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Theossicles (also calledauditory ossicles) are threeirregular bones in themiddle ear ofhumans and othermammals, and are among the smallestbones in the human body. Although the term "ossicle" literally means "tiny bone" (fromLatinossiculum) and may refer to any small bone throughout the body, it typically refers specifically to themalleus,incus andstapes ("hammer, anvil, and stirrup") of the middle ear.
The auditory ossicles serve as akinematic chain to transmit and amplify (intensify)sound vibrations collected from the air by theear drum to the fluid-filledlabyrinth (cochlea). The absence or pathology of the auditory ossicles would constitute a moderate-to-severeconductive hearing loss.
The ossicles are, in order from the eardrum to the inner ear (from superficial to deep): themalleus,incus, andstapes, terms that inLatin are translated as "thehammer,anvil, andstirrup".[1]
Studies have shown that ear bones in mammal embryos are attached to thedentary, which is part of the lowerjaw. These are ossified portions ofcartilage—calledMeckel's cartilage—that are attached to the jaw. As the embryo develops, the cartilage hardens to form bone. Later in development, the bone structure breaks loose from the jaw and migrates to the inner ear area. The structure is known as the middle ear, and is made up of thestapes,incus,malleus, andtympanic membrane. These correspond to thecolumella,quadrate,articular, andangular structures in the amphibian, bird or reptile jaw.[3]
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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 (oval window), the opening to the vestibule of the inner ear.
Sound traveling through the air is mostly reflected when it comes into contact with a liquid medium; only about 1/30 of the sound energy moving through the air would be transferred into the liquid.[4] This is observed from the abrupt cessation of sound that occurs when the head is submerged underwater. This is because the relative incompressibility of a liquid presents resistance to the force of the sound waves traveling through the air. The ossicles give the eardrum amechanical advantage via lever action and a reduction in the area of force distribution; the resulting vibrations are stronger but don't move as far. This allows more efficient coupling than if the sound waves were transmitted directly from the outer ear to the oval window. This reduction in the area of force application allows a large enough increase inpressure to transfer most of the sound energy into the liquid. The increased pressure will compress the fluid found in the cochlea and transmit the stimulus. Thus, the lever action of the ossicles changes the vibrations so as to improve the transfer and reception of sound, and is a form ofimpedance matching.
However, the extent of the movements of the ossicles is controlled (and constricted) by two muscles attached to them (thetensor tympani and thestapedius). 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) (seeacoustic reflex). These muscles are more highly developed inbats and serve to block outgoing cries of the bats duringecholocation (SONAR).
Occasionally the joints between the ossicles become rigid. One condition,otosclerosis, results in the fusing of the stapes to the oval window. This reduceshearing and may be treated surgically using a passivemiddle ear implant.[further explanation needed]
There is some doubt as to the discoverers of the auditory ossicles and several anatomists from the early 16th century have the discovery attributed to them with the two earliest beingAlessandro Achillini andJacopo Berengario da Carpi.[5] Several sources, includingEustachi andCasseri,[6] attribute the discovery of the malleus and incus to the anatomist and philosopherAchillini.[7] The first written description of the malleus and incus was byBerengario da Carpi in hisCommentaria super anatomia Mundini (1521),[8] although he only briefly described two bones and noted their theoretical association with the transmission of sound.[9]Niccolo Massa'sLiber introductorius anatomiae[10] described the same bones in slightly more detail and likened them both to little hammers.[9] A much more detailed description of the first two ossicles followed inAndreas Vesalius'De humani corporis fabrica[11] in which he devoted a chapter to them. Vesalius was the first to compare the second element of the ossicles to an anvil although he offered the molar as an alternative comparison for its shape.[12] The first published description of the stapes came inPedro Jimeno'sDialogus de re medica (1549)[13] although it had been previously described in public lectures byGiovanni Filippo Ingrassia at theUniversity of Naples as early as 1546.[14]
The termossicle derives fromossiculum, adiminutive of "bone" (Latin:os;genitiveossis).[15] The malleus gets its name from Latinmalleus, meaning "hammer",[16] the incus gets its name from Latinincus meaning "anvil" from incudere meaning "to forge with a hammer",[17] and the stapes gets its name from Modern Latin "stirrup", probably an alteration of Late Latinstapia related to stare "to stand" and pedem, an accusative of pes "foot", so called because the bone is shaped like a stirrup – this was an invented Modern Latin word for "stirrup", for which there was no classical Latin word, as the ancients did not use stirrups.[18]
