The cochlea is divided into three chambers by a basilar and reissner's membrane. The upper chamber is known as scala vestibuli, the lower chamber is scala tympani and the middle chamber is known as scala media. Endolymph is a potassium rich fluid and is present in scala media. Its composition is similar to intracellular fluid. The endolymph here is secreted by the stria vascularis which forms the lateral wall of scala media. Perilymph is a fluid present in scala vestibuli and scala tympani. Its composition is similar to extracellular fluid in that it is high in Na+ and low in K+.