Quick Study: some important 
Muscles of the Chin

Muscles of the chin

(Note: there's lots of other muscles in the face. I'm only going to highlight the ones that are most important in shaping facial expressions and are in context with what we're talking about.)

The mentalis is interesting: it gives you that that little ball of tissue right at the tip of your chin. It's origin is in the bones around your lower set of "two front teeth" (the mandiblular incisors). It's insertion, that is the part of the body it applies it's action to, is the skin of the chin. Muscles shorten when they contract, so, when the mentalis contracts, it pulls the skin and tissue of your chin right up almost into your lower lip. This is in the opposite direction of the the other two main muscles of the chin: the triangularis and the lip depressors - which both pull the corners of the mouth and lips down towards the chin. 

The Mentalis muscle of the chin

Try this. Place a finger on your chin just under your lower lip. Now whine like a dog. Yep, go "hmm hmm hmm" the way a sad puppy dog would. Think "frown" at the same time. You'll feel the mentalis pull right up to your lower lip. Feel it? That's the mentalis doing it's job. I think the name comes the expression a person makes while he/she is thinking too hard. Ya know, like Socrates going "Hmm, should I drink the hemlock  and die or should I tell a big old fib and live to be old and miserable?" I just know that's where that name came from. Had to. You can quote me on that. :-)

Now that you're frowning and shaking your head, you can observe the action of the triangularis: the muscle that pulls down the sides of your mouth.  It's origin is in the sides of the jaw (the mandible). It's insertion is in the orbicularis oris - the circular to oval muscle around the mouth. The Buccinator (bux-sin-a-toor) is shown here too. You'll learn a little more about it in the "dimple" section. Here's the triangularis and the orbicularis oris

The Triangularis attaches to the Orbicularis oris

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