are carried to and concentrated upon the liquid in the internal
ear (Fig. 154). From here the vibrations pass through the channels of the
cochlea and set into vibration the contents of the scala media and
different portions of the basilar membrane. This serves as a stimulus to
the fibers of the auditory nerve, causing them to transmit impulses which,
on passing to the brain, produce the sensation of hearing.

Much of the peculiar structure of the cochlea is not understood. Its
minute size and its location in the temporal bone make its study extremely
difficult. The connection of the scala vestibula with the scala tympani,
and this with the middle ear, is necessary for the passage of vibrations
through the internal ear. Its liquids, being practically incompressible
and surrounded on all sides by bones, could not otherwise yield to the
movements of the stapes. (See Practical Work.) The rods of Corti are
thought to act as dampers on the basilar membrane, to prevent the
continuance of vibrations when once they are started.

*Detection of Pitch.*--The method of detecting tones of different pitch is
not understood. Several theories have been advanced with reference to its
explanation, one of the most interesting being that proposed by Helmholtz.
This theory is based on our knowledge of sympathetic vibrations. The
basilar membrane, while continuous throughout, may be regarded as made up
of many separate cords of different lengths stretched side by side. A tone
of a given pitch will set into vibration only certain of these cords,
while tones of different pitch will set others into vibration.

Another theory is that the basilar membrane responds to all kinds of
vibrations and the analysis of sound takes place in the brain.

A third view is that the filaments from the hair cells, rather than the
basilar membrane, respond to the vibrations and in turn stimulate the
terminations of the nerve fibers.

[Fig. 155]

Fig. 155--*Diagram* showing how wax may plug the auditory canal and cause
deafness.

*Hygiene of the Ear.*--The ear, being a delicate organ, is frequently
injured by careless or rough treatment. The removal of the ear wax by the
insertion of pointed instruments has been found to interfere with the
natural method of discharge and to irritate the membrane. It should never
be practiced. It is unnecessary in the healthy ear thus to cleanse the
auditory