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Full text of "Diseases Of The Nose Throat And Ear"

CHAPTER 47

PHYSIOLOGY AND FUNCTIONAL
EXAMINATION OF THE EAR

For clarity the consideration of the physiology of the auditory and vestibular
apparatus in this chapter is followed immediately by an account of the methods
employed in their functional examination. In clinical practice, however, the
history of the patient's case is taken first and this is followed by otoscopic
examination before the functional tests are carried out.

PHYSIOLOGY OF THE AUDITORY APPARATUS

Airborne sound consists of vibrations of the atmosphere, that is, of alternate
phases of condensation and rarefaction. These vibrations of air are converted
into vibrations of the fluids of the inner ear, which in turn converts them
into nerve impulses to be transmitted along the auditory nerve.

The auricle to some extent collects the sound waves and they are trans-
mitted along the external acoustic meatus to the tympanic membrane which is
set in motion. The vibrations of the tympanic membrane are transmitted to the
malleus, incus and stapes. The movement of the footplate of the stapes in the oval
window (fenestra vestibuli) transmits the vibrations to the perilymph and
endolymph. The conversion of airborne to fluid borne sound is assisted slightly
by the lever system formed by the ossicles but to a much greater extent by the
marked difference in area between the tympanic membrane and footplate of
the stapes. During the compression phase of airborne sound the stapes is
pushed inwards; the fluids of the ear cannot be compressed with the result
that the round window membrane is forced outwards. This reciprocal action
of the oval and round windows is essential for transmission of sound to the
inner ear fluids. In the normal ear the presence of the t^ppanic membrane
and an air-containing middle ear prevents the compression wave of airborne
sound from reaching the round window and opposing the outward movement
of the round window membrane. This protection of the round window is lost
where there is a large perforation of the tympanic membrane and this is one
of the factors which may produce deafness.

Optimum movement of the tympanic membrane depends upon equal
pressure on its inner and outer aspects. This is achieved by the Eustachian
tube (auditory tube) which normally opens during each act of swallowing.
In this way middle ear pressure can be maintained at the same level as that
in the external acoustic meatus during changes in atmospheric pressure. Reflex
contraction of the tensor tympani and stapedius muscles, in response to loud
sound stimuli, dampens the movements of the ossicles and thus provides some
protection against excessive movement.

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