The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The mammalian middle ear contains three ossicles, which couple vibration of the eardrum into waves in the fluid and membranes of the inner ear. The hollow space of the middle ear has also been called the tympanic cavity, or cavum tympani. The eustachian tube joins the tympanic cavity with the nasal cavity (nasopharynx), allowing pressure to equalize between the middle ear and throat.
The primary function of the middle ear is to efficiently transfer acoustic energy from compression waves in air to fluid–membrane waves within the cochlea.
Ordinarily, waves travel in a system of fluids and membranes in the inner ear. This system should not be confused, however, with the propagation of sound as compression waves in a liquid.
The middle ear couples sound from air to the fluid via the oval window, using the principle of "mechanical advantage" in the form of the "hydraulic principle" and the "lever principle". The vibratory portion of the tympanic membrane is many times the surface area of the footplate of the stapes; furthermore, the shape of the articulated ossicular chain is like a lever, the long arm being the long process of the malleus, and the body of the incus being the fulcrum and the short arm being the lenticular process of the incus. The collected pressure of sound vibration that strikes the tympanic membrane is therefore concentrated down to this much smaller area of the footplate, increasing the force but reducing the velocity and displacement, and thereby coupling the acoustic energy.
The middle ear is able to dampen sound conduction substantially when faced with very loud sound, by noise-induced reflex contraction of the middle-ear muscles.
The middle ear contains three tiny bones known as the ossicles: malleus, incus, and stapes. The ossicles were given their Latin names for their distinctive shapes; they are also referred to as the hammer, anvil, and stirrup, respectively. The ossicles directly couple sound energy from the ear drum to the oval window of the cochlea. While the stapes is present in all tetrapods, the malleus and incus evolved from lower and upper jaw bones present in reptiles. See Evolution of mammalian auditory ossicles.
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