LBL has nearly doubled in length, extending well past the end of the red portion, but the lips are only slightly changed, stretching a bit at the end. LBL has pivoted down from (A). Row of curved bulges, like waves approaching a shoreline, appear below mouth corner (B). These are the signature wrinkles of the triangularis. The lowermost portion of the nasolabial fold deepens. The skin outside the fold is bulged out and pulled down, looking like a broad, rounded hook on the rim of the face (C).
Is this mouth active or relaxed? Creases are very similar to (B), but corner ends at (D) and is not pulled down. Key is difference between crease and gap (LBL).
Downward Puller II: Mentalis
The mentalis is the other half of the pout. It has a much larger role to play in the face than triangularis. It's another one of those muscles, like corrugator or orbicularis oris, that we use in our everyday expressive gestures. It's part of a great number of expressions, particularly those where there is some effort at restraint involved. This includes partly restrained anger and sadness and the stifled smile.
There's certainly no muscle on the face you can demonstrate to yourself easier: "shrug" the chin, pushing up the center of your lips, and you've just performed the basic action of mentalis. There's also no muscle on the face with a more distinctive signature wrinkle— look at what the shrug does to the chin. A little raised island with puckered skin is the unmistakable mark that the mentalis is active—nothing any other muscle does is at all similar. Incidentally, the facial shrug you've just performed is one of the most frequent ways we use the mentalis, a sort of lower-face version of shrugging the shoulders.
Though I've placed the mentalis in the "downward puller" category, it is a peculiar sort of downward puller. It creates the downcurved mouth indirectly, by pushing the center of the lips up. The lips are pushed up because the skin over the chin, which is where the mentalis inserts, is on its way toward the base of the teeth, which is where the muscle attaches. When the mentalis contracts, the ball of the chin tries to move up and under the base of the lower lip.
If the push is strong enough, then the lower lip thrusts further out than the upper in a pout. The feeling of the lip being the top of a platform is thus greatly intensified, with a sharper shadow underneath and more sense of overhang. Along with the changes in the chin, this is a particular detail that helps signal that the mentalis has acted. In fact, it can be more important, as it is possible to inadvertently suggest a pout when you use shadows under the lip that are too sharply defined.
The teamwork of mentalis and triangularis is what we see most often on the face. The action of the two muscles is perfectly complementary. Mentalis is the specialist in pushing the lips up in the middle, and triangularis in pulling the lips down at their ends. The muscles' joint action combines these two motions to push the mouth further up in the middle and further down at the ends, while preserving the signature wrinkles of the individual muscles— namely, the raised ball on the chin and curved wrinkling underneath the mouth corner.
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