development continues well into ADOLESCENCE,
refining the brain’s ability to interpret and catego-
rize the signals the senses send to it.
Functions of the ear: hearing and balance
Hearing (audition) occurs through air conduction
and BONEconduction of sound waves. The struc-
tures of the outer and middle ear facilitate air con-
duction. The outer ear, called the auricle or pinna,
is a structure of CARTILAGEand SKINthat extends
from the side of the head. Its somewhat dishlike
structure serves to “catch” sound waves traveling
through the air; its ridges and curves channel
those sound waves into the auditory canal. The
auditory canal funnels and focuses the sound
waves, directing them to the TYMPANIC MEMBRANE
or eardrum, which vibrates in response. The tym-
panic membrane marks the end of the outer ear
and the start of the middle ear, creating a sealed
chamber. Its vibration activates, in sequence, the
three tiny auditory ossicles, or bones, of the mid-
dle ear: first the malleus (hammer), then the incus
(anvil), and finally the stapes (stirrup). The flat of
the stapes rests against the oval window, a small
translucent membrane in the wall of the COCHLEA.
This point of contact represents the transition
from the middle ear to the inner ear and from an
environment of air to one of fluid.
The middle ear is pressurized, allowing the
tympanic membrane and the auditory ossicles to
vibrate freely and without resistance. The
EUSTACHIAN TUBE, a short canal of tissue, connects
the middle ear with the upper throat at the back
of the nose. Somewhat like an elongated valve, it
serves to equalize pressure between the middle
ear and the external environment. Swallowing
and yawning force air into the eustachian tube,
causing it to open (sometimes with a perceptible
pop). Unequal pressure between the middle ear
and the atmosphere causes the tympanic mem-
brane to bulge in the direction of the lower pres-
sure, altering its ability to convey the vibrations of
sound waves. Circumstances that prevent the
eustachian tube from opening to balance air pres-
sure, such as a cold that fills the nasal passages
and eustachian tubes with congestion, causes the
sensation of muffled hearing and pressure in the
ear. When pressure in the middle ear remains
lower than the atmospheric pressure for a pro-
longed time, the body attempts to compensate by
drawing fluid into the middle ear. Though the
fluid may relieve the sensation of pressure, it fur-
ther constrains middle ear function. Blocked
eustachian tubes establish ideal conditions for
middle ear INFECTION(OTITISmedia), allowing BAC-
TERIAto move into the middle ear. Until about age
10, the eustachian tubes are nearly horizontal. As
the child’s facial structure lengthens with maturity
the eustachian tubes shift and angle downward
from the ears to the throat, improving their ability
to drain congestion and remain open.
The vibration of the stapes against the oval
window amplifies the energy of the sound waves
and sets in motion the fluid (endolymph) within
the cochlea on the other side of the oval window.
Fluid further focuses and aligns the sound waves
into patterns. A second membrane, the round
window, dissipates excessive vibration into the
fluid of the inner ear (perilymph) on its other side.
The moving endolymph within the cochlea in
turn stimulates microscopic fibers along a mem-
brane that forms a structure within the cochlea
called the organ of Corti. The fibers resonate to
specific sound waves, activating NERVEimpulses in
specialized cells called HAIR cells. The cochlear
nerve carries the nerve impulses to the eighth cra-
nial nerve (vestibulocochlear nerve), which in
turn transmits them to the brain’s temporal lobe.
The temporal lobe filters, interprets, and analyzes
the nerve impulses, translating them into sound
messages including language.
The bony structures of the head and face also
conduct sound waves. Bone conduction bypasses
the outer and middle ears. Sound waves instead
travel as vibrations along the bones to the inner
ear, where they pass to the bony part of the
cochlea. The vibrations of the bony cochlea pass to
the endolymph, and the rest of the hearing
process unfolds. Sounds conveyed through bone
conduction are significantly restricted in tonal
range and volume because they bypass the ampli-
fying structures of the tympanic membrane and
auditory ossicles. The sound waves of one’s own
voice travel primarily through bone conduction
along the bones of the face, which explains why it
seems so different when heard as a recording in
which the sound waves travel by air conduction.
The inner ear also manages the body’s balance
and motion in relation to gravity. The structuresThe Ear, Nose, Mouth, and Throat 3