The significance of comparing the threefold aspect of consciousness to the three sides of a triangle consists in the fact that if any side of a triangle is removed no triangle remains. In like manner, none of the three attributes of consciousness could be wanting without the conscious state ceasing to exist as such. No one, for instance, could feel the pressure of a tight shoe without at the same time knowing it, and fixing his attention upon it. Neither could a person at any particular time know that the shoe was pinching him unless he was also attending to and feeling the experience.
CHAPTER XX
MIND AND BODY
Relation of Mind to Bodily Organism.—Notwithstanding the antithesis which has been affirmed to exist between mind and matter, yet a very close relation exists between mind and the material organism known as the body. There are many ways in which this intimate connection manifests itself. Mental excitement is always accompanied with agitation of the body and a disturbance of such bodily processes as breathing, the beating of the heart, digestion, etc. Such mental processes as seeing, hearing, tasting, etc., are found also to depend upon the use of a bodily organ, as the eye, the ear, the tongue, without which it is quite impossible for the mind to come into relation with outside things. Moreover, disease or injury, especially to the organs of sense or to the brain, weakens or destroys mental power. The size of the brain, also, is found to bear a certain relation to mental capacity; the weight of the average brain being about 48 ounces, while the brain of an idiot often weighs only from 20 to 30 ounces.
THE NERVOUS SYSTEM
Brain and Spinal Cord
Divisions of Nervous System.—This intimate connection between mind and body is provided for through the existence of that part of the bodily organism known as the nervous system, and it is this part, together with its associated organs of sense, that chiefly interests the student of psychology. A study of the character and functions of the various parts of the nervous system, and of the nervous substance of which these parts are composed, belongs to physiology rather than to psychology. As the student-teacher is given a general knowledge of the structure of the nervous system in his study of physiology, a brief description will suffice for the present purpose. The nervous system consists of two parts, (1) the central part, or cerebro-spinal centre, and (2) an outer part—the spinal nerves. The central part, or cerebro-spinal centre, includes the spinal cord, passing upward through the vertebrae of the spinal column and the brain. The brain consists of three parts: The cerebrum, or great brain, consisting of two hemispheres, which, though connected, are divided in great part by a longitudinal fissure; the cerebellum, or little brain; and the medulla oblongata, or bulb. The spinal nerves consist of thirty-one pairs, which branch out from the spinal cord. Each pair of nerves contains a right and left member, distributed to the right and the left side of the body respectively. These nerves are of two kinds, sensory, or afferent, (in-carrying) nerves, which carry inward impressions from the outside world, and motor, or efferent, (out-carrying) nerves, which convey impulses outward to the muscles and cause them to contract. There are also twelve pairs of nerves connected with the eye, ear, nose, tongue, and face, which, instead of projecting from the spinal cord, proceed at once from the brain through openings in the cranium. These are, therefore, known as cerebral nerves. In their general character, however, they do not differ from the projection fibres.