Classes of Fibres.—These fibres are supposed to be of four classes, as follows:
1. Sensory Cerebral and Spinal Fibres
These have already been referred to as spreading outward from the brain and spinal cord to different parts of the body. Their office is, therefore, to carry inward to the centres of grey matter impressions received from the outside world, thus setting up a connection between the various senses and the cortex of the brain.
2. Motor Cerebral and Spinal Fibres
These fibres connect the centres of grey matter directly with the muscles, and thus provide a means of communication between these muscles and the cortex of the brain.
3. Association Fibres
These connect one part of the cortex with another within the same hemisphere.
4. Commissural Fibres
These connect corresponding centres of the two hemispheres of the cerebrum.
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.