As a minute or two in front of a mirror will demonstrate, the mouth is capable of being stretched, squeezed, or curled into just about any shape. Obviously, it requires quite a muscular network to allow us to do all those funny things with the mouth.
There are twelve muscles in the network, but we can eliminate five right away as having nothing to do with facial expression (several are used, for example, mainly for eating).
Of the seven remaining muscles, one is already familiar—zygomatic major, the smiling muscle. There is also levator labii superioris, the sneering muscle; triangularis and mentalis, which work as a team to bend the mouth downward; risorius/platysma, which elongates the lips; depressor labii inferior is, which pulls the lower lip downward; and orbicularis oris, the muscle of the lips, which presses both lips together.
The mouth is freely suspended. It's only attachment to the skull is indirect—it's surrounded by muscles that attach to the skull at one end and the mouth at the other. Imagine a slit in the middle of a trampoline: like the mouth, it's not attached to anything solid, but it's part of a stretchable fabric attached to a rigid frame. The slit would have enough free play to take on an enormous variety of shapes as the trampoline stretched and sagged.
As a result of its suspension, the mouth can be pulled and pushed as if it were part of a rubber mask. In fact, actors sometimes use the lower part of the face as a mask, holding through muscle tension a facial shape completely different from their own.
There is another factor that affects our discussion of the mouth that was not an issue with the eye. Though the same muscle may be active when the mouth is both open or closed, the actual appearance of the mouth is quite different, as in a smile with the lips parted and one with the lips together. For this reason the mouth will be discussed in two separate sections.
Drawing the mouth is always a delicate matter. It sometimes seems as though the least little change—a little shading here, a break in the angle there—will transform our reading of the mouth from one mood to another. For example, it takes very little to suggest a smile; the slightest angling up of the corners will do. Unfortunately, it can also take very little to suggest a sneer or a pout!
It is generally more difficult for the artist to control the expression of the mouth than the eye. One problem is that certain relaxed mouths resemble those engaged in an expression.
For example, we may see people who, on first glance, seem to be permanently sneering or pouting— perhaps some of them are. In most cases, however, a careful look will reveal certain very subtle surface forms indicating that, in fact, no expression is involved. And if no expression is involved, we must avoid emphasizing the wrong wrinkles—those signature wrinkles that suggest that such an expression is in progress.
Lighting can also create problems. Shadows on and below the lower lip are important in defining if the lip is tense or thrusting out.
Finally, there is the perspective factor. When the head is tilted up or down, the curve of the mouth will be altered accordingly. Be careful not to curve the mouth in the direction you're used to seeing it in, rather than the one it's actually taking.
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