The ability to communicate through facial expressions had long intrigued painters, doctors, and philosophers. Thanks to Charles le Brun’s celebrated 1668 conference for the Royal Academy of Painting and Sculpture, the “expression of passions” became a doctrine. The 23 “expressive heads” – originally drawn (1698), later engraved (1727) – used to illustrate the conference became enormously successful. This “grammar” for expressing passions, or emotions, identified the face as the essential feature in the physical expression of passions.
Duchenne continued in that tradition, turning it into a veritable photographic-physiological school of visual perception. Thanks to photography, he was able to determine the muscular foundations of classic affects. Along the same lines as the experiments that Galvani performed with frogs’ legs, they were caused purely by electrical stimulation of the muscles. Charles Darwin later extended Duchenne’s reflections in his book The Expression of the Emotions in Man and Animals.
Unlike Lavater’s physiognomy, which was very popular at the time, Duchenne wasn’t the least bit interested in his subject’ profound nature. On the contrary: his experiments demonstrated that, with the help of electricity, it is possible to bring about the expression of various emotions on a person’s face, even if the person is not actually experiencing them.
For Duchenne, the face is a surface whose folds, lines and wrinkles can be deciphered like so many signs. Not only that, but he believed that expressions of emotions were a natural, universal language that preceded education. Rooted in the realm of nature, it took precedent over culture. So photography allowed him to establish the “face’s expressive lines,” in such a way that – as he so elegantly put it – an “orthography of physiognomy in movement,” could be composed (p. 62)
). “What reason alone had led us to sense, comes across clearly in my research. I have indeed noticed, in all of my experiments, that it is always a single muscle that performs the fundamental movement, representing a movement affecting the soul. This rule is so rigorous, that Man has been deprived of the power of changing it, or even modifying it.” Here are some examples:

Muscles of Joy and Loving Kindness

The zygomatic major muscle and the lower palpebral orbicularis (eyelid contractor) (pp. 55 to 64)

Duchenne de Boulogne’s best-known contribution to research in this field is having discerned the difference between the "fake smile" (figures 30 and 31)) and the "natural smile" (figure 32). The latter is known to this day as the “Duchenne smile.” Sincerity, it turns out, is found near the eyes.

“The expressive difference between these face depends exclusively on a particular movement of the lower eyelid. […] The muscle that produces this texture in the lower eyelid does not obey one’s will; it can only be put into play by true affection, by an emotion that is pleasing to the soul. Its inertia, during a smile, unmasks false friends. […] Not only does it bring cheer to the eye – and in that sense, it is the counterpart to the zygomatic major, for the smile – but it also, in certain circumstances, under the influence of affectionate sentiments, contracts partially. In that case, it brings loving kindness to the gaze” (p. 62-63).

Le Brun describes Laughter in these terms: “(…) produced by joy mixed with surprise, makes the eyebrows rise towards the middle of the eye, and bend towards the sides of the nose; the eyes are almost shut and sometimes appear wet, or shed tears, which make no alteration in the face; the mouth half open, shows the teeth; the corners of the mouth drawn back, cause a wrinkle in the cheeks, which appear so swelled as to hide the eyes in some measure; the nostrils are open, and all the face is of a red colour.” (1727, p. 2-3) (Encyclopaedia Britannica, or a Dictionary of the Arts, Sciences and Literature… 3rd Edition, Volume 14 (out of 18), Edinburgh, MDCCXCVII (1797) p. 15 (via Google books).

Darwin, who was a contemporary of Duchenne’s, studied his book attentively: “Praise his book. Known for excellent treatises, and very under-estimated, it seems to me, compared to other authors. A major step forward,” can even be seen in the margins of his personal copy. Because by applying evolutionary theory to expressing emotions, Darwin is also searching – in animals as much as in human beings – the involuntary nature of expressions.
The causes and effects of joy and laughter are described in great deal, beyond a single face. “Joy, when intense, leads to various purposeless movements – to dancing about, clapping the hands, stamping, &c.; and to loud laughter. Laughter seems primarily to be the expression of mere joy or happiness.” Focusing on the tickling sensation, whether physical or intellectual (caused by risible ideas), he reins in Duchenne’s findings: “Dr. Duchenne attributes the falseness of the expression altogether to the orbicular muscles of the lower eyelids not being sufficiently contracted; for he justly lays great stress on their contraction in the expression of joy. No doubt there is much truth in this view, but not, as it appears to me, the whole truth. The contraction of the lower orbiculars is always accompanied, as we have seen, by the drawing up of the upper lip.” (The Expression of the Emotions in Man and Animals, 1896, D. Appleton, NY p. 198 and beyond, openlibrary.org).

