Guillaume Duchenne, another nineteenth-century scientist, studied the physiology of facial expressions. The Duchenne smile combines the use of the zygomatic major and the orbicularis oculi muscles to produce a genuine smile. The zygomatic major raises the muscles at the corner of the mouth, and because they are voluntary, they can be used to convey a fake or forced smile. The orbicularis oculi, however, are involuntary. They raise the cheeks and produce crows feet around the eyes during an authentic Duchenne smile.
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