<P> The cochlea is a portion of the inner ear that looks like a snail shell (cochlea is Greek for snail .) The cochlea receives sound in the form of vibrations, which cause the stereocilia to move . The stereocilia then convert these vibrations into nerve impulses which are taken up to the brain to be interpreted . Two of the three fluid sections are canals and the third is a sensitive' organ of Corti' which detects pressure impulses which travel along the auditory nerve to the brain . The two canals are called the vestibular canal and the tympanic canal . </P> <P> The walls of the hollow cochlea are made of bone, with a thin, delicate lining of epithelial tissue . This coiled tube is divided through most of its length by an inner membranous partition . Two fluid - filled outer spaces (ducts or scalae) are formed by this dividing membrane . At the top of the snailshell - like coiling tubes, there is a reversal of the direction of the fluid, thus changing the vestibular duct to the tympanic duct . This area is called the helicotrema . This continuation at the helicotrema allows fluid being pushed into the vestibular duct by the oval window to move back out via movement in the tympanic duct and deflection of the round window; since the fluid is nearly incompressible and the bony walls are rigid, it is essential for the conserved fluid volume to exit somewhere . </P> <P> The lengthwise partition that divides most of the cochlea is itself a fluid - filled tube, the third duct . This central column is called the cochlear duct . Its fluid, endolymph, also contains electrolytes and proteins, but is chemically quite different from perilymph . Whereas the perilymph is rich in sodium ions, the endolymph is rich in potassium ions, which produces an ionic, electrical potential . </P> <P> The hair cells are arranged in four rows in the organ of Corti along the entire length of the cochlear coil . Three rows consist of outer hair cells (OHCs) and one row consists of inner hair cells (IHCs). The inner hair cells provide the main neural output of the cochlea . The outer hair cells, instead, mainly receive neural input from the brain, which influences their motility as part of the cochlea's mechanical pre-amplifier . The input to the OHC is from the olivary body via the medial olivocochlear bundle . </P>

What is the fluid-filled space behind the oval window