what is the fluid-filled space behind the oval window
Cochlea The lengthwise partition that divides most of the cochlea is itself a fluid-filled tube, the third duct. This central column is called the cochlear duct. Its fluid, endolymph, also contains electrolytes and proteins, but is chemically quite different from perilymph. Whereas the perilymph is rich in sodium ions, the endolymph is rich in potassium ions, which produces an ionic, electrical potential.