Download A -Outer Ear

(EAR)
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* INTRODUCTION
Ear, organ of hearing and balance. Only vertebrates, or animals with backbones, have
ears. Invertebrate animals, such as jellyfish and insects, lack ears, but have other
structures or organs that serve similar functions. The most complex and highly developed
ears are those of mammals.
Listen to this!
Sounds are waves in the air. The sound waves go into our ear-drum.
Our ear-drum moves with the sound waves. When our ear-drum moves it bump into three
tiny bones called the hammer, the anvil and the stirrup. As these bones bump against each
other, the stirrup moves in and out of a place that looks like a snail’s shell. This is called
the cochlea.
Inside the cochlea are liquid and nerves. As the stirrup moves in and
out of the opening in the cochlea, it makes waves in the liquid. The waves move across
the nerves. When that happens the nerves carry messages to our brain. Our brain tells us
there’s sound.
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* STRUCTURE OF THE HUMAN EAR
Structure of the Ear The human ear consists of three sections: the outer ear, the middle
ear, and the inner ear. The outer ear includes the auricle (pinna), the visible part of the ear
that is attached to the side of the head, and the waxy, dirt-trapping auditory canal. The
tympanic membrane (eardrum) separates the external ear from the middle ear, an air-
filled cavity. Bridging this cavity are three small bones—the malleus (hammer), the incus
(anvil), and the stapes (stirrup). The cochlea and semicircular canals make up the inner
ear.
Like the ears of other mammals, the human ear consists of three sections: the outer,
middle, and inner ear. The outer and middle ears function only for hearing, while the
inner ear also serves the functions of balance and orientation.
Keeping our balance
Our ears do more than help our hear. They have another jobthey help our to keep your balance.
They help us stay upright when we are moving and when we
are standing still.
Inside our ear three small hollow loops filled with liquid. These
are our balance tubes. At the bottom of each loop are tiny cells that have hairs sticking
out of them. Leading from each hair cell is a nerve.
When we move our head, the liquid in the loops moves across these hair
cells. The nerves send messages to our brain. Then our brain sends messages to the
muscles in our body. These messages help to keep our balance when we walk, run, skip
or jump
These are the parts of the
ear that help to keep our
balance.
A -Outer Ear
The outer ear is made up of the auricle, or pinna, and the outer auditory canal. The auricle
is the curved part of the ear attached to the side of the head by small ligaments and
muscles. It consists largely of elastic cartilage, and its shape helps collect sound waves
from the air. The earlobe, or lobule, which hangs from the lower part of the auricle,
contains mostly fatty tissue.
The outer auditory canal, which measures about 3 cm (about 1.25 in) in length, is a
tubular passageway lined with delicate hairs and small glands that produce a wax-like
secretion called cerumen. The canal leads from the auricle to a thin taut membrane called
the eardrum or tympanic membrane, which is nearly round in shape and about 10 mm
(0.4 in) wide. It is the vibration of the eardrum that sends sound waves deeper into the
ear, where they can be processed by complex organs and prepared for transmission to the
brain. The cerumen in the outer auditory canal traps and retains dust and dirt that might
otherwise end up on the eardrum, impairing its ability to vibrate.
The inner two-thirds of the outer auditory canal is housed by the temporal bone, which
also surrounds the middle and inner ear. The temporal bone protects these fragile areas of
the ear.
B- Middle Ear
The eardrum separates the outer ear from the middle ear. A narrow passageway called the
eustachian tube connects the middle ear to the throat and the back of the nose. The
eustachian tube helps keep the eardrum intact by equalizing the pressure between the
middle and outer ear. For example, if a person travels from sea level to a mountaintop,
where air pressure is lower, the eardrums may cause pain because the air pressure in the
middle ear becomes greater than the air pressure in the outer ear. When the person yawns
or swallows, the eustachian tube opens, and some of the air in the middle ear passes into
the throat, adjusting the pressure in the middle ear to match the pressure in the outer ear.
This equalizing of pressure on both sides of the eardrum prevents it from rupturing.
