Mouthgags
Mouthgags are not primarily used by dentists, as some may assume, but rather by ENT doctors. For surgery in or through the mouth, a permanent opening must be guaranteed and any accidental closure by fatique or reflex has to be made impossible. Also when inserting a stomach tube or any kind of sound through the mouth, mouthgags are used for the same reason. In many cases the patient is not conscious and thus cannot even intentionally open his mouth or keep it open by himself.
Here is a survey of some of the most common mouthgags. Jennings and Whitehead are also available with tongue depressor, which fixes the tongue in addition to the jaws.

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