Muscle of Agression

The procerus muscle or pyramidalis nasi (p. 27 à 35)

“However feebly the pyramidalis nasi exerts its action on the tip of the eyebrows, and the glabella between them, it brings hardness to even the gentlest gaze, and announces aggression. […] Figure 18, in which both of a subject’s pyramidalis have been simultaneously galvanized into contracting, displays an expression of cruelty and hatred that inspires repulsion. One fears this gaze greatly; only a fierce and cruel nature could bestow such an expression. […] Is it possible to see a fouler gaze? It declares a fierce instinct; it is the eye of the tiger.” (p. 33-34

Le Brun doesn’t address aggressiveness as such, but rather the emotions that are likely to precede it. “Hatred or Jealousy wrinkles the forehead; the eyebrows are sunk down and knit; the eye-ball is half-hid under the eye-brows, which turn towards the object; it should appear full of fire, as well as the white of the eye and the eye-lid; the nostrils are pale, open, more marked than ordinary, and drawn backwards so as to make wrinkles in the cheeks; the mouth is so shut as to show the teeth are closed; the corners of the mouth are drawn back and very much sunk; the muscles of the jaw appear sunk; the colour of the face is partly inflamed and partly yellowish; the lips pale or livid.”
“The effects of anger,” Le Brun points out, “show its nature. The eyes become red and inflamed; the eye-ball is flaring and sparkling; the eye-brows are sometimes elevated and sometimes sunk down equally; the forehead is very much wrinkled, with wrinkles between the eyes; the nostrils are open and enlarged; the lips pressing against one another, the under one rising over the upper one leaves the corners of the mouth a little open, making a cruel and disdainful grin.” (Encyclopaedia Britannica, p. 16)
Darwin, on the other hand, goes beyond movements of the nostrils and eyebrows to emphasize the baring of teeth. “The expression of a half-playful sneer graduates into one of great ferocity when, together with a heavily frowning brow and fierce eye, the canine tooth is exposed. […] The uncovering of the canine tooth is the result of a double movement. The angle or corner of the mouth is drawn a little backwards, and at the same time a muscle which runs parallel to and near the nose draws up the outer part of the upper lip, and exposes the canine on this side of the face. The contraction of this muscle makes a distinct furrow on the cheek, and produces strong wrinkles under the eye, especially at its inner corner. The action is the same as that of a snarling dog; […] I suspect that we see a trace of this same expression in what is called a derisive or sardonic smile.” (Darwin, pp. 250-251 (p. 271))

Muscle of fear, fright and terror

Platysma (neck muscle) (p. 101 to 108)
>figure 62 and figure 63

“We have seen that figures 56 and 57 express surprise, astonishment and amazement with tremendous realism, through a combined contraction of the depressor of the lower jaw and the occipitofrontalis muscle; but this muscular combination, which produces movements, lines and furrows that seem, at first glance, to be analogous to those painted by fear and terror, does not, however, reproduce those expressions. All that is required for it to appear is to pair the platysma with one of the motor muscles of the eyebrows.” (p. 105)
“In the presence of figures 61, 62 and 63, one can have no doubt: this man is scared near to death, in a frightened stupor; his features express fear and horror from the sight or the news of a terrible, life-threatening danger or an inescapable ordeal. ‒ Before the use of chloroform, the earliest surgical operations commonly brought out comparable expressions of fear and horror. […] This expression of fear can only be rendered perfectly by pairing the platysma and the occipitofrontalis with the depressors of the lower jaw. […] The enlargement of the palpebral opening and the haggard look that can be observed on these faces undoubtedly adds to the effect of their expressions.” (p. 107)

>figure 64 “Figures 61, 62 and 63, for example him, show him looking terrified by the idea of either danger, impending death, or a sentence of torture that is about to be carried out; but in figures 64 and 65, in addition to that terrible emotion of the soul, the expression of the horrible pain of his torture has also been added. ‒ This expression must be that of the damned.” (p. 108)

For Le Brun, the neck doesn’t have much to do with horror: “An object despised sometimes causes horror, and then the eye-brow knits, and sinks a great deal more. The eye-ball, placed at the bottom of the eye, is half covered by the lower eye-lid; the mouth is half open, but closer in the middle than the sides; which being drawn back, makes wrinkles in the cheeks; the face grows pale, and the eyes become livid; the muscles and the veins are marked.” (EB p. 15)
His description of fear, however, takes its tension into account: The violence of terror or fright alters all the parts of the face; the eye-brow rises in the middle; its muscles are marked, swelled, pressed one against the other, and sunk towards the nose, which draws up as well as the nostrils; the eyes are very open; the upper eye-lid is hid under the eye-brow; the white of the eye is encompassed with red; the eye-ball fixes toward the lower part of the eye; the lower part of the eyelid swells and becomes livid; the muscles of the nose and cheeks swell, and these last terminate in a point toward the sides of the nostrils; the mouth is very open, and its corners very apparent; the muscles and veins of the neck stretched; the hair stands on end; the colour of the face, that is the end of the nose, the lips, the ear, and round the eyes is pale and livid, and all ought to be strongly marked.” (EB 15-16)

Darwin, also connects the expression of horror to the one of astonishment. He even acknowledges having shown Duchenne’s images “to twenty-four persons (…), without any explanation being given,” as well as having queried numerous and far-flung correspondents in order to arrive at the same conclusion: “Fear is often preceded by astonishment; and is so far akin to it, that both lead to the sense of sight and hearing being instantly aroused.” (p. 315 / pp. 279/290 for the English).
But even though he uses Duchenne’s photographs to express terror, he admits to having doubts about the platysma’s decisive role: “The original was shown to fifteen persons; twelve answered terror or horror, and three agony or great suffering. From these cases and from an examination of the other photographs given by Dr. Duchenne, together with his remarks thereon, I think there can be little doubt that the contraction of the platysma does add greatly to the expression of fear. Nevertheless, this muscle ought hardly to be called that of fright, for its contraction is certainly not a necessary concomitant of this state of mind. A man may exhibit extreme terror in the plainest manner by death-like pallor, by drops of perspiration on his skin, and by utter prostration, with all the muscles of his body, including the platysma, completely relaxed.” (p. 300)