The middle ear is a narrow, air-filled chamber that extends vertically for about 15 mm
(about 0.6 in) and for nearly the same distance horizontally. Inside this chamber is a
linked chain of three ossicles, or very small bones. Both the Latin and common names of
these bones are derived from their shapes. They are called the malleus, or hammer; the
incus, or anvil; and the stapes, or stirrup, which is the tiniest bone in the body, being
smaller than a grain of rice.
The hammer is partly embedded in the eardrum, and the stirrup fits into the oval window,
a membrane that fronts the inner ear. Vibrations of the eardrum move the hammer. The
motion of the hammer moves the anvil, which in turn moves the stirrup. As sound
vibrations pass from the relatively large area of the eardrum through the chain of bones,
which have a smaller area, their force is concentrated. This concentration amplifies, or
increases, the sound just before it passes through the oval window and into the inner ear.
When loud noises produce violent vibrations, two small muscles, called the tensor
tympani and the stapedius, contract and limit the movement of the ossicles, thus
protecting the middle and inner ear from damage.
C-The Inner Ear
The chain of bones in the middle ear leads into the convoluted structures of the inner ear,
or labyrinth, which contains organs of both hearing and balance. The three main
structures of the inner ear are the cochlea, the vestibule, and the three semicircular canals.
The cochlea is a coiled tube that bears a close resemblance to the shell of a snail, which is
what the word means in Greek. Along its length the cochlea is divided into three fluidfilled canals: the vestibular canal, the cochlear canal, and the tympanic canal. The
partition between the cochlear canal and the tympanic canal is called the basilar
membrane. Embedded in the basilar membrane is the spiral-shaped organ of Corti. The
sensory cells in the organ of Corti have thousands of hairlike projections that receive
sound vibrations from the middle ear and send them on to the brain via the auditory
nerve. In the brain they are recognized and interpreted as specific sounds.
The vestibule, the second main structure of the inner ear, helps the body maintain balance
and orientation by monitoring the sensations of movement and position. Without a sense
of balance, even simple functions like walking would pose impossible challenges. With
no sense of orientation, people would not know if they were in a normal position, upside
down, or lying on their sides. Both balance and orientation depend on nerve impulses to
reach the brain when the body is unbalanced or disoriented. The brain, in turn, sends
messages to appropriate muscles, causing them to correct the imbalance or reposition the
body.
The vestibule is made up of two sacs, the utriculus and the sacculus. Special sensory
areas in the walls of the utriculus send impulses to the brain indicating the position of the
head. These sensory areas consist of hairlike projections embedded in gelatin. Covering
the surface of the gelatin are small mineral particles. Depending on the position of the
head, the gelatin and mineral particles exert varying pressures on the sensory cells. The
cells, in turn, send particular patterns of stimulation to the brain, where the patterns are
interpreted.
For example, when the head is upright, the gelatin and mineral particles press down on all
the hairlike cells equally. When the head is tilted straight forward by dropping the chin,
the gelatin and mineral particles pull on all the hairlike cells equally. If the head is tilted
to one side or the other, the cells receive unequal stimulation, varying with the direction
and amount of tilt. If the utriculus of both ears is destroyed by injury or disease, the head
will hang down limply unless its position can be judged with the eyes. The utriculus is
also used to detect the body’s starting or stopping. If a person stops suddenly, the gelatin
and mineral particles continue to move, exerting a forward pull on the hairlike cells. The
cells then send a specific pattern of nerve impulses to the brain.
The structure of the sacculus is similar to that of the utriculus, but its function is not well
understood. The sacculus may aid in determining body orientation, but it may also have a
function in hearing.
Arising from the utriculus is the third main structure of the inner ear, the three
semicircular canals. These canals direct body balance when the body moves in a straight
line or rotates in any direction. Each canal also contains sensory areas with sensory hair
cells that project into a cone-shaped cap of gelatin. Two of the semicircular canals are in
a vertical position and are used to detect vertical movement, such as jumping or falling.
The third canal is horizontal and detects horizontal movement, such as turning or
spinning.
The action of the canals depends on the inertia of the fluid inside. When the motion of the
body changes, the fluid lags behind, causing the hair cells in the canal to bend. The
bending of the hair cells sends nerve impulses to the brain, which in turn informs the
body of changes in the direction of movement.
