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

264                                          THE EAR

Vibrations of perilymph and endolymph produce movements of the basilar
membrane and shearing movements between the hair cells and tectorial
membrane of the organ of Corti and this initiates nerve impulses in the fibres
of the auditory nerve. In the case of low frequency sound there results a
synchronous nerve impulse with each sound wave and this simple analysis of
frequency is possible up to about 1000 impulses per second but is limited by
the rate at which nerve fibres can carry impulses. At higher sound frequencies
some alternative method of coding frequency is required. Different parts of
the basilar membrane respond maximally to sound of different frequency, the
basal end of the basilar membrane responding maximally to high frequencies
and the apical end to low frequencies. As each part of the organ of Corti in
relation to the basilar membrane has its own individual pattern of nerve
supply, nerve fibres in the auditory nerve will respond maximally to a limited
range of frequency and within this range is a peak response. A similar spatial
relationship of response to specific frequencies can be demonstrated in the
central auditory pathways.

Vibrations of the basilar membrane produce electrical potentials which
follow accurately the wave form and changes in intensity of the stimulating
sound. These are known as cochlear microphonics and can be readily picked
up by an electrode in the region of the round window. More elaborate
recording techniques will demonstrate action potentials in the auditory nerve
and other electrical changes which accompany stimulation of the cochlea.

FUNCTIONAL EXAMINATION  OF THE AUDITORY APPARATUS

In the great majority of cases of ear disease it is necessary to investigate the
condition of hearing and to ascertain whether deafness, if present, is due to
involvement of the sound-conducting apparatus or of the inner ear and its
central connections. Conductive deafness-mew be causedj3Y. Jesions of the.
external auditory meatus, the tympanic membrarfe,"fEeliiiddle ear cleft or
inner limit of the sound-conducting apparatus

being the footplate of the stapes within the oval window! Deafness due to
ks|onsjnjhe inner ear, auditory nerve or tfreir central connections 'is known

as

In a number of cases examination of the vestibular apparatus will also be
required.

Tests of Hearing

Whispered Voice Test

The distance at which the patient can hear a whispered voice with the
opposite ear occluded is measured and the result is recorded for each ear. If
the patient cannot hear a whispered voice a conversation voice is used and
the distance at which he can hear this is measured. If he cannot hear a
conversation voice, a raised conversation voice is used close to the ear and ;
may be necessary to shout into the ear. If it is necessary to raise the voic
allow the patient to hear, it is essential to prevent him from hearing wiffel
opposite ear. This 'masking' is most conveniently applied by a Barany a
box, a clockwork-driven source of loud noise ±Fis. 145). A loud shout i
ear tbHbe tested witn me Barany noise box in the opposite ear is still tbfr*
reliable test for total deafness. The whispered voice or conversation voice test