Considering how much can be expressed by the eyes and brow, it is surprising that there are only five muscles responsible for it all: two under the forehead and brow, and three surrounding the eye. Our control over these muscles is so fine-tuned that we can express virtually the whole range of emotions with just a little twist here, a little lift there: our perception is so practiced that we can instantly recognize the differences.
The Eyebrow Raiser: Frontalis
The frontalis lifts the eyebrows straight up the forehead, creating the familiar worry lines on the forehead. It's often thought of as the muscle of surprise (as in the expression, "that'll raise some eyebrows"), but it also contributes to fear and sadness.
It is a broad, flat muscle that lies across the entire width of the forehead, like a sweatband. Its fibers run vertically, dropping from the hairline, where they are fixed in place, to its free end, the skin underneath the eyebrows. You can trace its line of attachment by running your hand along the upper rim of your eye socket, starting at the outside and continuing across the glabella, the root of the nose. The frontalis attaches along this entire edge.
When the frontalis contracts, all the skin just above the eyes and nose is pulled straight up toward the hairline, with the eyebrows coming along for the ride. As the forehead rises, the skin caught in the way gathers into horizontal creases: the worry lines mentioned above.
You may have no crease lines, or you may have half a dozen or more, fairly evenly spaced, but the creases will never be exactly straight. Typically, they will have a long dip in the middle, and as they approach the side of the forehead, they will suddenly change direction, curve sharply downward, and die out. They are arched very similarly to the way the eyebrow is arched.
When the lift of the brow is only partial, fewer wrinkles, and fewer complete ones, will appear.
As the eyebrows are pulled upward, the skin below them, rather then being wrinkled, is stretched taut across the underlying bone. Bags and wrinkles are smoothed out, as though ironed flat. The upper edges of the orbit, the glabella, and nasal bones all show clearly through the tight skin. That portion of the skin that usually bags over the upper lid, partially or completely hiding it, is lifted free, and the entire rounded form of the lid shows clearly. The eye itself, however, does not necessarily open any wider. Its opening is a separate function.
The creases, the tightened skin, and the exposed lid, taken together, are important in showing that the frontalis has acted. The eyebrows move upward as well, but sometimes eyebrows simply grow high. The other effects, however, only occur when the frontalis contracts.
Outer half/Inner half
Portions of the frontalis muscle can act on their own. Many people are able to use just the outer half to raise up the outermost portion of*one eyebrow. Wrinkles form above this portion, resulting in an odd, rather quizzical expression. When just the inner half contracts, only the innermost ends of the eyebrows are raised, and wrinkles form across just the middle of the forehead. Most people can only move the inner portion in combination with the corrugator group in expressions of grief or anxiety.
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