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An ear is an organ used by an animal to detect sound which isn't always in
the same part of the body. The term may refer to the entire system
responsible for collection and processing of sound (the auditory system), or
merely the externally-visible part.
Problems with the ear or auditory processing system in the brain can lead to
The mammalian ear
Mammals, including humans, have two ears, one on each side of the head.
The outer ear is the external portion of the ear. The visible part is called
the pinna, or auricle, and functions to collect and focus sound waves. Many
mammals can move the pinna in order to focus their hearing in a certain
direction, in much the same way that they can turn their eyes. Humans have
generally lost this ability. From the pinna, the sound moves into the ear
canal, a simple tube running to the middle ear.
The middle ear includes the eardrum (tympanum or tympanic membrane) and the
ossicles, three tiny bones of the middle ear. Their Latin names are the
malleus, incus, and stapes, but they are also referred to in English as the
hammer, anvil, and stirrup respectively. These bones form the linkage
between the tympanic membrane and the oval window that leads to the inner ear.
The tympanum turns vibrations of air in the ear canal into vibrations of the ossicles.
Reptilian ears only have one bone - the stapes. The other two, unique to
mammals, are derived from bones of the jaw, and allow finer detection of sound.
The middle ear is hollow. If the animal moves to a high-altitude
environment, or dives into the water, there will be a pressure difference
between the middle ear and the outside environment. This pressure will pose
a risk of bursting or otherwise damaging the tympanum if it is not relieved.
This is the function of the Eustachian tubes - evolutionary descendants of
the gills - which connect the middle ear to the nasal cavity. The Eustachian
tubes are normally pinched off at the nose end, to prevent being clogged
with phlegm, but they may be opened by lowering and protruding the jaw.
The inner ear contains both the organ of hearing (the cochlea) and the
labyrinth or vestibular apparatus, the organ of balance located in the inner
ear that consists of three semicircular canals and the vestibule.
The cochlea is a hollow organ filled with a fluid called endolymph and lined
on the inside with hair cells - sensory cells which are topped with
hair-like structures, the stereocilia. All vibrations passing through the
middle ear enter the endolymph. Hair cells are varied in length, so that
they resonate with sounds of various frequency. Whenever a hair cell
resonates, it sends a nerve impulse to the brain, which is perceived as a
sound of whatever pitch the hair cell is associated with. A very strong
movement of the endolymph due to very loud noise may cause hair cells to
die. This is a common cause of partial hearing loss, and the reason why
anyone near guns or heavy machinery should wear earmuffs or earplugs.
The vestibular apparatus is filled with the same endolymph as the cochlea,
but instead of detecting sound, it detects rotation of the head. If a line
is drawn through the middle of each of the three semicircular canals,
perpendicular to the plane in which the canal lies, the three lines would be
perpendicular. They would represent three axes of rotation. Any rotation
could be represented as three simultaneous rotations about the three axes.
Spiders have hairs on their legs which are used for detecting sound.
Diseases and medical conditions of the ear and auditory system
* Acoustic neurinoma
* Balance disorders
* Benign Paroxysmal Positional Vertigo
* Ear infections
* Conductive hearing impairment
* Labyrinthine hydrops
* MŽniŹre's disease
* Neurofibromatosis Type 1
* Neurofibromatosis Type 2
* Noise-induced hearing loss
* Nonsyndromic hereditary hearing impairment
* Otitis externa
* Otitis media
* Perilymph fistula
* Sensorineural hearing loss
* Sudden deafness
* Usher syndrome
* Vestibular neuronitis