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in man and other terrestrial vertebrates, the part of the auditory system located between the external ear and the internal ear. The middle ear includes the air-filled tympanic cavity, which contains auditory ossicles and the auditory, or eusta-chian, tube. In man and some primates, it also includes mastoid cells. In most vertebrates, it is bounded on the outside by the tympanic membrane. The middle ear is separated from the internal ear by the cartilaginous or bony wall of the vestibule of the labyrinth. The auditory ossicles transmit sound vibrations from the tympanic membrane to the internal ear. In most animals, the middle ear is connected to the pharynx by the auditory tube. In many terrestrial vertebrates and especially in mammals it contains many additional structures that perform important acoustic functions. The middle ear is partly or completely reduced in many terrestrial and secondarily aquatic amphibians, in many mammals, and in some turtles and snakes.