F an electric current is applied to the closed eyelids we have a sensation of colour, if to the ears, a sensation of sound, if to the tongue, or nose, a sensation of taste or smell. What is apparently one and the same stimulus applied to different sense organs gives rise in the brain to different sensations. It seems easy to explain this by what we know of the anatomy and physiology of the brain. When our sense organs are stimulated, the stimulus is conveyed by means of sensory nerves to the brain and there gives rise to sensation, the sense organs are sensitive to various kinds of stimuli, the stimulus to the eyes gives the sensation of sight, to the ears the sensation of sound and so on. It seems then it must be the brain which gives the special character of our sensations to consciousness, for we have only to send the brain a stimulus through the optic nerve to get sight, through the auditory nerve to get sound and so on, and this notwithstanding that the stimulus has in itself none of the things which we distinguish as sensible qualities. Since it is out of sensations that all knowledge is built up, for there is nothing in the understanding which was not first in the senses, it seems to follow that our mind, that is the power or faculty we have of perceiving, remembering and imagining, must be produced by the brain, that is, be an effect of some process or other that goes on in the brain.
But it is not only by experiments such as this that we make the discovery that there is a close and intimate relation between the mind and the brain, a relation which makes the mind dependent on the brain. In ordinary discourse we use the words brain and mind as interchangeable terms, because it is quite evident that where there is lack of intelligence there is deficiency in the brain and where there is a high degree of intelligence there is a correspondingly high development of brain. The immense advance in recent times in our knowledge of the physiology of cerebral process serves more and more to emphasize the entire and absolute dependence of mind on brain, and particularly of the higher processes of mind on the development of the cells and fibres of the cerebral cortex. Physiologists tell us that there are the almost inconceivable number of something like 7000 million nerve cells in the brain of the newly born child and these, which neither increase nor decrease throughout life, undergo a development of interconnection by means of the branching fibres they send out in every direction. When through disease or malformation or any other cause this development is arrested we have mental deficiency or idiocy.
The brain is an organ of immense complexity and has a great deal more to do than only to turn stimuli into sensations. It is part of a complete nervous system and it is only a part of the brain that is concerned in mental process. A nervous system or something that closely corresponds to it seems to be a necessary acquirement of every living organism that moves freely, and its structure, however complex it may be in its higher development, is built up of a simple arrangement of cells and fibres that receive and transmit movement. Its function is to receive a stimulus and to transmit it to a centre where a responsive movement is prepared. The nervous system is therefore described as sensori-motor from the double function that it performs. There are two kinds of response to a stimulus—one immediate and automatic, the other conscious and willed. It is only those stimuli that are transmitted to the cerebral cortex, and only some of these, that give rise to consciousness; consciousness seems to occur just at the moment when the movement in the fibres reaches the cerebral cortex; some of these movements, not all, give rise to consciousness. If a movement gives rise to consciousness it occurs when a stimulus reaches the cortex and before it passes from the brain to the muscles to issue in actions. There appears to be a power of inhibiting or delaying the response to the stimulus while consciousness lasts and consciousness seems to have the function of giving us a choice of the direction which the response shall take.