specialized endings of the auditory nerve by vibrations of air.

As the different colors are due to different vibration frequencies of
the ether, so different pitches of sound are due to differences in the
rates of the air vibrations. The low bass notes are produced by the low
vibration frequencies. The high notes are produced by the high
vibration frequencies. The lowest notes that we can hear are produced by
about twenty vibrations a second, and the highest by about forty
thousand vibrations a second.

=Other Sense Organs.= We need not give a detailed statement of the facts
concerning the other senses. In each case the sense organ is some
special adaptation of the nerve-endings with appropriate apparatus in
connection to enable it to be affected by some special thing or force in
the environment.

In the case of taste, we find in the mouth, chiefly on the back and
edges of the tongue, organs sensitive to sweet, sour, salt, and bitter.
In the nose we have an organ that is sensitive to the tiny particles of
substances that float in the air which we breathe in through the nose.

In the skin we find several kinds of sense organs that give us the
sensations of cold and warmth, of pressure and pain. These are all
special and definite sensations produced by different kinds of organs.
The sense of warmth is produced by different organs from those which
produce the sense of cold. These organs can be detected and localized on
the skin. So, also, pain and touch or pressure have each its particular
organ.

Within the body itself we have sense organs also, particularly in the
joints and tendons and in the muscles. These give us the sensations
which are the basis of our perception of motion, and of the position of
the body and its members. In the semicircular canals of the inner ear
are organs that give us the sense of dizziness, and enable us to
maintain our equilibrium and to know up from down.

The general nature of the sense organs and of sensation should now be
apparent. The nervous system reaches out its myriad fingers to every
portion of the surface of the body, and within the body as well. These
nerve-endings are specially adapted to receive each its particular form
of stimulation. This stimulation of our sense organs is the basis or
cause of our sensations. And our sensations are the elementary stuff of
all our experience. Whatever thoughts we have, whatever ideas or images
we have, they come originally from our sens