*HEARING
Sound is a series of vibrations moving as waves through air or other gases, liquids, or
solids. A ringing bell, for example, sets off vibrations in the air. Detection of these
vibrations, or sound waves, is called hearing. The detection of vibrations passing through
the ground or water is also called hearing. Some animals can detect only vibrations
passing through the ground, and others can hear only vibrations passing through water.
Humans, however, can hear vibrations passing through gases, solids, and liquids.
Sometimes sound waves are transmitted to the inner ear by a method of hearing called
bone conduction. For example, people hear their own voice partly by bone conduction.
The voice causes the bones of the skull to vibrate, and these vibrations directly stimulate
the sound-sensitive cells of the inner ear. Only a relatively small part of a normal
person’s hearing depends on bone conduction, but some totally deaf people can be helped
if sound vibrations are transferred to the skull bones by a hearing aid.
Humans hear primarily by detecting airborne sound waves, which are collected by the
auricles. The auricles also help locate the direction of sound. Although some people have
auricular muscles so well-developed that they can wiggle their ears, human auricles,
when compared to those of other mammals, have little importance. Many mammals,
especially those with large ears, such as rabbits, can move their auricles in many
directions so that sound can be picked up more easily.
After being collected by the auricles, sound waves pass through the outer auditory canal
to the eardrum, causing it to vibrate. The vibrations of the eardrum are then transmitted
through the ossicles, the chain of bones in the middle ear. As the vibrations pass from the
relatively large area of the eardrum through the chain of bones, which have a smaller
area, their force is concentrated. This concentration amplifies, or increases, the sound.
When the sound vibrations reach the stirrup, the stirrup pushes in and out of the oval
window. This movement sets the fluids in the vestibular and tympanic canals in motion.
To relieve the pressure of the moving fluid, the membrane of the oval window bulges out
and in. The alternating changes of pressure in the fluid of the canals cause the basilar
membrane to move. The organ of Corti, which is part of the basilar membrane, also
moves, bending its hair like projections. The bent projections stimulate the sensory cells
to transmit impulses along the auditory nerve to the brain.
A Loudness, Pitch, and Tone
Human ears are capable of perceiving an extraordinarily wide range of changes in
loudness, the tiniest audible sound being about 1 trillion times less intense than a sound
loud enough to cause the ear pain. The loudness or intensity of a noise is measured in a
unit called the decibel. The softest audible sound to humans is 0 decibels, while painful
sounds are those that rise above 140 decibels.
Besides loudness, the human ear can detect a sound’s pitch, which is related to a sound’s
vibration frequency, or the number of sound waves passing into the ear in a given period.
The greater the frequency, the higher the pitch. The maximum range of human hearing
includes sound frequencies from about 15 to about 18,000 waves, or cycles, per second.
Because the human ear cannot hear very low frequencies, the sound of one’s own
heartbeat is inaudible. At the other end of the scale, a highly pitched whistle producing
30,000 cycles per second is not audible to the human ear, but a dog can hear it.
The third characteristic of sound detected by the human ear is tone. The ability to
recognize tone enables humans to distinguish a violin from a clarinet when both
instruments are playing the same note. The least noticeable change in tone that can be
picked up by the ear varies with pitch and loudness.
Another sonic phenomenon, known as masking, occurs because lower-pitched sounds
tend to deafen the ear to higher-pitched sounds. To overcome the effects of masking in
noisy places, people are forced to raise their voices.
* DISEASES OF THE HUMAN EAR
Some diseases of the ear can cause partial or total deafness. In addition, most diseases of
the inner ear are associated with a disturbance of balance. Ear problems should be
evaluated by specially trained physicians called otolaryngologies, who treat conditions
ranging from eardrum injuries caused by physical trauma to bony deposits in the inner ear
caused by the aging process.
The auricle and the opening into the outer auditory canal may be missing at birth.
Acquired malformations of the outer ear include scarring from cuts and other wounds.
Othematoma, known popularly as cauliflower ear, is a common result of injury to the ear
cartilage followed by internal bleeding and excessive production of ear tissue.
Inflammation of the outer ear may result from any condition that causes inflammation of
the skin, such as dermatitis, burns, and frostbite. Erysipelas, a skin disease caused by
bacteria, and seborrhea, a skin disease caused by the malfunction of the skin’s oil glands,
are common afflictions of the auricle. In the outer auditory canal, foreign bodies such as
insects, as well as abnormal buildups of cerumen, cause ear disturbances and should be
removed by a physician.
