he 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.

=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 num