The ear is composed of three parts:
1. The outer ear, which includes the part of the ear that is visible, plus the ear canal connected to the eardrum.
2. The middle ear, which includes the eardrum, the ear bones (ossicles), and the air spaces behind the eardrum and in the mastoid cavities.

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3. The inner ear, which contains the nerve endings that facilitate hearing and equilibrium.
The middle ear is what bothers travelers on airplanes because it is, in part, an air pocket vulnerable to changes in air pressure. On the ground, when you swallow, your ears make a little click or popping sound. This noise marks the passage of a small air bubble up from the back of your nose, through the eustachian tube, and into your middle ear. According to the American Council of Otolaryngology, "the air in the middle ear is constantly being absorbed by its membranous lining, but it is frequently re-supplied through the eustachian tube during the process of swallowing. In this manner air pressure on both sides of the eardnim stays about equal. If, and when, the air pressure is not equal, the ear feels blocked."
If the eustachian tube is blocked, no air can be replenished in the middle ear; any air present absorbs and a vacuum occurs, sucking the eardrum inward. Blocked eustachian tubes can cause a loss of hearing and pain. A clear and properly functioning eustachian tube is the key to problem-free ears on plane flights; if it can open wide enough and often enough, the eustachian tube can moderate changing air-pressure conditions. When you ascend on an airplane, it is to less pressure, so the air expands in the middle ear. The eustachian tube works much like a flutter valve on an automobile. When you ascend, the air in your ear is forced through the tube in a steady stream without any problem. When you descend, it is to greater air pressure. A vacuum forms even faster in the middle ear, making it harder for the air to go back through the membranous part of the eustachian tube. According to Dr. Andrew F. Home, in the Office of Aviation Medicine of the Federal Aviation Administration, the ear popping is caused when the valve of the eustachian tube opens and closes. On ascent, the air runs through the eustachian tube in a steady stream; on descent, the air must contend with the membranous part of the eustachian tube. Without the steady air flow, it takes longer to equalize air pressure inside and outside your ear. Airplane pilots are taught how to counteract differences in air pressure. The simple act of swallowing pulls open the eustachian tube, which is wh