Basilar membrane is a membranous septum that divides the cochlea into two parts, an upper part (scala vestibuli) and a lower part (scala media). Organ of corti is the auditory receptor that lies on the basilar membrane, and extends throughout cochlear duct.
The pitch of the sound is determined by the wave frequency (number of waves per second). The brain determines the pitch of a sound, by assessing the part of basilar membrane, which is more vibrating. More vibration means, more hair cells (present in the organ of corti, of the internal ear) fire in that region.
Thus, the blank can be filled with cochlea.