Final answer:
The cochlea is divided into upper and lower compartments by the bony spiral lamina and the membranous cochlear partition, with the organ of Corti located atop the basilar membrane.
Step-by-step explanation:
The two structures that together divide the cochlea into upper and lower compartments are the bony spiral lamina and the membranous cochlear partition. A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct, separated by these structures. The organ of Corti, containing the mechanoreceptor hair cells, sits atop the basilar membrane, which is part of the cochlear partition between the organ of Corti and the scala tympani. As sound is transduced into neural signals within the cochlea, the position where the basilar membrane vibrates is determined by the frequency of the incoming waves, with higher frequencies affecting the base and lower frequencies affecting the tip of the cochlea.