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Middle Ear.A thin, flat membrane, called the tympanic membrane or eardrum, stretches across the inner end of the external auditory canal. Beyond the eardrum is a flattened, rectangular, air-filled space called the middle ear cavity or tympanic cavity. Three tiny bones, called the ossicles, traverse this cavity as a chain of levers transmitting vibrations of the eardrum to the inner ear. Because of their shape, the ossicles are designated as the malleus of hammer, the incus or anvil, and the stapes or stirrup. The malleus is fastened to the center of the tympanic membrane. It is anchored to the walls of the cavity by tendons and is connected to the incus which, in turn, is fastened to the stapes. The footplate of the stapes, which is shaped like the footplate of an ordinary stirrup, fits into the oval window, an opening in the bony wall of the inner ear. Tiny muscles regulate sound transmission by tensing the bones. A membrane vibrates best when air pressures are equal on its two sides. The tympanic membrane has this advantage since the tympanic cavity is supplied with the air from the nose and throat by way of the Eustachian tube which enters into its lower forward angle. This tube is opened by swallowing or yawning, which permits air to enter the tympanic cave and maintain atmospheric pressure.
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