of communication to and from the upper brain and the mid-brain on the one hand and the rest of the body on the other. Secondly, it is itself a central office for the maintenance of certain vital functions, such as lung-breathing, heart-beating, saliva-secreting, swallowing, etc., all involuntary and unconscious in the sense that consciousness is not necessary to their performance.

The next higher division, or midbrain, is a large region from which the conscious will issues its edicts regulating all voluntary bodily movements. It is also the seat of certain special senses, such as sight.

Lastly, the surface brain, known as the cortex, is the interpretative and reflective center, the abode of memory, intellect and will.