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ANATOMY OF THE EAR
The ear is the organ of hearing and balance and consists
of three parts: the outer ear, the middle ear, and the inner ear. The outer
ear and middle ear are the apparatus for the collection and transmission of
sound. The inner ear is responsible for analyzing sound waves, and also
contains the mechanism by which the body keeps its balance.
The outer ear is comprised of the pinna and ear canal; the
middle ear: the eardrum, hammer, anvil, stirrup, and eustachian tube; and
the inner ear: the vestibule, semicircular canals, and cochlea. Sensory
impulses from the inner ear pass to the brain via the vestibulocochlear
nerve.
OUTER EAR
The outer ear consisting of the pinna (also called
the auricle) is the visible part of the ear and is composed of folds
of skin and cartilage. The pinna leads into the ear canal (also called
the meatus) and is about 1 inch (or 2.5cm) long in adults and closed
at its inner end by the tympanic membrane or (eardrum). The
part of the canal nearest the outside is made of cartilage. The cartilage is
covered with skin that produces wax, and the tiny hairs in the
canal traps dust, pollen, pollution, and small foreign bodies.
MIDDLE EAR
The middle ear is a small cavity between the eardrum and
the inner ear. It conducts sound to the inner ear by means of a chain of
three tiny, linked, movable bones called ossicles. They link
the eardrum to an oval window in the bony wall on the opposite inner
side of the middle-ear cavity. The bones are named because of their shapes.
The malleus, or hammer, is joined to the inside of the eardrum. The
incus, or anvil, has one broad joint with the malleus (which
lies almost parallel to it) and a delicate joint to the third bone, the
stapes, or stirrup. The base of the stapes fills the oval
window which leads to the inner ear.
The middle ear is cut off from the outside by the eardrum,
but it is not completely airtight. A ventilation passage,
called the eustachian tube, runs forward and down into the back of the nose.
The eustachian tube is normally closed, but it opens by muscular contraction
when yawning and swallowing.
The middle ear acts as a transformer. It passes the
vibrations of sound from compression and de-compression of the outside air.
The air is a thin medium that carries the sound into the inner ear where the
fluid in the inner ear, a thicker medium, resonates the sound vibration.
INNER EAR
The inner ear is an extremely intricate
series of structures contained deep within the bones of the skull. It
consists of a maze of winding passage ways, collectively known as the
labyrinth. The front part, the cochlea, is a tube
resembling a snail's shell and is related to hearing. The rear part which is
three semicircular canals and two other organs is concerned with balance.
The semicircular canals are set at right angles to each other and are
connected to a cavity known as the vestibule. These canals
contain hair cells bathed in fluid. Some of these cells are sensitive to
gravity and acceleration and others respond to head positions and
movement (side to side, up and down, or tilted). Posture or direction
information is registered by the relevant cells and conveyed by nerve fibers
to the brain.
DISORDERS OF THE EAR
The ear is susceptible to a large number of disorders,
some of which can lead to deafness. Vertigo (dizziness
associated with a disturbance of balance) is not common, but may occur in
some disorders of the inner ear.
INFECTIONS
Infection is the most common cause of ear disorders.
Infection may occur in the ear canal, leading to otitis externa,
or may affect the middle ear, causing otitis media, which
often leads to perforation of the eardrum. Persistent middle-ear effusion
(buildup of fluid within the middle ear), often due to infection, is the
most common cause of hearing difficulties in children.
Middle-ear infection can spread to cause mastoiditis
(infection of the mastoid process, the bone behind the ear), or
brain abscess, but these complications have become extremely rare since the
introduction of antibiotics.
SINUS
Sinus infection of the inner ear may cause
labyrinthitis with severe vertigo and/or sudden
hearing loss. This is not uncommon with people who often travel by air.
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