=The Neuron.=--The centres of grey matter are composed of aggregations, or masses, of very small nerve cells called neurons. A neuron may range from 1/300 to 1/3000 of an inch in diameter, and there are several thousand millions of these cells in the nervous system. A developed neuron consists of a cell body with numerous prolongations in the form of white, thread-like fibres. The neuron with its outgoing fibres is the unit of the nervous system. Neurons are supposed to be of three classes, sensory to receive stimulations, motor to send out impulses to the muscles, and association to connect sensory and motor centres. [Illustration: A Neuron in Stages of Development] These neurons, as already noted, are collected into centres, and the outgoing fibres give connection to the cells, the number of connections for each neuron depending upon its outgoing fibres. Some of these connections are already established within the system at birth, while others, as we shall see more fully later, are formed whenever the organism is brought into action in our thinking and doing. To speak of such connections being formed between nerve centres by means of their outgoing fibres does not necessarily mean a direct connection, but may imply only that the fibres of one cell approach nearly enough to those of another to admit of a nervous impulse passing from the one cell to the other. This is often spoken of as the establishment of a path between the centres. =The Nerve Fibres.=--The nerve fibres which transmit impressions to and from the centres of grey matter average about 1/6000 of an inch in thickness, but are often of great length, some extending perhaps half the length of the body. Large numbers of these fibres unite into a sheath or single nerve. It is estimated that the number of fibres in a single nerve number in most cases several thousand, those in the nerve of sight being estimated at about one hundred thousand. The fibres in the white substance of the brain are estimated at several hundred million. =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. [Illustration]