THE CORTEX
Cortex the Seat of Consciousness.—Experiments in connection with the different nerve cords and centres have demonstrated that intelligent consciousness depends upon the nerve centres situated in the cortex of the cerebrum. For instance, a sensory impulse may be carried inward to the cells of the spinal cord and upward to the cerebellum without any resulting consciousness. When, however, the stimulus reaches a higher centre in the cortex of the brain, the mind becomes conscious, or interprets the impression, and any resulting action will be controlled by consciousness, through impulses given to the motor nerves. It is for this reason that the cortex is called the seat of consciousness, and that mind is said to reside in the brain.
Localization of Function.—In addition, however, to placing the seat of consciousness in the cortex of the brain, psychologists also claim that different parts of the cortex are involved in different types of conscious activity. Sensations of sight, for instance, involve certain centres in the cortex, sensations of sound other centres, the movements of the organs of speech still other centres. Some go so far as to claim that each one of the higher intellectual processes, as memory, imagination, judgment, reasoning, love, anger, etc., involves neural activity in its own special section of the cortex. There seems no good evidence, however, to support this view. The fact seems rather that in all these higher processes, quite numerous centres of the cortex may be involved. The following figure indicates the main conclusions of the psychologists in reference to the localization of certain important functions in distinct areas of the cortex.
REFLEX ACTS
Nature of Reflex Action.—While a lower nerve centre is not a seat for purposeful consciousness, these centres may, in addition to serving as transmission points for cortical messages, perform a special function by immediately receiving sensory impressions and transmitting motor impulses. A person, for instance, whose mind is occupied with a problem, may move a limb to relieve a cramp, wink the eye, etc., without any conscious control of the action. In such a case the sensory impression was reported to a lower sensory centre, directly carried to a lower motor centre, and the motor impulse given to perform the movement. In the same way, after one has acquired the habit of walking, although it usually requires conscious effort to initiate the movements, yet the person may continue walking in an almost unconscious manner, his mind being fully occupied with other matters. Here, also, the complex actions involved in walking are controlled and regulated by lower centres situated in the cerebellum. In like manner a person will unconsciously close the eyelid under the stimulus of strong light. Here the impression caused by the light stimulus, upon reaching the medulla along an afferent nerve, is deflected to a motor nerve and, without any conscious control of the movements, the muscles of the eyelid receive the necessary impulse to close. Actions which are thus directed from a lower centre without conscious control, are usually spoken of as reflex acts. Acts directed by consciousness are, on the other hand, known as voluntary acts. The difference in the working of the nervous mechanism in consciously controlled and in reflex action may be illustrated by means of the accompanying figures.
The heavy lines in Figure 1 on the opposite page show that the sensory-motor arc is made through the cortex, and that the mind is, therefore, conscious both of the sense stimulus and also of the resulting action. Figure 2 shows the same arc through a lower centre, in which case the mind is not directly attending to the impression or the resulting action.
Function of Consciousness.—The facts set forth above serve further to illustrate the purposeful character of consciousness as man interprets and adjusts himself to his surroundings. So long, for instance, as the individual walks onward without disturbance, his mind is free to dwell upon other matters, cortical activity not being necessary to control the process of walking. If, however, he steps upon anything which perhaps threatens him with a fall, the rhythmic interplay between sensory and motor activity going on in the lower centres is at once disturbed, and a message is flashed along the sensory nerve to the higher, or cortical, centres. This at once arouses consciousness, and the disturbing factor becomes an object of attention. Consciousness thus appears as a means of adaptation to the new and varying conditions with which the organism is confronted.