bers 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]

=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.

THE CORTEX

=Cortex the Seat of Consciousness.=--Experiments in connection with the
different nerve cords and centres have demonstrated that intelligent
consciousness depends upon the nerve centres situated in the cortex of
the