The frontalis is the muscle underneath the forehead. Its basic action is simple: it lifts the eyebrows straight upward, creating rows of wave-shaped wrinkles. The expressions of fear, surprise, and sadness all involve some degree of this brow lifting. But the fact that so many older people have these wrinkles has a more mundane explanation: raising the eyebrows is the most common of all the conversational expressions. We raise our brows frequently and unconsciously to accompany our talk, with the same sort of meaning as a hand gesture.
The wrinkles of frontalis are arched like the eyebrows, with a dip in the middle, and a descending leg on the side. The higher the eyebrows are raised, the deeper the wrinkles. Some of the wrinkles may be broken, or only part way across, particularly if the eyebrows are only part-way raised. The topmost wrinkle is the most shallow. The eyebrow does not glide evenly up the forehead. The outer end is "pinned" to the face and does not move with the rest. So as it rises, the eyebrow changes in shape from a shallow arc to a deeper one. As the brow rises, it also stretches the skin around the eye. Most of the upper eyelid is exposed. The skin of the upper orbit and glabella is smoothed, and the bony forms show more clearly underneath (A). The eye may be opened more, as at (B).
Raising just one eyebrow is a skill some people have. A slightly quizzical look may result. This action's made possible by the fact that the different sections of the frontalis can be controlled separately. In this case (C), the right half is being contracted; the left half is relaxed. The arching "signature wrinkles" fade out as soon as they reach the midline (D). Note the difference between the two upper lids and the two upper eye sockets.
The Frowning Muscle:
The corrugator is the most human of the expression muscles. Some might argue in favor of the zygomatic major, the smiling muscle, but it's too easy to smile with nothing behind it. No one tries to cry, to be afraid, or to scowl with concentration—all expressions in which the corrugator is the main protagonist. They're responses to the ups and downs of life. The smile can be a mask; when we use the corrugator, it's often because the world has broken through.
The corrugator has thus acquired a very bad reputation. When novelists talk about someone's "troubled brow," "knit brow," or "darkened brow," the corrugator is responsible. Almost all the actions of the corrugator are negative in effect. In concert with other muscles, the corrugator is what makes us look sad, troubled, or angry. By itself, it simply makes us look perplexed.
The corrugator's basic action is to pull the eyebrows down and bring them closer together. Actually, two muscles, the corrugator and the procerus (or pyr-amidalis) do this; since they always contract simultaneously, they can be considered functionally as one muscle (the corrugator group).
The corrugator group has fibers that radiate upward from the bridge of the nose and go into the skin of the lower, middle brow. The corrugator itself has its two fixed ends anchored to the skull at the outer corners of the glabella. From this point the two muscle bodies, strips about as wide across as your little finger, run diagonally outward and upward to a point just above the middle of the eyebrow. Here, the free ends are knotted into the skin.
The procerus is fan-shaped. The base of the fan is fixed onto the nasal bone where the bone meets the cartilage. The upper, movable end is attached to the skin in the area between the two eyebrows.
Look in a mirror, frown, and you can trace the various effects of the contraction of the corrugator group on the brow. The overall sense of a frown from the front view is that the two eyebrows are unsuccessfully attempting to join up. The inner ends of the eyebrows have moved together and downward; the skin in between is bunched and wrinkled ("corrugated"). Usually, the entire eyebrow appears tipped downwards; this is the classic effect cartoonists seize on and exaggerate when drawing an angry face.
If you look at the picture of corrugator from the side, you'll note that the movement of the brow is more than just down and together; it's also forward. The bunching of the skin above the eye creates a sort of shelf, projecting outward.
The movement of the eyebrows also affects the area between the eyebrows, where two—three, at the most—very deep, vertical wrinkles tend to form. These wrinkles, inside of each inner-brow lump, are quite straight for most of their length, but they often curve under the brow lump before fading out. These wrinkles are permanently etched into the face of mo^t older people, particularly men.
The downward pressure and forward pressure of the brow drags the skin below along with it. A new, sharply defined, skin fold appears above the eye, pulled across the upper lid. The lid itself is pressed down further on the eye, partly closing it. Especially in the innermost corner of the eye socket, the eye appears more shadowed.
The minimum elements necessary to recognizably depict a frowning brow are these: inward-slanting eyebrows, vertical in-between creases, and extra-low level of the brow. The other, more subtle changes enhance the effect.
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