200 EXPRESSION OF JOY: CHAP. VIII. ll-'l A' 4;< surface; but the surface on which we sit offers a marked exception to this rule. According to Gratiolet,7 certain nerves are much more sensitive to tickling than others. From the fact that a child can hardly tickle itself, or in a much less degree than when tickled by another person, it seems that the precise point to be touched must not be known; so with the mind, something unexpected—a novel or incongruous idea which breaks through an habitual train of thought—appears to be a strong ele- ment in the ludicrous. The sound of laughter is prodiiced by a deep inspira- tion followed by short, interrupted, spasmodic contrac- tions of the chest, and especially of the diaphragm.8 Hence we hear of "laughter holding both his sides." From the shaking of the body, the head nods to and fro. The lower jaw often quivers up and down, as is likewise the case with some species of baboons, when they are much pleased. During laughter the mouth is opened more or less widely, with the corners drawn much backwards, as well as a little upwards; and the upper lip is somewhat raised. The drawing back of the corners is best seen in moderate laughter, and especially in a broad smile— the latter epithet showing how the mouth is widened. In the accompanying figs. 1—3, Plate III., different degrees of moderate laughter and smiling have been' photographed. The figure of the little girl, with the hat, is by Dr. Wallich, and the expression was a genuine one; the other two are by Mr. Rejlander. Dr. Duchenne repeatedly insists ° that, under the emotion of joy, the 7 * Be la Physionomie/ p. 186. 8 Sir C. Bell (Anat. of Expression, p. 147) makes some remarks on the movement of the diaphragm during laughter. ft *Mecanisme de la Physionomie Humaine,' Album, Leg-ende vi.