CENTRAL part of the osseous labyrinth and responds to gravity and movements of head. Cochlea is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth. Perilymph is an extracellular fluid located within the cochlea (part of the inner ear) in two of its three compartments: the SCALA tympani and scala vestibuli. The MAJOR CATION (positively charged ion) in perilymph is sodium. Endolymph is the fluid contained in the membranous labyrinth of the inner ear.