There is something fantastic in those experiments on decapitated animals of which the brain has been destroyed, and the face of which may be taken up in the hand like a mask of flesh. While applying an electric current to the motor nerves, one sees the features reanimate themselves, and a series of expressions pass over them one after another--attention, joy, rage, as though the electric apparatus applied to the facial nerve represented the commands of the brain or psychic impressions which in reality no longer exist. The mechanical part of expression is, therefore, much simpler than one thinks. When a psychic operation takes place in the nerve-centres, the tension propagates itself along the nervous lines of least resistance. The more sensitive we are, the more graceful, beautiful, expressive, and fascinating is the curving of the lips produced by a smile. Peasants and coarse, dull persons cannot smile, with them the stimulus increases until it bursts out in a noisy, vacant laugh. The nerve-paths are constructed in such a way that the brain does not need trouble itself about the muscular movements. It is the intensity of the excitement which produces the expression; the stronger it is, the more numerous are the paths through which the nervous tension forces its way; as it increases, it overcomes all obstacles and resistance confronting it in other paths impassable till that movement, and contracts muscles which till then had remained neutral. The effects of the passions are reflected principally in the muscles of the face and respiration. No other function has to adapt itself more continuously than this last to the needs of the organism, standing as it does in close connection with all changes taking place in the nerve-centres. The muscles most vividly expressing passion are nearly all respiratory muscles. IV Our nervous system is so constituted that during violent emotion its activity discharges itself in all directions, and herein must we seek the reason of the resemblance between such different conditions as laughter and weeping, pain and pleasure. _It is the quantity, not the quality, of the stimulus which has weight on the scale of the expressions._ This statement of mine will appear clearer if we study the phenomena produced by tickling. When monkeys are touched in the arm-pit they twist and writhe, laugh, and emit sounds like human cries. The nerve-centres are very susceptible to the mechanical excitation of certain nerves, to contacts which fill us with the most pleasant and delicious sensations, or burst like a storm upon the organism. We have heard of persons who let themselves be tickled to death, and there are many sensitive natures that can scarcely bear the most ardent enjoyments of life.