Wiktionary
risorius
n. A facial muscle that draws the angle of the mouth outward and is used when smiling.
Wikipedia
Risorius
The risorius is a muscle of facial expression which arises in the fascia over the parotid gland and, passing horizontally forward, superficial to the platysma, inserts onto the skin at the angle of the mouth. It is a narrow bundle of fibers, broadest at its origin, but varies much in its size and form.
Usage examples of "risorius".
After that, you can tell a fake smile because the risorius and platysma muscles pull the lower lip down and out, squaring it and exposing the lower teeth.