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RESEARCH.

RISORIUS/PLATYSMA.
 

                        The Risorius is the muscle that appears during extreme distress.  The risorius attaches to the muscular knot at the mouth corner, along with the Triangularis and Zygomatic Major.  The risorius pulls the mouth corner towards it's 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.  A broad thin sheath rising up from the collarbones and upper chest and attaching all along the lower part of the face.  The platysma covers the front half of the neck and shoulders then stops abruptly before reaching the back.
                        When the risorius contracts, the lower lip is the most affected.  The signature wrinkle of the risorius/platysma is a bracket like fold that appears under the mouth, starting at either mouth corner, it is the deepest at this point.  ( see fig 21)  In an open mouth expression, the teeth will be heavily exposed.  The risorius is the muscle responsible for exposing the lower row of teeth.  The final result of the risorius/platysma is to produce the "square mouth" shape that is crucial to the act of crying, fear and anger.

Fig . 21 - The Risorius/Platysma in action.