It remains to determine the character of this sudden relaxation of the strain of attention more precisely. As a sudden collapse, it is clearly to be distinguished from the gradual breakdown due to “mental fatigue” and nervous exhaustion. The psycho-physical energy concentrated for the special purpose of meeting the strain is by no means used up, but has to find some way of escape. Here, no doubt, we seem to come across Mr. Spencer’s ingenious idea that laughter is an escape of nervous energy which has suddenly been set free. It is no less evident that the redundant energy follows the direction of the risible muscles because no other commanding object for the attention presents itself at the moment. The innervation of these muscles is not a mere diversion of attention: it is a _dispersion_ of the energies which for the maintenance of attention ought to {69} be concentrated. We are never less attentive during our waking life than at the moment of laughter. Yet even here, I think, the theory of a convenient waste-pipe arrangement is not adequate. There is, I take it, in the case a relief of sur-charged nerve-centres, which process would seem to be better described by the figure of a safety-valve arrangement. It is not difficult to surmise why the liberated energy should follow this particular nervous route. There is no doubt that the motor apparatus, by the disturbances of which all such interruptions of the smooth flow of respiration are brought about, is very readily acted on by emotional agencies. Altered respiration, showing itself in altered vocalisation, is one of the first of the commonly recognised signs of emotional agitation; and this effect has been rendered more clear and precise by recent experiments. We should expect, then, that the collapse of strained attitudes, with the great change in feeling-tone which this must carry with it, would deeply affect the respiration. We know, however, more than this. Severe efforts of attention are in general accompanied by a partial checking of respiration, an effect which seems to be alluded to in the French expression, an effort “de longue haleine”. On the other hand, the termination of such an effort is apt to be announced by the sigh of relief. Now, though the movements of laughter are not the same as those of sighing, they resemble the latter in their initial stage, that of deepened inspiration. May we not conclude, then, that laughter is likely to occur as another mode of physiological relief from the attitude of mental strain? And supposing, as seems certain, that laughter in its moderate degrees, by bringing a new briskness into the circulation, relieves the congested capillaries of the brain, may we not go farther and say that nature has {70} probably come to our aid by connecting with the mental upheavals and the cruel strains here referred to, which pretty certainly involve a risky condition of the cerebral system of capillaries, a mode of