=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. [Illustration: Pair of Spinal Nerves] =Nervous Substance.=--Nervous substance is divided into two kinds--grey, or cellular, substance and white, or fibrous, substance. The greater part of the grey matter is situated as a layer on the outside of the cerebrum, or great brain, where it forms a rind from one twelfth to one eighth of an inch in thickness, known as the cortex. It is also found on the surface of the cerebellum. Diffuse masses of grey matter are likewise met in the other parts of the brain, and extending downward through the centre of the spinal cord. The function of the grey matter is to form centres to which the nerve fibres tend and carry in stimulations, or from which they commence and carry out impulses.