452
UNIT THREE
twenty-two times greater than that which sound waves exert
on the tympanic membrane.
The middle ear also has two small skeletal muscles
attached to the auditory ossicles and controlled involuntarily.
One of them, the
tensor tympani,
is inserted on the medial
surface of the malleus and is anchored to the cartilaginous
wall of the auditory tube. When it contracts, it pulls the mal-
leus inward. The other muscle, the
stapedius,
is attached to
the posterior side of the stapes and the inner wall of the tym-
panic cavity. It pulls the stapes outward
(f
g. 12.10)
. These
muscles are the effectors in the
tympanic refl
ex,
elicited in
about one-tenth second following a loud, external sound.
When the refl ex occurs, the muscles contract, and the mal-
leus and stapes move. As a result, the bridge of ossicles in
the middle ear becomes more rigid, reducing its effectiveness
in transmitting vibrations to the inner ear.
The tympanic refl ex reduces pressure from loud sounds
that might otherwise damage the hearing receptors. Ordinary
vocal sounds also elicit the tympanic refl
ex, as when a person
speaks or sings. This action muffl
es the lower frequencies of
such sounds, improving the hearing of higher frequencies,
common in human vocal sounds. In addition, the tensor
cavity by tiny ligaments and are covered by mucous mem-
brane. These bones bridge the tympanic membrane and
the inner ear, transmitting vibrations between these parts.
SpeciF
cally, the malleus is attached to the tympanic mem-
brane, helping to maintain its conical shape. When the
tympanic membrane vibrates, the malleus vibrates in uni-
son with it. The malleus vibrates the incus, and the incus
passes the movement on to the stapes. Ligaments hold the
stapes to an opening in the wall of the tympanic cavity
called the
oval window
(F
g. 12.9). Vibration of the stapes,
which acts like a piston at the oval window, moves a fl
uid
within the inner ear. These vibrations of the fl
uid stimulate
the hearing receptors.
In addition to transmitting vibrations, the auditory
ossicles form a lever system that helps increase (amplify)
the force of the vibrations as they pass from the tympanic
membrane to the oval window. Also, because the ossicles
transmit vibrations from the large surface of the tympanic
membrane to a much smaller area at the oval window, the
vibrational force strengthens as it travels from the outer to
the inner ear. As a result, the pressure (per square milli-
meter) that the stapes applies at the oval window is about
Temporal
bone
Malleus
Incus
Incus
Stapes
Stapes
Tendon of
stapedius m.
Stapedius
tendon
Stapedius m.
Tendon of
tensor tympani m.
Tensor tympani m.
Oval window
Round
window
Auditory
tube
Tympanic
membrane
Tympanic
cavity
(a)
(b)
FIGURE 12.10
Two small muscles attached to the (
a
) malleus and (
b
) stapes, the tensor tympani and the stapedius, are ef
ectors in the tympanic
reF
ex. ±igure 12.9 does not show these muscles. (
m.
stands ²or muscle.)
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