=Function of Parts.=--Because the various cells are thus brought into relation, the whole nervous system combines into a single organism, which is able to receive impressions and provides conditions for the mind to interpret these impressions and, if necessary, react thereon. When, for instance, a stimulus is received by an end organ (the eye), it will be transmitted by a sensory nerve directly inward to a sensory centre, or cell, in the cortex of the brain. In such a case it may be interpreted by the mind and a line of action decided upon. Then by means of associating cells and fibres a motor centre may be stimulated and an impulse transmitted along an outgoing motor nerve to a muscle, whereupon the necessary motor reaction will take place. A pupil may, for instance, receive the impression of a word through the ear or through the eye and thereupon make a motor response by writing the word. The arrows in the accompanying figure indicate the course of the stimulus and the response in such cases. 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. [Illustration: REFLEX ACTS]