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The spiral organ of Corti rests upon the basilar membrane, which is thin, homogeneous, and covered on its surfaces above and below with a layer of epithelium. The periosteum, which forms the outer wall of the cochlear duct of the ear, is greatly thickened here and altered in character, and is now called the spiral ligament. It projects inwardly from below as a triangular prominence, the basilar crest,mycontentbreak which is attached to the outer edge of the basilar membrane.