Individual differences |
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|Gray's||subject #231 1044|
|Chain of ossicles and their ligaments, seen from the front in a vertical, transverse section of the tympanum.|
The ear ossicles (also called auditory ossicles) are the three smallest bones in the human body. They are contained within the middle ear space and serve to transmit sounds from the air to the fluid filled labyrinth (cochlea). The absence of the auditory ossicles would constitute a moderate to severe hearing loss.
The ossicles are, in order from the eardrum to the inner ear, the hammer, anvil, and stirrup, so named because of the shape of the bones. They are also commonly referred to by the equivalent Latin terms: malleus, incus, and stapes respectively.
- The malleus articulates with the incus and is attached to the tympanic membrane (eardrum), from which vibrational energy is passed.
- The incus is connected to both the other bones.
- The stapes articulates with the incus and is attached to the membrane of the fenestra ovalis, the elliptical or oval window or opening between the middle ear and the vestibule of the inner ear.
As sound waves vibrate the tympanic membrane (eardrum), it in turn moves the nearest ossicle, the malleus to which it is attached. The malleus then transmits the vibrations, via the incus, to the stapes, and so ultimately to the membrane of the fenestra ovalis, the opening to the vestibule of the inner ear.
The ossicles give the eardrum mechanical advantage via lever action and a reduction in the area of force distribution; the resulting vibrations would be much smaller if the sound waves were transmitted directly from the outer ear to the oval window. However, the extent of the movements of the ossicles is controlled (and constricted) by certain muscles attached to them (the tensor tympani and the stapedius). It is believed that these muscles can contract to dampen the vibration of the ossicles, in order to protect the inner ear from excessively loud noise (theory 1) and that they give better frequency resolution at higher frequencies by reducing the transmission of low frequencies (theory 2) (see acoustic reflex).
Occasionally the joints between the ossicles become rigid. One condition, Otosclerosis, results in the fusing of the stapes to the oval window. This reduces hearing and may be treated surgically.
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