Emotion always causes numerous and intense bodily effects. Furious anger may cause frowning brows, grinding teeth, contracted jaws, clenched fists, panting breath, growling cries, bright redness of the face or sudden paleness. None of these effects is voluntary; we may not even be conscious of them.
Fright may produce a wild beating of the heart, a death-like pallor, a gasping motion of the lips, an uncovering or protruding of the eye-balls, a sudden rigidity of the body as if "rooted" to the spot.
Grief may cause profuse secretion of tears, swollen, reddened face, red eyes and other familiar symptoms.
Shame may cause that sudden dilation of the capillary blood-vessels of the face known as "blushing."
[Bodily Effects of Perception]
The sight of others laughing or yawning makes us laugh or yawn. The sound of one man coughing will become epidemic in an audience. The thought of a sizzling porter-house steak with mushrooms, baked potatoes and rich gravy makes the mouth of a hungry man "water."
Suppose I show you a lemon cut in half and tell you with a wry face and puckered mouth that I am going to suck the juice of this exceedingly sour lemon. As you merely read these lines you may observe that the glands in your mouth have begun to secrete saliva. There is a story of a man who wagered with a friend that he could stop a band that was playing in front of his office. He got three lemons and gave half of a lemon to each of a number of street urchins. He then had these boys walk round and round the band, sucking the lemons and making puckered faces at the musicians. That soon ended the music.
A distinguished German scientist, named Pavlov, has recently demonstrated in a series of experiments with dogs that the sight of the plate that ordinarily bears their food, or the sight of the chair upon which the plate ordinarily stands, or even the sight of the person who commonly brings the plate, may cause the saliva to flow from their salivary glands just as effectively as the food itself would do if placed in their mouths.