Actions of risorius/platysma. When the risorius/platysma contract, the mouth corners are drawn sideways. The action has much more effect on the lower lip than the upper. The pull is primarily at lower lip level. The lower lip is stretched into a taut, even band, with straight-line upper and lower edges. The lip becomes slightly more recessed and as a result is surrounded with a soft fold, stretched between mouth corners—the signature wrinkle. The entire front row of lower teeth are bared. In the most visible contribution of platysma, raised bands appear on the front and sides of the neck (G).
A. Rectangular inset below lower lip, where skin is flattened against chin.
B. Soft, rope-like bulges appear here, extending down to ¡aw.
C. Middle portion of upper lip mostly unchanged; outer legs hinge outward and stretch slightly down to mouth corner.
D. Wide mouth. Brackets around mouth form when action is strong, deepest at corners. Upper teeth hidden.
E. Inner outline of relaxed mouth.
F. Inner outline of tensed mouth. This is the "square mouth" shape; note corner angle.
Lower-Lip Curl: Depressor Labii Inferioris
Finally, we arrive at a muscle that has no reputation, few enemies, and not coincidentally a very minor role to play in facial expression. The impressively named depressor labii inferioris (depressor of the lower lip) is the muscle that curls the lower lip downward, and if that doesn't strike you as a very dramatic movement, you're correct. I've included it because it is a key muscle in the act of speaking and does appear, in combination with other, more expressive muscles, in a few of the expressions I've illustrated.
There are two strands of this muscle, one on the left, one on the right, and each runs from the lower lip down to the chin. The two strands contract together and curl the entire lower lip straight down, exposing more red area and showing the entire lower row of teeth.
The lower lip forms a thick square U with a well-defined edge running along most of its lower margin. The ris-orius/platysma, which also exposes the lower teeth, stretches the lip and widens the mouth, two things that do not happen with the lower lip curl. The way the teeth are uncovered is different as well: the depressor labii inferioris bares the teeth down to the gums, further than with the risorius.
The chin may wrinkle slightly.
In this chapter I've laid out the expressive vocabulary we have available to us—the raw materials from which facial expressions are made. At times, just a simple change from the neutral—an open mouth, a down-turned eye—can enliven the face enough for some purposes. To understand how the face communicates more complex physical and emotional states—pain, fatigue, sorrow—we will next explore how the various elements combine. In some cases, muscles that have been described in solitary action will be seen teaming up with other muscles, creating facial changes that retain elements of both.
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