What is the inner fluid of the inner ear called, which is also separated by a thin membrane?

The inner fluid of the inner ear referred to in the question is known as endolymph fluid. This fluid is found within the membranous labyrinth of the inner ear, which is a complex structure consisting of the cochlea, vestibule, and semicircular canals.

Endolymph is unique in its composition, containing high levels of potassium ions, which plays a crucial role in the function of the hair cells in the inner ear that are responsible for converting sound vibrations into nerve impulses. The separation of endolymph from perilymph fluid, which is found in the space surrounding the membranous labyrinth, by a thin membrane (the membranous labyrinth itself) is significant. This separation is essential for maintaining proper electrochemical gradients needed for hearing and balance.

Understanding the role and location of endolymph provides insight into how the inner ear functions, particularly in relation to hearing and equilibrium. This focused distinction between endolymph and other fluids like perilymph, cochlear fluid, or auditory fluid emphasizes its importance in the physiology of the auditory system.