As sound waves enter the ear, they travel through the outer ear and the external auditory canal and then strike the eardrum, causing it to vibrate. The central part of the eardrum is connected to a small bone of the middle ear called the malleus (hammer). As the malleus vibrates, it transmits the sound vibrations to the other two small bones or ossicles of the middle ear, the incus and stapes. As the stapes moves, it pushes a structure called the oval window in and out. This action is passed on to the cochlea, which is a fluid-filled snail-like structure that contains the receptor organ for hearing. The cochlea contains the spiral organ of Corti, which is the receptor organ for hearing. It consists of tiny hair cells that translate the fluid vibration of sounds from its surrounding ducts into electrical impulses that are carried to the brain by sensory nerves. As the stapes rocks back and forth against the oval window, it transmits pressure waves of sound through the fluid of the cochlea, sending the organ of Corti in the cochlear duct into motion. The fibers near the cochlear apex resonate to lower-frequency sound while fibers near the oval window respond to higher-frequency sound.