453
CHAPTER TWELVE
Nervous System III
The
osseous labyrinth
is a bony canal in the temporal
bone; the
membranous labyrinth
is a tube that lies within
the osseous labyrinth and has a similar shape
(f
g. 12.11
a
)
.
Between the osseous and membranous labyrinths is a fl uid
called
perilymph,
secreted by cells in the wall of the bony
canal. In the membranous labyrinth is a slightly different
uid called
endolymph.
The parts of the labyrinths include a
cochlea
(kok
le-ah)
that functions in hearing and three
semicircular canals
that
provide a sense of equilibrium. A bony chamber called the
vestibule,
between the cochlea and the semicircular canals,
houses membranous structures that serve both hearing and
equilibrium.
The cochlea is shaped like a snail shell, coiled around
a bony core (modiolus) with a thin, bony shelf (spiral lam-
ina) that wraps around the core like a spiral staircase (F g.
12.11
b
). The shelf divides the bony labyrinth of the cochlea
into upper and lower compartments. The upper compart-
ment, called the
scala vestibuli,
leads from the oval window
to the apex of the spiral. The lower compartment, the
scala
tympani,
extends from the apex of the cochlea to a mem-
brane-covered opening in the wall of the inner ear called the
round window.
These compartments constitute the bony
labyrinth of the cochlea, and they are F
lled with perilymph.
At the apex of the cochlea, a small opening (helicotrema)
connects the fl
uids in the chambers (
f gs.
12.11
b
and
12.12
).
A portion of the membranous labyrinth in the cochlea,
called the
cochlear duct
(scala media), lies between the
two bony compartments and is F
lled with endolymph. The
cochlear duct ends as a closed sac at the apex of the cochlea.
The duct is separated from the scala vestibuli by a
vestib-
ular membrane
(Reissner’s membrane) and from the scala
tympani by a
basilar membrane
(see fig. 12.12). Clinical
Application 12.4 describes an effective treatment for hearing
loss called a cochlear implant.
The basilar membrane extends from the bony shelf of the
cochlea and forms the fl
oor of the cochlear duct. It has many
thousands of stiff, elastic F
bers that lengthen from the base
of the cochlea to its apex. Vibrations entering the perilymph
at the oval window travel along the scala vestibuli and pass
through the vestibular membrane to enter the endolymph of
the cochlear duct, where they move the basilar membrane.
After passing through the basilar membrane, the vibrations
enter the perilymph of the scala tympani, and their forces
tympani muscle steadily pulls on the tympanic membrane. This
is important because a loose tympanic membrane would not be
able to effectively transmit vibrations to the auditory ossicles.
The muscles of the middle ear take 100 to 200 milliseconds to con-
tract. For this reason, the tympanic re±
ex cannot protect the hearing
receptors from the e²
ects of very rapidly repeated loud sounds, such
as from an explosion or a gunshot. On the other hand, this protective
mechanism can reduce the e²
ects of intense sounds that arise slowly,
such as the roar of thunder.
Auditory Tube
An
auditory tube
(aw
di-to
re tub) (eustachian tube) con-
nects each middle ear to the throat. This tube allows air to
pass between the tympanic cavity and the outside of the body
by way of the throat (nasopharynx) and mouth. It helps main-
tain equal air pressure on both sides of the tympanic mem-
brane. This is necessary for normal hearing (see F g. 12.10).
The function of the auditory tube becomes noticeable
during rapid change in altitude. ±or example, as a person
moves from a high altitude to a lower one, the air pressure
on the outside of the tympanic membrane steadily increases.
As a result, the tympanic membrane may be pushed inward,
out of its normal position, impairing hearing.
When the air pressure difference is great enough, some
air may force its way up through the auditory tube into the
middle ear. This equalizes the pressure on both sides of the
tympanic membrane, which moves back into its regular posi-
tion, causing a popping sound as normal hearing returns. A
reverse movement of air ordinarily occurs when a person
moves from a low altitude to a higher one.
The auditory tube is usually closed by valvelike fl
aps in
the throat, which may inhibit air movements into the middle
ear. Swallowing, yawning, or chewing aid in opening the
fl aps and can hasten equalization of air pressure.
Inner (Internal) Ear
The inner ear is a complex system of intercommunicating
chambers and tubes called a
labyrinth
(lab
i-rinth). Each ear
has two such regions—the osseous labyrinth and the mem-
branous labyrinth.
Signs of a middle ear infection (otitis media) in an infant or toddler are
hard to miss—irritability, screaming, fever, or tugging on the a²
ected
ear. Viewing the painful ear with an instrument called an otoscope
reveals a red and bulging tympanic membrane.
Ear infections occur because the mucous membranes that line
the auditory tubes are continuous with the linings of the middle ears,
creating a conduit for bacteria infecting the throat or nasal passages
to travel to the ear. The bacteria that most commonly cause middle ear
infection are
Streptococcus pneumoniae, Haemophilus inf
uenzae,
and
Moraxella catarrhalis.
This route to infection is greater in young chil-
dren because their auditory tubes are shorter than in adults.
Physicians treat acute otitis media with antibiotics. Recurrent
infections may cause hearing loss and interfere with learning, so chil-
dren with recurrent otitis media may be fitted with tympanostomy
tubes, inserted into affected ears during a brief surgical procedure.
The tubes form a small tunnel through the tympanic membrane so
the ears can drain. By the time the tubes fall out, the child has usually
outgrown the susceptibility to ear infections.
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