Final answer:
The fluid that fills the membranous labyrinth of the inner ear is called endolymph, which plays an essential role in the process of hearing and balance.
Step-by-step explanation:
The fluid that fills the membranous labyrinth of the inner ear is known as endolymph. The inner ear consists of bony structures filled with this fluid that plays a critical role in both hearing and balance. When sound waves reach the inner ear, they cause the oval window to move, which in turn generates pressure waves in the fluid. The fluctuations in endolymph stimulate the hair cells in the organ of Corti, transforming sound waves into electrical signals that the brain interprets as sound.
The perilymph is another fluid that surrounds the membranous labyrinth, filling the space between the bony labyrinth and the membranous labyrinth. Sound vibrations are transmitted through the perilymph before influencing the endolymph-contained structures.