The Eustachian tube (or auditory tube) is a tube that links the pharynx to the middle ear.
In adults the Eustachian tube is approximately 35 mm long.
Some medical books call this the pharyngotympanic tube.
Normally the Eustachian tube is closed, but it can open to let a small amount of air through to equalize the pressure between the middle ear and the atmosphere.
When this happens we hear a small pop, an event familiar to airplane travelers or drivers in mountainous regions.
Yawning or swallowing can pull on muscles in the neck, causing the tube to open.
Some people are born with the ability to contract just these muscles voluntarily, similar to people who can wiggle their ears.
Without this airway, the middle ear would be isolated from the atmosphere, and could be easily damaged by pressure changes.