It is the main hearing organ. The cochlea forms three chambers.

(i) Upper − scala vestibule

(ii) Middle − scala media

(iii) Lower − scale tympani

The floor of the scala media is basilar membrane while its roof is Reissner’s membrane. Reissner’s membrane gives out a projection called tectorial membrane. The organ of corti, a hearing organ, is located on the basilar membrane. Organ of corti contains receptor hair cells. The upper scala vestibule and lower scala tympani contain perilymph.