The fluid contained within the membranous labyrinth is called endolymph.

Here's a breakdown:

* Membranous labyrinth: This is the intricate network of fluid-filled sacs and ducts within the bony labyrinth of the inner ear.

* Endolymph: This is a potassium-rich fluid that fills the membranous labyrinth. It's essential for the function of the sensory receptors within the inner ear, specifically for hearing and balance.

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