diness and general efficiency of
muscular contraction. Though it has no known sensory or intellectual
functions, it is very closely connected with the cerebrum, receiving a
tremendous bundle of axons from different parts of the cerebrum, by
way of the brain stem. Possibly these are related to motor activity.
The phrenologists taught that the cerebellum was the center for the
sexual instinct, but there is no evidence in favor of this guess.

Different Levels of Reaction

Let a noise strike the ear and start nerve currents in along the
auditory nerve, passing through the lowest and intermediate centers
and reaching the auditory-sensory area of the cortex. When this last
is aroused to activity, we have a sensation of sound, which is the
first conscious reaction to the external stimulus. Axons running from
the auditory-sensory to the near-by cortex give a perception of some
fact indicated by the external stimulus, and this perception is a {66}
second and higher conscious reaction, which, to be sure, ordinarily
occurs so quickly after the first that introspection cannot
distinguish one as first and the other as second; but the facts of
brain injury, already mentioned, enable us to draw the distinction.
The perceived fact may call up a mental image, or a recognition of
some further fact less directly signified by the noise; these would be
reactions of still higher order. Much of the cortex is apparently not
very directly connected with either the sensory or the motor areas,
and probably is concerned somehow in the recognition of facts that are
only very indirectly indicated by any single sensory stimulus, or with
the planning of actions that only indirectly issue in muscular
movement.

On the sensory and intellectual side, the higher reactions follow the
lower: sensation arouses perception and perception thought. On the
motor side, the lower reactions are aroused by the higher. Thus the
speech center arouses the motor centers for the speech organs,
combining the action of these into the speaking of a word; and in a
similar way, it seems, the intention to speak a sentence expressing a
certain meaning acts as a stimulus to call up in order the separate
words that make the sentence. A general plan of action precedes and
arouses the particular acts and muscular movements that execute the
plan.

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EXERCISES

1. Outline of the chapter. Fill in sub-topics under each of the
following heads:

A. Mental processes