Why do you open your mouth when applying mascara?

That face you make when inserting contacts, putting on eyeliner, or applying mascara is a bizarre human quirk right up there with sneezing in sunlight and contagious yawning. Why do you feel a compulsion to open your mouth when it comes to messing with your eyes? There’s no definitive answer, but the leading theory is pretty compelling.

To understand so-called “mascara mouth,” you need to understand a few things about the anatomy of your head. The muscle mainly responsible for opening and closing your mouth is called the lateral pterygoid, and that’s controlled by the central sensation highway known as the trigeminal nerve. The muscle that lifts and lowers your upper eyelids, on the other hand, has the Harry Potter-esque name of levator superioris, and that’s controlled by the oculomotor nerve.

The trigeminal nerve and the oculomotor nerve originate really close together in the brainstem. The leading theory behind mascara mouth is that, to put it simply, those wires can get crossed. When you activate the oculomotor nerve to keep your eyes open to avoid making a mess of mascara or contacts, you end up activating the trigeminal nerve and opening your jaw. Bam, mascara mouth.

There are other theories as well: some say that opening your mouth simply stretches your facial skin to make a sturdier surface for makeup. That could be true, as well. But there’s one scientific phenomenon that lends some credence to the nerve theory: in some children, a condition called Marcus Gunn Jaw-winking syndrome makes them wink one or both eyelids every time they move their jaw. Mascara mouth is no syndrome, but it could be a mild version of this neurological fluke.

So the next time, try to keep your mouth closed when putting in contacts etc. You will find it more difficult (subconsciously) than it really looks. 😮

This post is dedicated to LX.

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