By use of the electrical potentials of the cochlea, the pathway of sound conduction to the ear was investigated in two species of dolphins, Tursiops truncatus and Lagenorkynchus obliquidens. The observations show that the external auditory meatus is vestigial and that neither this meatus nor the tympanic membrane plays any part in sound reception. The ossicular chain has an essential function, though the outermost ossicle, the malleus, can be removed without seriously affecting the conductive process. It is concluded that auditory stimulation in the dolphin occurs by bone conduction. Acoustic vibrations in the water reach the ear through the tissues of the head, and set up differential motions between the stapes and the cochlear capsule. These differential motions involve the cochlear fluids, which are given mobility by the presence of gas in the region of the round window. The vibrations of the basilar membrane and stimulation of the hair cells of the organ of Corti then follow in the usual way.

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