The open-mouthed smile expressed as a pattern of light and dark, the light seen as falling from above. The deepest shadows are planes that undercut; deep shadows also notch at lip corner and under top edge of upper lip. (A) shows the undercut of cheek, where the nasolabial fold is deepest. (B) shows notch and ropelike mound alongside: (C) is a cartoon version of the same thing.
This drawing isolates the frame that frames the laugh. The sense is of two ropes, starting where the nasolabial fold ends and "melting" into the chin. The dimple marks the outs-ide of the frame. The "rope" is more distinct in the laugh. An extra line forms (C) as the chin folds against neck.
A laugh is a smile with space between the teeth. The one thing about the mouth that is different is the outer legs of the lower lip—these angle upward much more steeply (A), as does the nasolabial fold (B). The sharp turn in the upper lip is clear in this laugh. Note that the full row of teeth and tips of lower teeth are exposed; a distressed mouth will show a full row of lower teeth.
The Stretched Lower Lip: Risorius (and Platysma)_
You may see people on the street corrugating their corrugators, flexing their frontalis, or even trying out a little one-sided sneering; but don't look for any demonstrations of the risorius; that is, unless you're living in a combat zone. The risorius, the muscle of the stretched lower lip, is not only a muscle of the distressed face, it's the only one that appears only in extreme distress, and not otherwise. We don't see it in the face of someone who's merely sad, for example, but if they get even sadder and begin to cry, that's when the risorius will appear. Just being afraid or angry isn't enough, but we have to be scared out of our wits, or angry enough to be dangerous. It's sort of the ambulance chaser of the facial musculature, waiting for something cataclysmic to occur before it stirs into action.
At one time, risorius was thought to be a muscle of pleasurable expressions. Even now there is no firm agreement among anatomists on the role of the risorius; some say that other fibers, parallel and slightly deeper, do most of the job of stretching the lower lip, and the risorius is a minor player at best. Others say that the risorius is the main agent, and the deeper muscle, the platysma, the neck tenser, is the sidekick. Since this is a matter for the anatomists to sort out, we'll compromise, and say that the risorius/platysma does the job.
The risorius is quite simple in its construction. It attaches to the muscular knot at the mouth corner, along with the triangularis and zygomatic major. Each of these muscles acts on the corner in a different way: triangularis pulls it down and out, zygomatic major up and back. The risorius pulls the mouth corner toward its own origin, a layer of connective fiber overlaying the back of the jaw.
The platysma, however, is as much a muscle of the neck as of the face. Think of it as a sort of an unrolled turtleneck, a broad, thin sheath rising up from the collarbones and upper chest and attaching all along the lower part of the face. Actually, it's only the front half of the turtleneck. Platysma covers the front half of the neck and shoulders, then stops abruptly before reaching the back. The part we're interested in comes up from the side of the neck and runs underneath the risorius to attach to the mouth corner.
You can see how closely lower-lip stretching and neck tensing are linked
Roman mask of tragedy. Since ancient times the square mouth shape created by risorius/platysma has appeared in art as the personification of horror.
by trying it on yourself. As soon as you begin to stretch your lower lip sideways, you can feel a corresponding tension in the neck; if the action is strong enough, you'll see thin vertical pillars rise up from the surface of the neck folded by the action of platysma.
When the lip stretcher contracts, the lower lip is the one most strongly affected. In the smile, the upper lip is stretched straight and the lower lip bends; with the lip stretcher, the lower lip is stretched straight and taut, and the upper lip bends. The sideways pull tends to shove the lower lip back into the face, pressing it up against the teeth and flattening its subtle curves into a single ribbon. The entire lower lip becomes uniform—a straight line above, a nearly parallel straight margin below.
The signature wrinkle of the risorius/platysma is a bracketlike fold that appears under the mouth, starting at either mouth corner. In shape it's like an extremely elongated, square U, depressed into the face. It's deepest where it begins at the mouth corner, creating a crease that angles downward. It appears when the action is strong.
A strong action of the lip stretcher, as occurs in crying, will also bunch up the face to either side of the mouth. Two wide, ropelike mounds, similar to those that appear in laughing (but less well defined), frame the mouth and seem to hook underneath the chin.
In all the open mouth expressions, the way the teeth show tells you a lot about the nature of the face. The risorius exposes the lower row of teeth, not the upper, and these are seen all the way across the mouth, not just in the center. When all the lower teeth are showing, it's a bad sign. The upper teeth, however, remain hidden even if the mouth is open wide. In the expression of anger, where the risorius is active, the upper teeth are seen because the upper lip has been lifted by the sneering muscle.
The net result of the actions of the risorius/platysma is to produce the "square mouth" shape that is a crucial component of the faces of crying, fear, and anger. Anatomists have speculated that the lip stretching action is related to the preparation of the mouth for screaming, and that our ancestors long ago found the shape an efficient one to produce a piercing, attention-getting sound.
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