I. 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.
II. Structure of the Human 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.
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 fluid-filled 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.
III. 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 hairlike 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.
IV. 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 otolaryngologists, 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
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.
V. Hearing and Balance in Other Vertebrates
Every healthy vertebrate is capable of receiving sounds, maintaining balance, and establishing orientation. Animals higher up on the evolutionary ladder have more complex organs, but all vertebrates have semicircular canals, and all except fish have a cochlea or a similar structure.
A. Fish
In fish, some areas of skin can detect waves of pressure and ripples originating from a distant object. This is a form of hearing. Fish also have pear-shaped organs called neuromasts with sensory cells that have hairlike projections embedded in a gelatin-like substance. Waves of pressure cause the gelatin-like substance to move, bending the hairlike projections and stimulating the sensory cells. Some fish have shallow pits in the skin, each of which contains a single neuromast. In other fish the pits are joined, forming a long open groove, called a lateral line. In most fish the groove is covered over to form a canal. Pressure changes are received by the neuromasts through openings along the canal. Fish also have a simple equivalent to the human inner ear, or labyrinth.
In the region of the head, the neuromast pits are modified to form a pouchlike structure similar to the labyrinth of mammals. The structure has three semicircular canals and a stonelike statolith, which functions in the same way as the mineral particles in the utriculus of mammals. When the fish changes position, the statolith presses on nerve cells, causing impulses to be sent to the brain.
Many fish hear by bone conduction. When sound waves travel through the water, they make the bones of the skull vibrate. The vibration makes the fluid in the labyrinth vibrate. This in turn stimulates the sensory cells.
In some fish, like carp and catfish, hearing is improved by the presence of an air bladder that picks up sound waves and transmits them through small bones to the labyrinth. These bones, like the bones in the middle ear of mammals, intensify the vibrations. Fish equipped with these structures have keen hearing.
B. Amphibians
Some amphibians, such as frogs, have a middle ear that consists of an eardrum and a single bone called the columella. Vibrations of the eardrum are amplified by the columella and transmitted to the inner ear. Frogs also have a eustachian tube connecting the middle ear to the throat. Since amphibians have no outer ear, the eardrum is on the surface of the body. In frogs it can be seen as a circular area on each side of the head.
Other amphibians, such as salamanders, do not have a middle ear or eardrum. These animals hear by picking up vibrations on the ground through the skull or the front legs and shoulder blades. The vibrations are then transferred to the inner ear.
C. Reptiles
In certain reptiles the inner ear is highly developed and quite similar to that of mammals. It has a fluid-filled cochlear canal, a vestibular canal, and a tympanic canal. Many reptiles also have an outer ear. It consists of a narrow tubular passageway, called the outer auditory canal, that runs from the surface of the skin to the middle ear.
Snakes do not have a middle ear cavity, an eardrum, an ear opening, or an eustachian tube. They are therefore deaf to airborne sounds, but can hear vibrations or sounds carried through the ground. A columella, sometimes consisting of two bones, rather than the one bone of amphibians, connects the lower jaw and the inner ear. When the snake lowers its head to the ground, the lower jaw picks up vibrations and transmits them through the columella to the inner ear.
D. Birds
Birds have highly developed hearing. Although the structure of their ears is similar to that of reptiles, birds have the added capability of distinguishing the pitch of a sound and the direction from which it comes. They can recognize calls of other birds of their own kind, and some species, such as catbirds, can imitate the calls of other birds. In owls, which rely heavily on hearing to locate their prey, the cochlea is long and coiled, suggesting a highly developed sense of hearing.
VI. Hearing and Balance in Invertebrates
Invertebrates do not have ears, but they do possess systems that provide them with adequate sensory input. “Hearing” in marine invertebrates is related to their sense of touch and their ability to detect water currents or waves of pressure in the water. A disturbance in the water, such as that produced by a moving object, stimulates the animal as if it had actually been touched.
Sponges display a sense of balance by maintaining their normal upright position, and their response to water currents indicates that they have a sense of touch. Scientists do not understand the mechanisms that control these functions. More highly developed marine invertebrates, such as jellyfish and sea anemones, feature a special structure called the statocyst, which is a small fluid-filled sac lined with sensory hairs and containing one or more stonelike statoliths. When the animal is in its normal position, the statolith presses against the sensory cells at the bottom of the sac. When a wave causes the animal to lean to one side, the statolith moves to another part of the sac, stimulating the cells in that region. The statolith also enables the sea anemone to detect vibrations that pass under it through the sea bottom.
Many mollusks, crustaceans, and worms have statoliths. In some crustaceans, such as the shrimp and the lobster, the statoliths are grains of sand that the animal itself places in a pit or closed sac. Whenever the animal molts, or grows a new shell, it instinctively replaces the lost grains of sand.
Many insects maintain their orientation by means of a series of pits on the abdomen that contain sensory organs. Two types of hearing organs are known in insects: hair sensilla and tympanic organs.
Various insects, such as mosquitoes and cockroaches, have hair sensilla, which may be located almost anywhere on the body. A hair sensillum is basically a sensory cell associated with a nerve fiber. Because hair sensilla are very light, air particles set into vibration by sound waves cause the sensilla to move back and forth. The movement sends impulses along the nerve fibers. In the mosquito the sensilla are located on the antennae, or feelers. Because the male mosquito's sensilla respond strongly to the whining sound produced by the female's wings, they help him locate a mate.
Grasshoppers, crickets, and many moths have more complicated hearing organs called tympanic organs. Tympanic organs are usually paired, and may be located on the abdomen, thorax, or legs. A tympanic organ consists of a thin area of the insect's shell that acts like a membrane and vibrates when sound strikes it. An air space and a sensitive rodlike structure under the membrane lead to a sensory cell. Vibrations of the membrane are transferred to the rod, which causes impulses to be sent out by the sensory cell. Insects apparently use their hearing to recognize other insects of their own kind. Since tympanic organs can determine the direction of a sound, they are probably useful for finding mates.
"Ear," Microsoft® Encarta® Online Encyclopedia 2001 http://encarta.msn.com © 1997-2001 Microsoft Corporation. All rights reserved.
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