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tympanic cavity[tim′pan·ik ′kav·əd·ē]
a cavity in the middle ear of terrestrial animals and man. It develops from the cavity of the first gill slit. It has become lost in caudate and legless amphibians and burrowing snakes. The tympanic cavity is located between the tympanic membrane, the bony labyrinth of the inner ear, and one or more cranial bones; it contains the auditory ossicles. The tympanic cavity is filled with air and communicates with the pharyngeal cavity via the auditory or Eustachian tube. In some terrestrial vertebrates, especially mammals, the tympanic cavity enlarges and its functions intensify because of the formation of eardrums and additional auditory cavities in the adjacent parts of the cranium. The development of cancellous bone in these parts helps to make “spatial hearing” more acute, especially in aquatic and subterranean animals. The connection of the tympanic cavity to resonance chambers attuned to the sound frequencies most important in the life of animals increases the selective sensitivity of the organ of hearing.
G. N. SIMKIN