They provide the necessary conditions for intelligent and purposeful
action on the part of the body.
2. They supply the basis for the higher mental activities, as perception,
memory, thought, imagination, and emotion.
Intelligent action is impossible without a knowledge both of the bodily
organs and of the body's surroundings. Protection and the regulation of
the work of an organ necessitate a knowledge of its condition, while the
adapting and adjusting of the body to its surroundings require a knowledge
of what those surroundings are. The dependence of all the higher forms of
mental activity upon sensations is recognized by psychologists and is
easily demonstrated by a study of the manner in which we acquire
knowledge. "Without sensation there can be no thought."
*Steps in the Production of Sensations.*--The steps in the production of
sensations are not essentially different from those in the production of
reflex action. First of all, external stimuli act upon the fiber
terminations in the sense organs, or elsewhere, starting impulses in the
neurons. These pass into the central nervous system and there excite
neurons which in turn discharge impulses into the cerebrum. The result is
to arouse an activity of the mind--a sensation.
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