A Middle Ear Disorders
Diseases of the middle ear include perforation of the eardrum and infection. Perforation
of the eardrum may be caused by injury from a sharp object, a blow to the ear, or by
sudden changes in atmospheric pressure.
Infection of the middle ear, whether acute or chronic, is called otitis media. Acute otitis
media with effusion includes all acute infections of the middle ear caused by pus-forming
bacteria, which usually reach the middle ear by way of the eustachian tube. Bacterial
infection of the mastoid process, a cone-shaped, honeycombed projection of bone behind
the auricle, may occur as a complication of middle ear infections. Hearing impairment
often follows because newly malformed tissues affect the mobility of the eardrum and the
ossicles. Painful swelling of the eardrum may require a surgical incision to permit
drainage of the middle ear. Since the use of penicillin and other antibiotics became
widespread, mastoid complications have become much less frequent. Sometimes acute
otitis media with effusion leads to a chronic infection that does not respond readily to
antibacterial agents.
Acute and chronic nonsuppurative otitis media, which do not involve the formation or
discharge of pus, are caused by closure of the eustachian tube due to conditions such as a
head cold, diseased tonsils and adenoids, inflammation of the sinuses, or riding in
airplanes without pressurized cabins. The chronic form can also result from bacterial
infection. Because the watery discharge impairs hearing, chronic otitis media in young
children may interfere with language development. A variety of treatments are employed,
including use of antibiotics and antihistamines, removal of tonsils and adenoids, and
insertion of tubes into the middle ear to allow drainage.
About 1 in 100 adults has hearing loss due to a condition called otosclerosis or
otospongiosis, in which an abnormal amount of spongy bone is deposited between the
stapes and the oval window. As a result, the stapes becomes immobilized and can no
longer transmit sensations to the inner ear. If the condition progresses, surgical removal
of the bony deposit is necessary, followed by reconstruction of the connection between
the stapes and the oval window. Sometimes the surgeon will replace the stapes with a
mechanical piston-like device. Even after successful surgery, deposits of bony tissue may
again build up and cause hearing loss several years later.
B Inner Ear Diseases
George von Békésy United States physicist George von Békésy won the 1961 Nobel
Prize in medicine or physiology. His research on the mechanics of the inner ear led to
advances in reconstructive surgery and improvements in the design of hearing aids.
Diseases of the inner ear can affect the sense of balance and cause symptoms of motion
sickness. Anemia, tumors of the acoustic nerve, exposure to abnormal heat, disturbances
of the circulatory system, skull injuries, poisoning, emotional disorders, and hyperemia,
or increased blood flow, may also cause these symptoms. Ménière’s disease results from
abnormalities in the semicircular canals and produces nausea, hearing loss, a disturbed
sense of balance, and tinnitus, or a persistent ringing in the ears. Destruction of the inner
ear by cryosurgery or ultrasound is sometimes used to combat intractable dizziness.
Damage to the organ of Corti in the inner ear accounts for the condition of many people
who are either totally deaf or severely hearing-impaired. Scientists have addressed the
difficulties of such people by developing an electronic device called a cochlear implant.
This device is more sophisticated than a hearing aid, which merely increases the volume
of the sounds that pass through the normal hearing organs. The cochlear implant works
by translating sound waves into electric signals. These signals are relayed to electrodes
that have been surgically implanted in the cochlea so that the auditory nerve is directly
stimulated. After successful surgery, once deaf or severely hearing-impaired patients can
usually detect a wide range of sounds, but results depend on factors that include the
health of the auditory nerves and the duration of deafness. Nonetheless, lip-reading
ability often improves, and implant users have varying degrees of success in using the
telephone.
Otalgia, or earache, is not necessarily associated with ear disease; occasionally it is
caused by impacted teeth, sinus disease, inflamed tonsils, infections in the nose and
pharnyx, or swelling of the lymph nodes in the neck. Tinnitus may also result from these
conditions. Permanent tinnitus is most often caused by prolonged exposure to loud noise,
which damages the hair cells of the cochlea. A sound masker, worn like a hearing aid,
may offer relief to some sufferers by blocking the perception of ringing in the ears.