ternal ear the
impression is transmitted to the brain by the nerve, (13.)

971. The auditory nerve, like the optic, has but one function, that of
special sensibility. The nerves which furnish the ear with ordinary
sensibility, proceed from the fifth pair.

HYGIENE OF THE ORGANS OF HEARING.

972. Hearing, like the other senses, is capable of very great
improvement. By cultivation, the blind are enabled to judge with great
accuracy the distance of bodies in motion, and even the height of
buildings. It is also capable of improvement when all the other senses
are perfect. Thus the Indian will distinguish sounds that are
inaudible to the white man.

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969. What parts of the ear are essential in order to hear sounds? What
follows loss of hearing? 971. What is the office of the auditory
nerve? What nerves convey ordinary sensibility to the ear? 972-978.
_Give the hygiene of the organs of hearing._ 972. Is this sense
capable of improvement? How does this sense aid the blind? Is it also
capable of improvement when all the other senses are perfect? In whom
is this illustrated?

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_Note._ Let the anatomy and physiology of the organs of hearing be
reviewed, from fig. 148, or from anatomical outline plate No. 10.

973. Acute hearing requires perfection in the structure and functions
of the different parts of the ear, and that portion of the brain from
which the auditory nerve proceeds. Deafness is by no means unfrequent.
We will now advert to some of the common causes of imperfect hearing.

974. The structure or functional action of the brain may be deranged
by inflammation, by compression, or by debility, and produce deafness.
The first is seen during inflammatory affections of the brain, and in
fevers; the second is seen in accidental injuries of the head; the
third is seen in old age, and after severe diseases of the head, and
fevers. In these cases, applications to, and operations upon, the ear
do no good. The only remedy is to remove, if possible, the diseased
condition of the brain.

975. Imperfect hearing may be produced by the destruction of the
membrana tympani, or removal of the bones of the ear, or the parts
within the labyrinth. In these instances, medical treatment is of no
avail, as the destroyed parts cannot be restored.

976. Hearing may be rendered defective by a diminution of the
vibratory character of the membrana tympani. This may result from a