The risorius/platysma is a muscle of extreme circumstances. The act of stretching the lower lip—creating the "square mouth"—is not one we do casually or conversationally, like lowering the brows. When the lower lip
The risorius (A) inserts into the muscular knot of the corner of the mouth at the same location as zygomatic major and triangularis. It originates on a connective layer over the upright leg of the jaw. When it contracts, it pulls the mouth corner straight backward.
The platysma (B) is a thin shroud that hangs down from the lower face and covers the front of the neck. It may extend down past The collarbones onto the chest. The fibers of its side portion (C) run parallel and beneath those of the risorius; it too attaches to the mouth corner. Besides pulling the mouth corner straight back, it tenses the skin of the neck, raising thin, vertical wrinkles.
stretches, the neck tenses. This connection has prompted anatomists to consider the link between the risorius and the neck-tenser, platysma. Their respective roles are still uncertain, but one action definitely calls forth the other.
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