572
UNIT 3
Regulation and Integration of the Body
15
implanted in the eardrum permits pus to drain into the external
ear. Te tube falls out by itself within a year.
Te tympanic cavity is spanned by the three smallest bones in
the body: the
auditory ossicles
(Figure 15.24 and
Figure 15.25
).
Tese bones, named for their shape, are the
malleus
(mal
9
e-us;
“hammer”); the
incus
(ing
9
kus; “anvil”); and the
stapes
(sta
9
pēz;
“stirrup”). Te “handle” of the malleus is secured to the eardrum,
and the base of the stapes fits into the oval window.
±iny ligaments suspend the ossicles, and mini synovial joints
link them into a chain that spans the middle ear cavity. Te in-
cus articulates with the malleus laterally and the stapes medially.
Te ossicles transmit the vibratory motion of the eardrum to the
oval window, which in turn sets the fluids of the internal ear into
motion, eventually exciting the hearing receptors.
±wo tiny skeletal muscles are associated with the ossicles
(Figure 15.25). Te
tensor tympani
(ten
9
sor tim
9
pah-ni) arises
from the wall of the pharyngotympanic tube and inserts on the
malleus. Te
stapedius
(stah-pe
9
de-us) runs from the posterior
wall of the middle ear to the stapes. When loud sounds assault
the ears, these muscles contract reflexively to limit the ossicles’
vibration and minimize damage to the hearing receptors.
Internal Ear
Te
internal ear
is also called the
labyrinth
(“maze”) because
of its complicated shape (see Figure 15.24). It lies deep in the
temporal bone behind the eye socket and provides a secure site
for all of the delicate receptor machinery.
temporal bone. Te entire canal is lined with skin bearing hairs,
sebaceous glands, and modified apocrine sweat glands called
ceruminous glands
(sĕ-roo
9
mĭ-nus). Tese glands secrete
yellow-brown waxy
cerumen
, or earwax (
cere
5
wax), which
provides a sticky trap for foreign bodies and repels insects.
In many people, the ear is naturally cleansed as the ceru-
men dries and then falls out of the external acoustic meatus.
Jaw movements as a person eats, talks, and so on, create an un-
noticeable conveyor-belt effect that moves the wax out. In other
people, cerumen builds up and becomes compacted.
Sound waves entering the external acoustic meatus eventu-
ally hit the
tympanic membrane
, or
eardrum
(
tympanum
5
drum), the boundary between the outer and middle ears. Te
eardrum is a thin, translucent, connective tissue membrane,
covered by skin on its external face and by mucosa internally.
Shaped like a flattened cone, its apex protrudes medially into
the middle ear.
Sound waves make the eardrum vibrate. Te eardrum, in
turn, transfers the sound energy to the tiny bones of the middle
ear and sets them vibrating.
Middle Ear
Te
middle ear
, or
tympanic cavity
, is a small, air-filled,
mucosa-lined cavity in the petrous portion of the temporal
bone. It is flanked laterally by the eardrum and medially by a
bony wall with two openings, the superior
oval window
and the
inferior
round window
. Superiorly the tympanic cavity arches
upward as the
epitympanic recess
, the “roof” of the middle ear
cavity. Te
mastoid antrum
, a canal in the posterior wall of the
tympanic cavity, allows it to communicate with
mastoid air cells
housed in the mastoid process.
Te anterior wall of the middle ear abuts the internal carotid ar-
tery (the main artery supplying the brain) and contains the open-
ing of the
pharyngotympanic (auditory) tube
(formerly called
the eustachian tube). Te pharyngotympanic tube runs obliquely
downward to link the middle ear cavity with the nasopharynx (the
superiormost part of the throat), and the mucosa of the middle ear
is continuous with that lining the pharynx (throat).
Normally, the pharyngotympanic tube is flattened and
closed, but swallowing or yawning opens it briefly to equalize
pressure in the middle ear cavity with external air pressure. Tis
is important because the eardrum vibrates freely only if the
pressure on both of its surfaces is the same; otherwise sounds
are distorted. Te ear-popping sensation of the pressures equal-
izing is familiar to anyone who has flown in an airplane.
Homeostatic Imbalance
15.12
Otitis media
(me
9
de-ah), or middle ear inflammation, is a fairly
common result of a sore throat, especially in children, whose
pharyngotympanic tubes are shorter and run more horizon-
tally. Otitis media is the most frequent cause of hearing loss in
children. In acute infectious forms, the eardrum bulges and be-
comes inflamed and red. Most cases of otitis media are treated
with antibiotics. When large amounts of fluid or pus accumulate
in the cavity, an emergency
myringotomy
(lancing of the ear-
drum) may be required to relieve the pressure, and a tiny tube
Pharyngotym-
panic tube
Tensor
tympani
muscle
Tympanic
membrane
(medial view)
Stapes
Malleus
View
Superior
Anterior
Lateral
Incus
Epitympanic recess
Stapedius
muscle
Figure 15.25
The three auditory ossicles and associated
skeletal muscles.
Right middle ear, medial view.
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