One reason we yawm is to equalize pressure. Our outer ear has an external auditory meatus - that's the tunnel that leads into your ear where you have to clean earwax out of. The external auditory meatus terminates at the tympanic membrane (eardrum). On the other side of the tympanic membrane is the middle ear, or tympanic cavity. The pressure in the tympanic cavity is not necessarily the same as the atmospheric pressure in the external auditory meatus. This can cause a reduction in hearing, or if the difference is large enough, pain. The tympanic cavity has a tube, called the Eustachian tube, that runs down to the pharynx (I believe into the nasopharynx, but it might be the oropharynx instead). When we yawn, air 'inflates' the tube and allows air to pass up into the tympanic cavity, equalizing its pressure with atmospheric pressure.
This is why people are advised to yawn when climbing or descending in planes.
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PS: Looked up the anatomy of the ear in my A&P text and found 2 things.
First, it is the nasopharynx that the Eustachian tube connects to.
Second, I see no mention of the Eustachain tube being "deflated" and yawning causing it to "inflate". I'm pretty sure my professor said that though, and she has been correct on other things she's stated but weren't in the book (but she's also been wrong about a thing or two).
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PPS: Looks like my professor was right.
Quote:
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”Pressurization of the middle ear can and should be vigorous on the surface [as opposed to during a dive], when no negative pressure gradient is present across the middle ear. This means that it is possible (and desirable) for an individual to pre-pressurize the middle ear and to inflate the Eustachian tube prior to descent.” (http://faculty.washington.edu/ekay/MEbaro.html)
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