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An ear is an organ used by an animal to detect sound. The term may refer to the entire system responsible for collection and processing of sound (the auditory system[?]), or merely the externally-visible part.
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.
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.
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.