200 EXPRESSION OF JOY: Chap. VIII. 



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 produced 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. Eejlander. Dr. Duchenne 

 repeatedly insists 9 that, under the emotion of joy, the 



7 * De la Phvsionomie,' p. 186. 



8 Sir C. Bell (Anat, of Expression, p. 147) makes some 

 remarks on the movement of the diaphragm during 

 laughter. 



9 ' Mecanisme de la Phvsionomie Humaine,' Album, 

 Legende vi. 



