force or substance. Vibrations of ether affect the eye. Vibrations of
air affect the ear. Liquids and solutions affect the sense of taste.
Certain substances affect the sense of smell. Certain organs in the skin
are affected by low temperatures; others, by high temperatures; others,
by mechanical pressure. Similarly, each sense organ in the body is
affected by a definite kind of force or substance.

This affecting of a sense organ is known technically as _stimulation_,
and that which affects the organ is known as the _stimulus_.

Two important consequences ordinarily follow the stimulation of a sense
organ. One of these is movement. The purpose of stimulation is to bring
about movement. To be alive is to respond to stimulation. When one
ceases to respond to stimulation, he is dead. If we are to continue
alive, we must constantly adjust ourselves to the forces of the world in
which we live. Generally speaking, we may say that every nerve has one
end in a sense organ and the other in a muscle. This arrangement of the
nerves and muscles shows that man is essentially a sensitive-action
machine. The problems connected with sensitivity and action and the
relation of each to the other constitute a large part of the field of
psychology.

We said just now, that a nerve begins in a sense organ and ends in a
muscle. This statement represents the general scheme well enough, but
leaves out an important detail. The nerve does not extend directly to a
muscle, but ordinarily goes by way of the brain. The brain is merely a
great group of nerve cells and fibers which have developed as a central
organ where a stimulation may pass from almost any sense organ to
almost any muscle.