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.
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.
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.