king, they are the states of mind experienced as the
_direct_ result of impulses reaching the brain. In a sense, just as
impulses passing to the muscles cause motion, impulses passing to the
brain cause sensations. The feeling which results from the hand's touching
a table is a sensation and so also is the pain which is caused by an
injury to the body. The mental action in each case is due to impulses
passing to the brain. Care must be exercised by the beginner, however, not
to confuse sensations with the nervous impulses, on the one hand, or with
_secondary_ mental effects, such as emotion or imagination, on the other.
Sensations are properly regarded as the first conscious effects of the
afferent impulses and as the _beginning stage_ in the series of mental
processes that may take place on account of them.

In some way, not understood, the mind associates the sensation with the
part of the body from which the impulses come. Pain, for example, is not
felt at the brain where the sensation is produced, but at the place where
the injury occurs. This association, by the mind, of the sensations with
different parts of the body, is known as "localizing the sensation."

*Sensation Stimuli.*--While the sensations are dependent upon the afferent
impulses, the afferent impulses are in turn dependent upon causes outside
of the nervous system. If these are removed, the sensations cease and they
do not start up again unless the exciting influences are again applied.
Any agency, such as heat or pressure, which, by acting on the neurons of
the body, is able to produce a sensation, may be called a _sensation
stimulus_. It has perhaps already been observed that the stimuli that lead
to voluntary action, as well as those that produce reflex action of the
muscles, cause sensations at the same time. From this we may conclude that
sensation stimuli are the same in character as those that excite motion.
On the other hand, it should be noted that sensations are constantly
resulting from stimuli that are of too mild a nature to cause motion.

*Classes of Sensations.*--Perhaps as many as twenty distinct sensations,
such as pain, hunger, touch, etc., are recognized. If these are studied
with reference to their origin, it will be seen that some of them result
from the action of definite forms of stimuli upon the neurons terminating
in sense organs; while the others, as a rule, arise from the action of
indefinite stimuli upon neurons in parts of