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The middle ear

The middle ear (tympanic cavity) is an air-filled cavity which contains three of the smallest bones in our body: the hammer (malleus), the anvil (incus), and the stirrup (stapes). Ligaments attach the malleus to the eardrum, while the stapes is in contact with another membrane, the oval window (fenestra vestibuli) of the inner ear. The incus is in between the two and all three form a lever system, held in place by tiny muscles.

The three bones transfer the sound vibrations from the eardrum to the oval window, and the leverage is arranged in a manner to amplify the mechanical force. In addition, the original force from the eardrum concentrates on the much smaller oval window, creating a further increase in strength. The mechanical leverage and membrane size differences result in a strengthening of the vibrations by approximately twenty times.

When loud noise threatens to damage the middle and inner ear, two of the small muscles, the tensor tympani and the stapedius, contract and limit the movement of the three bones, preventing violent vibrations. This muscle reflex is, however, sometimes too slow to counter rapid or explosive loud noises and permanent damage may still occur.

An air-filled cavity has the disadvantage that the air expands and wants to get out whenever the outside air pressure decreases. To prevent damage to the middle ear, it requires a pressure relief valve. The Eustachian tube, a narrow passageway to the back of the throat, acts as such a valve. It equalizes the pressure between the middle ear cavity and the outside world. Unfortunately, this tube becomes easily blocked when you have a cold. Driving up a mountain or going on a flight may painfully remind you of this part of the human anatomy. Yawning or swallowing, usually helps in opening the tube and relieving the pressure.

Have you noticed that babies on board an airliner often cry during take-off or landing? If you are travelling with a small child, give him or her something to swallow or chew during this phase of flight, even if you think the child doesn’t have a cold. Better have the Eustachian tube open up and relief the air pressure than spoiling a flight with a child in pain.

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