The cochlea is shaped like the coiled shell of a snail. Inside, it contains a bony core and a thin bony shelf (spiral lamina) that winds around the core like the threads of a screw. The shelf divides the bony labyrinth of the cochlea into upper and lower compartments. The upper compartment, called the scala vestibuli, leads from the oval window to the apex of the spiral. The lower compartment, themycontentbreak scala tympani, extends from the apex of the cochlea to a membrane-covered opening in the wall of the inner ear called the round window. The round window is actually situated below and a little to the back of the oval window, from which it is separated by a rounded elevation, the promontory. The membranous labyrinth of the cochlea is represented by the cochlear duct (scala media). It lies between the two bony compartments and ends as a closed sac at the apex of the cochlea. The cochlear duct is separated from the scala vestibuli by a vestibular membrane (Reissner’s membrane) and from the scala tympani by a basilar membrane. The basilar membrane extends down from the bony shelf to form the floor of the cochlear duct.