canal, as the wax is passed by a natural process to where it is
easily removed by a damp cloth. If the natural process is obstructed,
clean warm water and a soft linen cloth may be employed in cleansing the
canal, without likelihood of injury. Clean warm water may also be
introduced into the auditory canal as a harmless remedy in relieving
inflammation of the auditory canal and of the middle ear. Children's ears
are easily injured, and it goes without saying that they should never be
pulled nor boxed.

It frequently happens that a mass of wax collects in the auditory canal
and closes the passage so completely as to cause deafness (Fig. 155). This
may come about without pain and so gradually that one does not think of
seeking medical aid. Such masses are easily removed by the physician, the
hearing being then restored. Both for painful disturbances of the ear and
for the gradual loss of hearing, the physician should be consulted.

*The Hearing of School Children.*--School children not infrequently have
defective hearing and for this reason are slow to learn. The hearing is
easily tested with a watch, the normal ear being able to hear the watch
tick at a distance of at least two feet. Pupils with defective hearing
should, of course, have medical attention, and in the classroom should be
seated where they can hear to the best advantage.

*Summary.*--Sound waves constitute the external stimuli for the sensation
of hearing. They consist of progressive vibratory movements of the air
that originate in vibrating bodies. Through the larynx and the ear, sound
waves are utilized by the body in different ways, but chiefly as a means
of communication. The larynx produces sound waves which are reenforced and
modified by the air passages. The ear supplies suitable conditions for the
action of sound waves upon nerve cells. Both the ear and the larynx are
constructed with special reference to the nature and properties of sound
waves, and they illustrate the body's ability to adjust itself to, and to
make use of, its physical environment.

*Exercises.*--1. For what different purposes are sound waves employed in
the body?

2. How do sound waves originate? How are they transmitted? How do they
differ from the waves on water?

3. How are sound waves able to act as nerve stimuli?

4. Describe two methods of reenforcing sound waves. Which method is
employed in the body?

5. Name all the parts of the body that are directly or indirectl