Middle ear

The middle ear is the portion of the ear internal to the eardrum, and external to the oval window of the cochlea. The middle ear contains three ossicles, which amplify vibration of the eardrum into pressure waves in the fluid in 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.

The function of the middle ear is to efficiently transfer sound energy from air to liquid.

Ossicles

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 amplify vibration of the eardrum, into pressure waves of the fluid in the cochlea (or inner ear) by a factor of 1.3. Since the size of the eardrum area is about 17 fold larger than those of the oval window, the sound pressure is concentrated on a smaller area, leading to a total amplification of at least 22. The eardrum is fused to the malleus, which connects to the incus, which in turn connects to the stapes. Vibrations of the stapes footplate introduce pressure waves in the inner ear. The auditory ossicles can also reduce sound pressure(the inner ear is very sensible against overstimulation), by uncoupling each other through particular muscles.

The ratio in area between the tympanic membrane and the oval window results in an effective amplification of approximately 14 dB, peaking at a frequency of around 1 kHz. The combined transfer function of the outer ear and middle ear gives humans a peak sensitivity to frequencies between 1 kHz and 3 kHz.

Muscles

The movement of the ossicles may be stiffened by two muscles, the stapedius and tensor tympani, which are under the control of the facial nerve and trigeminal nerve, respectively. These muscles contract in response to loud sounds, thereby reducing the transmission of sound to the inner ear. This is called the acoustic reflex.

Nerves

Of surgical importance are two branches of the facial nerve which also pass through the middle ear space. These are the horizontal and chorda tympani branches of the facial nerve. Damage to the horizontal branch during surgery can lead to partial, unilateral facial paralysis.

Comparative anatomy

Mammals are unique in having three ear bones. The incus and stapes develop from bones of the jaw, and allow finer detection of sound.

Some mammals, such as the cat, have an enlarged middle ear encased in a thin, bulbous bone; this structure is known as a bulla.

Disorders of the middle ear

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 one of the functions of the Eustachian tubes — evolutionary descendants of the gills — which connect the middle ear to the nasopharynx. 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; this is why yawning helps relieve the pressure felt in the ears when on board an aircraft.

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