The cochlea
(a) The spiral duct of the cochlea is divided into two channels by the flexible basilar membrane and is filled with fluid (yellow). The inner hair cells, which sit on the membrane, connect directly to the auditory nerve.
(b) When sound enters the cochlea (here shown uncoiled) it agitates the fluid, causing a ripple to travel along the basilar membrane. This movement (which is grossly exaggerated here) is detected by sensory hair cells, supported on the membrane.