PHYSIOLOGY OF THE ORGANS OF HEARING.
963. HEARING is that function by which we obtain a knowledge of the
vibratory motions of bodies, which constitute sounds. The precise
function of all the different parts of the ear is not known.
964. The function of that part of the external ear which projects from
the head is to collect sounds and reflect them into the meatus.
965. The membrana tympani serves to facilitate the transmission of
sounds, and also to moderate their intensity. It is so arranged that
it can be relaxed or tightened.
_Observation._ This membrane, when healthy, has no opening; and it
must be apparent that the apprehension which is often expressed, that
insects will penetrate further, is groundless. The pain is owing to
the extreme sensibility of the membrana tympani.
966. The supposed office of the tympanum is to transmit the vibrations
made on the membrana tympani to the internal ear. This is effected by
the air which it contains, and by the chain of small bones that are
enclosed in this cavity.
967. The use of the Eustachian tube is to admit air into the tympanum,
which renders the pressure on both sides equal, and thus its membrane
is kept in a proper state of tension.
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963-971. _Give the physiology of the organs of hearing._ 963. What is
hearing? Are the precise functions of the different parts of the ear
known? 964. What is the function of the external ear? 965. Of the
membrana tympani? What observation in reference to this membrane? 966.
What is the supposed office of the middle ear? 967. What is the use of
the Eustachian tube?
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_Observation._ When near a cannon, or a field-piece, about being
discharged, by opening the mouth the impression upon the auditory
nerve will be diminished, and the unpleasant sensation lessened. This
is the result of the air in the middle ear escaping through the
Eustachian tube, when the vibrations of the membrana tympani are
violent.
[Illustration: Fig. 148. A view of all the parts of the ear. 1, The tube
that leads to the internal ear. 2, The membrana tympani. 3, 4, 5, The
bones of the ear. 7, The central part of the labyrinth, (vestibule.) 8,
9, 10, The semicircular canals. 11, 12, The channels of the cochlea. 13,
The auditory nerve. 14, The channel from the middle ear to the throat,
(Eustachian tube.)]
968. But little is known of the functions of the internal ear; its
parts are filled with a watery fluid, in which the filaments of the
auditory nerve terminate.
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What observation in this connection? 968. What is the function of the
internal ear?
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969. Many of the parts just enumerated aid in hearing, but are not
absolutely essential to this sense. But if the vestibule and auditory
nerve are diseased or destroyed, no sound is then perceived. If this
sense is destroyed in early life, the person also loses the power of
articulating words. Hence a man born deaf is always dumb.
970. The transmission of sound through the different parts of the ear
will now be explained by aid of fig. 148. The vibrations of air are
collected by the external ear, and conducted through the tube (1) to
the membrana tympani, (2.) From the membrane vibrations pass along the
chain of bones, (3, 4, 5.) The bone 5 communicates with the internal
ear, (7, 8, 9, 10, 11, 11, 11, 12, 12, 12.) From the internal 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
thickening of this membrane, or from an accumulation of wax upon its
outer surface. The increased thickness is usually the result of
inflammation, either acute or chronic. The proper treatment is such as
is efficient to remove inflammatory action.
_Observations._ 1st. The introduction of heads of pins into the ear is
a frequent cause of chronic inflammation of the membrana tympani.
Hence this practice should never be adopted, and if acquired, should
be abandoned.
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973. On what does acute hearing depend? 974. State effects on the
hearing in some conditions of the brain. How relieved? 975. Of the
effect on hearing when the bones of the ear or the labyrinth are
destroyed? Is medical treatment of any avail? 976. What conditions of
the drum of the ear may impair hearing? How relieved? What is said of
the introduction of pins to cleanse the ear?
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2d. The accumulations of viscid wax may be softened by dropping some
animal oil into the ear, and then removing it by ejecting warm soap
suds a few hours subsequent to the use of the oil. This may be
repeated for several successive days.
977. Hearing may be impaired by obstruction of the Eustachian tube.
The closure of this canal diminishes the vibratory character of
the air within the tympanum, in the same manner as closing the
opening in the side of a drum. For the same reason, enlarged
tonsils, inflammation and ulceration of the fauces and nasal
passages during and subsequent to an attack of scarlet fever, and
the inflammation attending the "sore throat" in colds, are common
causes of this obstruction.
978. The treatment of such cases of defective hearing, is to have the
tonsils, if enlarged, removed by a surgeon; for the inflammation and
thickening of the parts remedial means should be applied, directed by
a skilful physician. The nostrums for the cure of deafness are
generally of an oleaginous character, and may be beneficial in cases
of defective hearing caused by an accumulation of wax upon the drum of
the ear, but in this respect they are no better than the ordinary
animal oils.
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What is the remedy where there is an accumulation of wax? 977. What is
the effect on hearing if the Eustachian tube is obstructed? 978. What
is the treatment when deafness is caused by inflammation or ulceration
the fauces? What is said of the nostrums used for deafness?
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