On the other hand, all are familiar with the truth that bodily activity deadens emotion.  Under great irritation we get relief by walking about rapidly.  Extreme effort in the bootless attempt to achieve a desired end greatly diminishes the intensity of the desire.  Those who are forced to exert themselves after misfortunes, do not suffer nearly so much as those who remain quiescent.  If any one wishes to check intellectual excitement, he cannot choose a more efficient method than running till he is exhausted.  Moreover, these cases, in which the production of feeling and thought is hindered by determining the nervous energy towards bodily movements, have their counterparts in the cases in which bodily movements are hindered by extra absorption of nervous energy in sudden thoughts and feelings.  If, when walking along, there flashes on you an idea that creates great surprise, hope, or alarm, you stop; or if sitting cross-legged, swinging your pendent foot, the movement is at once arrested.  From the viscera, too, intense mental action abstracts energy.  Joy, disappointment, anxiety, or any moral perturbation rising to a great height, will destroy appetite; or if food has been taken, will arrest digestion; and even a purely intellectual activity, when extreme, will do the like.

Facts, then, fully bear out these a priori inferences, that the nervous excitement at any moment present to consciousness as feeling, must expend itself in some way or other; that of the three classes of channels open to it, it must take one, two, or more, according to circumstances; that the closure or obstruction of one, must increase the discharge through the others; and conversely, that if to answer some demand, the efflux of nervous energy in one direction is unusually great, there must be a corresponding decrease of the efflux in other directions.  Setting out from these premises, let us now see what interpretation is to be put on the phenomena of laughter.

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That laughter is a display of muscular excitement, and so illustrates the general law that feeling passing a certain pitch habitually vents itself in bodily action, scarcely needs pointing out.  It perhaps needs pointing out, however, that strong feeling of almost any kind produces this result.  It is not a sense of the ludicrous, only, which does it; nor are the various forms of joyous emotion the sole additional causes.  We have, besides, the sardonic laughter and the hysterical laughter, which result from mental distress; to which must be added certain sensations, as tickling, and, according to Mr. Bain, cold, and some kinds of acute pain.