In the basic action of the mentalis, the skin on the chin, pulled toward the base of the teeth, is flattened. The lips lift and project out like a shelf (A). The strongest lift is in the center, but the entire lip is thinned (B). LBL is straightened; a sharp, bright edge appears on lower lip (C). Corners appear pulled down because the center is lifted. A sharp shadow appears under lips—the hollow is more drastic than on a relaxed mouth (D). Raised, shield-shaped "island/' the signature wrinkle, always appears on the chin, with pockmarked texture (E).
Triangularis and Mentalis contracting together result in the facial shrug—the facial equivalent to a shoulder shrug. In its more subtle versions it's called the pout. The flattened lips and puckered-up chin of mentalis are added to the down corners and hooklike furrows of triangularis. Lower lip dominates upper and sticks out further and appears larger.
No muscle on the face gets more of a workout than the muscle of the lips, orbicularis oris. The other muscles we've talked about pull on the mouth at its edges; orbicularis oris is what's inside the lips themselves. When we talk, it finetunes the lip edges to form the sounds of speech. It's made up of several specialized layers that can shape the mouth just so. It's also one of the principal muscles we use when we eat. But our chief reason for including it here is the way it acts to compress the lips in expressions such as anger and grief.
Most of the actions of the orbicularis oris act to tighten the lips in some fashion. In this, it's similar to the circu lar muscle of the eye, orbicularis oculi, which creates a squint when contracted. In the case of the mouth, the equivalent of the squint is the tight-lipped look, where the lips themselves are thinned to nearly a single line.
The orbicularis oris runs, just under the skin, in a great oval around the mouth. It's precisely underneath where a juice glass rests when we drink. The muscle stretches from the base of the nose, to the corners of the mouth, to the little crease on the chin under the lower lip. With it's own thickness added to that of the skull beneath, it can at times be sensed through the skin, in form something like the bell of a trumpet, with its small end forward, centered on the mouth.
The attachments of orbicularis oris to the head are unlike those of any other muscle. It's not so much attached as suspended. You can find its attachment on your own face if you pinch the skin of your cheek just beyond the mouth corner, with one finger inside and one outside. The thickened spot you feel is a sort of muscular knot, where a half dozen or so muscles converge on the outer end of the orbicularis oris. By braiding together with the mouth muscle, they provide a flexible anchorage for it to pull against. The knot also helps define the appearance of the corner. In the closed mouth we see the knot as a crescent-shaped mound just beyond the end of the line between the lips. The mound adds a very subtle extra curve on the far outline in three-quarter views.
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