The Great Nerve Centers
The great nerve centers which play an important part in the production and expression of mental states are those of the brain and spinal cord, respectively.
THE SPINAL CORD.
The spinal cord is that cord or rope of nerve substance which is inclosed in the spinal column or “backbone.” It leaves the lower part of the skull and extends downward in the interior of the spinal column for about eighteen inches. It is continuous with the brain, however, and it is difficult to determine where one begins and the other ends. It is composed of a mass of gray matter surrounded by a covering of white matter. From the spinal cord, along its length, emerge thirty-one pairs of spinal nerves which branch out to each side of the body and connect with the various smaller nerves, extending to all parts of the system. The spinal cord is the great central cable of the nervous telegraphic system, and any injury to or obstruction of it cripples or paralyzes those portions of the body the nerves of which enter the spinal cord below the seat of the injury or obstruction. Injuries or obstructions of this kind not only inhibit the sensory reports from the affected area, but also inhibit the motor impulses from the brain which are intended to move the limbs or parts of the body.
THE GANGLIA OR “TINY BRAINS.”
What are known as ganglia, or tiny bunches of nerve cells, are found in various parts of the nervous system, including the spinal nerves. These groups of nerve cells are sometimes called “little brains,” and perform quite important offices in the mechanism of thought and action. The spinal ganglia receive sensory reports, and issue motor impulses, in many cases, without troubling the central brain regarding the matter. These activities are known as “reflex nervous action.”
What is known as reflex nervous action is one of the most wonderful of the activities of the nervous and mental mechanism, and the knowledge thereof usually comes as a surprise to the average person, for he is generally under the impression that these activities are possible only to the central brain. It is a fact that not only is the central brain really a trinity of three brains, but that, in addition to these, every one has a great number of “little brains” distributed over his nervous system, any and all of which are capable of receiving sensory reports and also of sending forth motor impulses. It is quite worth while for one to become acquainted with this wonderful form of neuro-mental activity.
A cinder enters the eye, the report reaches a ganglion, a motor impulse is sent forth, and the eyelid closes. The same result ensues if an object approaches the eye but without actually entering it. In either case the person is not conscious of the sensation and motor impulse until the latter has been accomplished. This is reflex action. The instinctive movement of the tickled foot is another instance. The jerking away of the hand burnt by the lighted end of the cigar, or pricked by the point of the pin, is another instance. The involuntary activities, and those known as unconscious activities, result from reflex action.
More than this, it is a fact that many activities originally voluntary become what is known as “acquired reflexes,” or “motor habits,” by means of certain nervous centers acquiring the habit of sending forth certain motor impulses in response to certain sensory reports. The familiar movements of our lives are largely performed in this way, as, for instance, walking, using knife and fork, operating typewriters, machines of all kinds, writing, etc. The squirming of a decapitated snake, the muscular movements of a decapitated frog, and the violent struggles, fluttering, and leaps of the decapitated fowl, are instances of reflex action. Medical reports indicate that in cases of decapitation even man may manifest similar reflex action in some cases. Thus we may see that we may _feel_ and _will_ by means of our “little brains” as well as by the central brain or brains. Whatever mind may be, it is certain that in these processes it employs other portions of the nervous system than the central brain.
THE THREE BRAINS.
What is known as the brain of man is really a trinity of three brains, known respectively as (1) the _medulla oblongata_, (2) the _cerebellum_, and (3) the _cerebrum_. If one wishes to limit the mental activity to conscious intellectual effort, then and then only is he correct in considering the cerebrum or large brain as “the brain.”
_The Medulla Oblongata._–The medulla oblongata is an enlargement of the spinal cord at the base of the brain. Its office is that of controlling the involuntary activities of the body, such as respiration, circulation, assimilation, etc. In a broad sense, its activities may be said to be of the nature of highly developed and complex reflex activities. It manifests chiefly through the sympathetic nervous system which controls the vital functions. It does not need to call on the large brain in these matters, ordinarily, and is able to perform its tasks without the plane of ordinary consciousness.
_The Cerebellum._–The cerebellum, also known as “the little brain,” lies just above the medulla oblongata, and just below the rear portion of the cerebrum or great brain. It combines the nature of a purely reflex center on the one hand, with that of “habit mind” on the other. In short, it fills a place between the activities of the cerebrum and the medulla oblongata, having some of the characteristics of each. It is the organ of a number of important acquired reflexes, such as walking, and many other familiar muscular movements, which have first been consciously acquired and then become habitual. The skilled skater, bicyclist, typist, or machinist depends upon the cerebellum for the ease and certainty with which he performs his movements “without thinking of them.” One may be said never to have thoroughly acquired a set of muscular movements such as we have mentioned, until the cerebellum has taken over the task and relieved the cerebrum of the conscious effort. One’s technique is never perfected until the cerebellum assumes control and direction of the necessary movements and the impulses are sent forth from below the plane of ordinary consciousness.
_The Cerebrum._–The cerebrum, or “great brain” (which is regarded as “the brain” by the average person), is situated in the upper portion of the skull, and occupies by far the larger portion of the cavity of the skull. It is divided into two great divisions or hemispheres. The best of the modern authorities are agreed that the cerebrum has zones or areas of specialized functioning, some of which receive the sensory reports of the nerves and organs of sense, while others send forth the motor impulses which result in voluntary physical action. Many of these areas or zones have been located by science, while others remain as yet unlocated. The probability is that in time science will succeed in correctly locating the area or zone of each and every class of sensation and motor impulse.
The area of thought, memory, and imagination has not been clearly located, except that these mental states are believed to have their seat in the _cortex_ or outer thin rind of gray brain matter which envelopes and covers the mass of brain substance. It is, moreover, considered probable that the higher processes of reasoning are performed in or by the cortex of the frontal lobes. The cortex of a person of average intelligence, if spread out on a flat surface, measures about four square feet. The higher the degree of intelligence possessed by a lower animal or human being, as a rule, the deeper and more numerous are the folds or convolutions of the cortex, and the finer its structure. It may be stated as a general rule, with but very few exceptions, that the higher the degree of intelligence in a lower animal or human being, the greater is the area of its cortex in proportion to the size of the brain. The cortex, it must be remembered, is folded into deep furrows or convolutions, the brain in shape, divisions, and convolutions resembling the inner portion of an English walnut. The interior of the two hemispheres of the cerebrum is composed largely of connective nerves which doubtless serve to produce and maintain the unity of function of the mental processes.
While physiological psychology has performed great work in discovering brain-centers and explaining much of the mechanism of mental processes, it has but touched the most elementary and simple of the mental processes. The higher processes have so far defied analysis or explanation in the terms of physiology.