What is the function ascribed to Eustachian tube?

The Eustachian tube is the tube that connects middle ear cavity with the pharynx.

It helps in equalising the pressure on either sides of the ear drum. There is a valve present at the pharyngeal opening. This usually remains closed. It opens up during yawning, swallowing, or due to abrupt change in altitude to let the air enter or leave the tympanic cavity to equalise the pressure.