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 ear drum separates this space from the ear canal. The area is pressurized. The ear drum 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 the tympanic cavity. This maintains a constant pressure. Sometimes, this pressure is not equalized with the environment outside the head, and this is often the reason why some people experience discomfort in airplanes and at higher elevations The cavity plays a very important in a person's ability to hear. Inside the middle ear, three small bones form a chain. These are known as the ossicles, and they conduct sound vibrations from the ear drum to the inner ear. Once in the fluid filled inner ear, sounds are converted into nerve impulses and are sent to the brain.