<P> The middle ear couples sound from air to the fluid via the oval window, using the principle of "mechanical advantage" in the form of the "hydraulic principle" and the "lever principle". The vibratory portion of the tympanic membrane (eardrum) is many times the surface area of the footplate of the stapes (the third ossicular bone which attaches to the oval window); furthermore, the shape of the articulated ossicular chain is like a lever, the long arm being the long process of the malleus, the fulcrum being the body of the incus, and the short arm being the lenticular process of the incus . The collected pressure of sound vibration that strikes the tympanic membrane is therefore concentrated down to this much smaller area of the footplate, increasing the force but reducing the velocity and displacement, and thereby coupling the acoustic energy . </P> <P> The middle ear is able to dampen sound conduction substantially when faced with very loud sound, by noise - induced reflex contraction of the middle - ear muscles . </P> <P> The middle ear is hollow . In a high - altitude environment or on diving into water, there will be a pressure difference between the middle ear and the outside environment . This pressure will pose a risk of bursting or otherwise damaging the tympanum if it is not relieved . If middle ear pressure remains low, the ear drum may become retracted into the middle ear <cite>. One of the functions of the Eustachian tubes that connect the middle ear to the nasopharynx is to help keep middle ear pressure the same as air pressure . The Eustachian tubes are normally pinched off at the nose end, to prevent being clogged with mucus, but they may be opened by lowering and protruding the jaw; this is why yawning or chewing helps relieve the pressure felt in the ears when on board an aircraft . </P> <P> Otitis media is an inflammation of the middle ear . </P>

Besides connecting the pharynx to the ear what other purpose does the eustachian tube serve