The lips themselves are part of a muscle the orbicularis oris. Like the muscle that circles the eye, the orbicularis oris tightens in on itself when it contracts, narrowing and compressing the lips. Pursed lips appear in suppressed anger, sadness, and simple nervousness. Open, taut lips appear in loud speaking, singing, and angry shouting. The tight-lipped look involves the disappearance of much of the red margin. When both lips tighten with the mouth closed, there is also a bulging-out around the lips, as though we were trying to hold in a mouthful of air.
A. Relaxed mouth.
B. Entire muscle tensed. Lips curl under themselves and tense. Tension straightens LBL slightly; kinks remain.
C. Vertical across-the-pull wrinkles on upper lip. Filtrum disappears.
D. Thicker parts of lip— entire lower lip, middle of upper—still show. With thinner lips, nothing would show.
E. Length of lips doesn't change, only width.
When we tense and press our lips in sadness or anger, the lips narrow, but the mouth does not shorten. You can see why if you press your lips together and at the same time probe your cheek with a finger. On the side of your face, you'll feel muscles deep underneath the surface tightening as you press in on your lips. The corner of the mouth is being fixed in place by some of the muscles attaching to it, and the result is it stays put while the rest of the mouth shrinks.
The lips narrow by curling in on themselves, not so much disappearing into the mouth as condensing, like a towel being wrung out. The result is two skinny, tightly curved edges, with only a trace of red lip still showing. If you perform the lip press and trace the surface from the base of your nose to the LBL with your finger, you'll feel it curve out and then in again: the reverse of its relaxed curve. There's a feeling of fullness to the surface, as though one were holding a mouthful of air or water. This same description holds true for the surface below the lips, sometimes even more so. This is an important part of the look of pressed lips and helps distinguish an active, tensed mouth from one that merely happens to have thin lips.
Shading the mouth's surroundings so that they look properly curved is the key to capturing the action of the orbicularis oris. Note also the relative brightness of the lower lip compared to the upper lip; as in many of our facial expressions, when the lips are tightened, the upper edge of the lower lip becomes more of a shelf, catching the light from above. The light on the lips changes very sharply from the shadow on the upper lip to the bridge edge of the lower lip.
Very often when the orbicularis oris tightens to compress the mouth, it's joined by the action of triangularis and mentalis. The three muscles complement each other's actions perfectly, locking the mouth into a vise of muscular pressure. I've dubbed this combination the three-muscle press, and it's a key part of the look of suppression in the stifled smile, the stifled sob, and suppressed anger.
There are times when the mouth is open and the orbicularis oris contracts. When that occurs, the effect of its contraction is not to close the mouth, or to press the lips together, but rather to thin the lips by curling them inward and tightening them. This action straightens the inner margin or the lips, particularly the upper lip; it's an important part of the look of the mouth in angry shouting.
The orbicularis oris also has a specialized branch (many anatomists treat it as a separate muscle) that pulls the mouth corners toward each other, as in saying the word flirt or fool. This O shaped mouth is often a part of the expression of surprise.
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