In yesterday's New York Times, the science section included a review of Eric Finzi's Face of Emotion: How Botox Affects Our Mood and Relationships. According to Finzi, injecting botulinum toxin into someone's face can cure her depression by preventing her from frowning. If someone can't frown, she's happier.
Facial expressions don't merely express emotions—they create them. Finzi cites recent research and thinkers like Darwin and William James to describe this as a physical, neurological effect.
Shakespeare's Henry V uses this effect at the siege of Harfleur when he tells his soldiers to "[d]isguise fair nature with hard-favored rage." They should make their faces resemble weapons and violent seascapes:
lend the eye a terrible aspect,
Let it pry through the portage of the head
Like the brass cannon, let the brow o'erwhelm it
As fearfully as doth a gallèd rock
O'erhang and jutty his confounded base,
Swilled with the wild and wasteful ocean.
If he can get them to do this, and to "set the teeth and stretch the nostril wide," then their spirits will rise to their "full height," and they will be ready to charge "[o]nce more unto the breach."