utdrag ur G B Duchennes Méchanisme de la Physionomie Humaine Paris 1862 Översättning till The mechanism of human facial expression av R Andrew Cuthbertson 1990
Plate 31: Slightly stronger electrical excitation of both mm. zygomaticus major: development of the same fundamental and secondary expressive lines of joy, with mild contraction of some fibers of the muscle called the sphincter of the eyelids: false laughter.
Plate 32: The same subject as in plates 30 and 31, showing that natural laughter really is constituted by the association of m. zygomaticus major and the inferior part of m. orbicularis oculi. Natural laughter, by the voluntary contraction of the two mm. zygomaticus major and of the inferior part of mm. orbicularis oculi.
Plate 33: The same subject, showing that the wrinkles radiating from the lateral canthus of the eyelids, due to the action of mm. zygomaticus major, disappear when mm. frontalis is stimulated at the same time. Maximal stimulation of these muscles gives a false incomplete expression of agreeable surprise, or admiration.
Plate 34: Showing, in the same subject, that the combination of the muscles of joy and pain, at certain degrees of contraction, will only produce a grimace. Strong electrical contraction of m. zygomaticus major and of m. corrugator supercilii: grimace.
Plate 35: To study the expressive lines produced by the strong and isolated contraction of m. zygomaticus major in a young woman. (This woman was depressed at the time of the experiment, which we can see from the slight lowering of the angle of the mouth on the left side). On the right, the fairly strong electrical excitation of m. zygomaticus major and mild excitation of the palpebral part of m. orbicularis oculi.: fundamental lines belonging to the muscle of joy: false laughter.
The m. zygomaticus major is the only muscle that expresses joy to all its degrees and in all its nuances, from the simple smile to the raucous laugh. It never portrays any other expression.
The best name we could find from its expressive action is the muscle of joy, though this is not completely justified when it contracts in isolation.