The cochlea is a spiral-shaped portion of the inner ear osseous labyrinth, located in the petrous portion of the temporal bone. It contains the cochlear duct, a portion of the membranous labyrinth that senses and processes sound. Its spiraled contour rotates a total of approximately two and three-quarter turns (990 degrees) around a central portion of bone called the modiolus. This spiraled tube is subdivided into three fluid-filled tubes by two membranes, the spiral lamina/basilar membrane and Reissner’s membrane. Two of the tubes, the scala tympani and scala vestibuli, considered the lower and upper sections respectively, are filled with perilymph. The middle tube is called the scala media (or cochlear duct), which is filled with endolymph and contains the organ of Corti, the primary sensory organ for hearing.