nd regulated by lower centres
situated in the cerebellum. In like manner a person will unconsciously
close the eyelid under the stimulus of strong light. Here the impression
caused by the light stimulus, upon reaching the medulla along an
afferent nerve, is deflected to a motor nerve and, without any conscious
control of the movements, the muscles of the eyelid receive the
necessary impulse to close. Actions which are thus directed from a lower
centre without conscious control, are usually spoken of as reflex acts.
Acts directed by consciousness are, on the other hand, known as
voluntary acts. The difference in the working of the nervous mechanism
in consciously controlled and in reflex action may be illustrated by
means of the accompanying figures.

[Illustration: FIG 1]

[Illustration: FIG 2]

The heavy lines in Figure 1 on the opposite page show that the
sensory-motor arc is made through the cortex, and that the mind is,
therefore, conscious both of the sense stimulus and also of the
resulting action. Figure 2 shows the same arc through a lower centre, in
which case the mind is not directly attending to the impression or the
resulting action.

=Function of Consciousness.=--The facts set forth above serve further to
illustrate the purposeful character of consciousness as man interprets
and adjusts himself to his surroundings. So long, for instance, as the
individual walks onward without disturbance, his mind is free to dwell
upon other matters, cortical activity not being necessary to control the
process of walking. If, however, he steps upon anything which perhaps
threatens him with a fall, the rhythmic interplay between sensory and
motor activity going on in the lower centres is at once disturbed, and a
message is flashed along the sensory nerve to the higher, or cortical,
centres. This at once arouses consciousness, and the disturbing factor
becomes an object of attention. Consciousness thus appears as a means of
adaptation to the new and varying conditions with which the organism is
confronted.

CHARACTERISTICS OF NERVOUS MATTER

=A. Plasticity.=--One striking characteristic of nervous matter is its
plasticity. The nature of the connections within the nervous system have
already been referred to. Mention has also been made of the fact that
numerous connections are established within the nervous system as a
result of movements taking place within the organism during life. In
other words, the movements wit