Regulation and Integration of the Body
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
(Figure 15.24 and
Tese bones, named for their shape, are the
kus; “anvil”); and the
“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
from the wall of the pharyngotympanic tube and inserts on the
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.
is also called the
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
mĭ-nus). Tese glands secrete
, or earwax (
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
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.
, 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
. Superiorly the tympanic cavity arches
upward as the
, the “roof” of the middle ear
, 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
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.
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
(lancing of the ear-
drum) may be required to relieve the pressure, and a tiny tube
The three auditory ossicles and associated
Right middle ear, medial view.