In the cochlea, both the bony labyrinth and the cochlear duct are coiled in a shape resembling that of a snail shell. Resting along the basilar membrane, which forms the base of the cochlear duct, is an arrangement of sensory cells and supporting cells known as the organ of Corti. This cluster of cells varies in thickness, so that different regions within the cochlea are sensitive to different wavelengths of sound. When sound waves are conducted across the bones of the middle ear, they cause the oval window (a membranous opening between the middle and inner ears) to move in and out along with the stapes of the middle ear, to which it is attached. The motion of the oval window sets up a wave in the perilymph filling the scala vestibuli of the cochlea. This wave is transmitted across Reissner's membrane (the roof of the cochlear duct) into the endolymph of the cochlear duct. It then passes through the tectorial membrane, which forms a roof to protect the organ of Corti, into the organ of Corti. The organ of Corti contains sensory cells with hairlike projections, called hair cells, that are deformed by the progress of the wave. The hair cells trigger nerve impulses that travel along the cochlear nerve, a branch of the auditory nerve, to the brain, where they are interpreted as sound. The sound wave then passes into the perilymph of the scala tympani, where it causes a second membrane-covered opening into the middle ear, the round window, to bulge outward and dampen the wave in the perilymph. The exact physical mechanism of hearing—i.e., the traveling of waves along the basilar membrane—was first correctly explicated by the Hungarian-American physicist Georg von Békésy in the mid-20th century.