An opening resembling a window. The fenestra ovalis (fenestra vestibuli), or oval window, is the opening between the middle ear and the vestibule of the inner ear. It is closed by a membrane to which the stapes is attached. The fenestra rotunda (fenestra cochleae), or round window, is the opening between the scala tympani of the cochlea and the middle ear. Sound vibrations leave the cochlea through the fenestra rotunda which, like the fenestra ovalis, is closed by a membrane.