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eardrum, a thin layer of connective tissue and epithelium covering the opening of the tympanic cavity in the ear of terrestrial vertebrates and man. The tympanic membrane is absent in caudate and legless amphibians and burrowing snakes. Sound waves on striking the membrane cause it to vibrate. The vibrations are then transmitted to the inner ear mostly by the auditory ossicles. The membrane is divided into three concentric zones in mammals and man because of the uneven action of forces on some of its parts: medial (which moves without becoming deformed), intermediate (the most delicate and strongly vibrated), and lateral (hinged, movable).
G. N. SIMKIN