[31] See Wundt, Physiolog. Psychologie (4te Auflage), Bd. i., pp. 434–5. According to this authority the propagation of the stimulation may be either direct from one sensory fibre to another, or indirect, involving muscular contractions and muscular sensations.
[32] See Külpe, Outlines of Psychology, p. 148.
[33] See the article on “The Psychology of Tickling, Laughing, and the Comic,” by G. S. Hall and A. Allin, in the American Journal of Psychology, vol. ix., p. 1 ff. These returns do not make it quite clear whether “ticklishness” is taken to mean the non-laughing as well as the laughing varieties.
[34] My references to Dr. Robinson’s views are partly to the article in the Dictionary already quoted, and partly to notes of lectures given before the British Association and the West Kent Medical Society, which he has been so kind as to show me. I have made much use of his interesting and often brilliant suggestions in dealing with the subject of ticklishness.
[35] Both of these are included by Dr. Richet among the most sensitive parts (loc. cit.).
[36] How far the results are complicated by the action of the muscles which serve to erect the separate hairs on the body, and are said by Lister to contract near a tickled surface, I am not sure.
[37] E.g., Külpe, op. cit., 147.
[38] Expression of the Emotions, p. 201.
[39] In using the expression “ticklish period,” I do not imply that ticklishness necessarily disappears after a certain period of maximal intensity. Like play, it probably persists in a certain number of persons as a susceptibility to which the laws of propriety leave but little scope for exercise.
[40] Op. cit., pp. 201, 202. The restriction I have added enables us to include the case of the sole of the foot.