is the middle ear is a fluid-filled cavity.
Middle ear. Also known as the tympanic cavity, the middle ear is an air-filled, membrane-lined space located between the ear canal and the Eustachian tube, cochlea, and auditory nerve. The eardrum separates this space from the ear canal. The area is pressurized. The eardrum acts as a natural boundary between the middle ear and the ear canal. Pressure in the middle ear is maintained through the Eustachian tubes, which are closed when not in use. Each time a person swallows, the Eustachian tubes open and allow fresh air to enter into the tympanic cavity.