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	What is Eustachian Tube? 1 Tympanic Membrane 　2 Maleus Incus Stapes 3 Semicircular canals 4 Cochlea 5 Cochlear Nerve 　　 6 Oval Window 7 Eustachian Tube 　　8 Orifice 9 Round Window　　　 10 Middle Ear Cavity The Eustachian tube is only channel of communication between the nasopharynx and the middle ear for the purpose of equalizing the pressure of the external air and that contained in the middle ear ( See the top figure). The natural ventilator of the middle ear is the eustachian tube. The middle ear is aerated only when the eustachian tube is opened, which take place during the act of deglutition. The tube is otherwise closed, so intended by nature to protect the middle ear from unnecessary exposure from the nasopharynx. Large pressures in the middle ear cavity can affect the inner ear by way of the cochlear windows(Round window and Oval window). When you travel by plane, you may sometimes feel ear fullness. But this feeling will disapper by swallowing or chewing. Your Eustachian Tube is working in this way.
	Shigeru Ishikawa (ishikawa@po.incl.ne.jp)