The defensive movements are such as the following:—retraction of the foot and leg when the sole is tickled; the bending of the head to the shoulder when the neck is tickled; the rendering of the body concave on the side which is attacked; the thrusting away of the hand of the tickler; wriggling and fencing with the arms when a child is tickled lying on his back. These movements appear to {57} introduce important modifications into the sensations excited by tickling. Dr. Louis Robinson tells us that the flexing of the foot when tickled transforms an unpleasant sensation into a rather pleasant one. We may now pass to the point of chief importance for our present study, the conditions of the laughter-reaction during a process of tickling. This reaction is clearly the typical form of childish risibility. It has been already more or less clearly implied, that we cannot mark off the laughter in this case as an effect determined by any assignable differences in the characteristics of the sensations involved. Dr. Louis Robinson thinks that the tickling which provokes laughter is a special variety involving the stimulation of the deeper-lying nerves. Dr. Leonard Hill, who has specially tested this point for me, writes, “There is no difference in response to deep and superficial tickling”; and again, “I am sure that the most delicate superficial stimulation can provoke laughter”. This certainly seems to agree with ordinary observation. One of the most laughter-provoking forms of tickling consists of a series of pianissimo touches. Again, in speaking of ticklish areas of the skin, we must be careful not to restrict the titillation which calls forth laughter to any assignable region. It is undeniable that there are areas which more readily respond, in the case of children generally, to the tickling provocation. The armpits perhaps will occur to most readers; and it is noticeable that Darwin speaks of the anthropoid apes giving out “a reiterated sound, corresponding with our laughter, when they are tickled, especially under the armpits”.[38] This {58} fact, however, does not imply that the area of sensibility is circumscribed. Dr. Leonard Hill assures me, as a result of his investigations, that laughter under favourable conditions may be excited by tickling _any part_ of the body. Dr. L. Robinson in a letter explains to me that he agrees with Dr. L. Hill here. He finds that if a child is in a ticklish mood, the tickling of any part or even the threat of doing so will suffice to provoke laughter. On the other hand, we cannot speak of any part of the surface as one, the tickling of which will uniformly call forth laughter. Here again, as we shall see, the influence of mental agencies modifies the result.