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1. In man and the higher vertebrates, the organ of hearing is very complicated, and is divisible into three parts: the external ear, which includes the pinna or auricle and meatus or external opening; the middle ear, drum, or tympanum; and the internal ear, or labyrinth. The middle ear is a cavity connected by the Eustachian tube with the pharynx, separated from the opening of the external ear by the tympanic membrane, and containing a chain of three small bones, or ossicles, named malleus, incus, and stapes, which connect this membrane with the internal ear. The essential part of the internal ear where the fibers of the auditory nerve terminate, is the membranous labyrinth, a complicated system of sacs and tubes filled with a fluid and lodged in a cavity, called the bony labyrinth, in the periotic bone. The membranous labyrinth does not completely fill the bony labyrinth, but is partially suspended in it in a fluid (the perilymph). The bony labyrinth consists of a central cavity, the vestibule, into which three semicircular canals and the canal of the cochlea (spirally coiled in mammals) open. The vestibular portion of the membranous labyrinth consists of two sacs, the utriculus and sacculus, connected by a narrow tube, into the former of which three membranous semicircular canals open, while the latter is connected with a membranous tube in the cochlea containing the organ of Corti. By the help of the external ear the sonorous vibrations of the air are concentrated upon the tympanic membrane and set it vibrating, the chain of bones in the middle ear transmits these vibrations to the internal ear, where they cause certain delicate structures in the organ of Corti, and other parts of the membranous labyrinth, to stimulate the fibers of the auditory nerve to transmit sonorous impulses to the brain. Songs . . . not all ungrateful to thine ear.Tennyson. Dionysius . . . would give no ear to his suit.Bacon. Friends, Romans, countrymen, lend me your ears.Shak. Ear |
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