reflex contractions only occur from repeated shocks to the nerve-centres—­that is, through summation of successive stimuli.  That this result is also due in some degree to an alternating increase in the sensibility of the various areas in question from altered supply of blood is reasonably certain.  As a consequence of this summation-process there would result in many cases and in cases of excessive nervous discharge the opposite of pleasure, namely:  pain.  A number of instances have been recorded of death resulting from tickling, and there is no reason to doubt the truth of the statement that Simon de Montfort, during the persecution of the Albigenses, put some of them to death by tickling the soles of their feet with a feather.  An additional causal factor in the production of tickling may lie in the nature and structure of the nervous process involved in perception in general.  According to certain histological researches of recent years we know that between the sense-organs and the central nervous system there exist closely connected chains of conductors or neurons, along which an impression received by a single sensory cell on the periphery is propagated avalanchelike through an increasing number of neurons until the brain is reached.  If on the periphery a single cell is excited the avalanchelike process continues until finally hundreds or thousands of nerve-cells in the cortex are aroused to considerable activity.  Golgi, Ramon y Cajal, Koelliker, Held, Retzius, and others have demonstrated the histological basis of this law for vision, hearing, and smell, and we may safely assume from the phenomena of tickling that the sense of touch is not lacking in a similar arrangement.  May not a suggestion be offered, with some plausibility, that even in ideal or representative tickling, where tickling results, say, from someone pointing a finger at the ticklish places, this avalanchelike process may be incited from central centres, thus producing, although in a modified degree, the pleasant phenomena in question?  As to the deepest causal factor, I should say that tickling is the result of vasomotor shock.” (A.  Allin, “On Laughter,” Psychological Review, May, 1903.)

The intellectual element in tickling conies out in its connection with laughter and the sense of the comic, of which it may be said to constitute the physical basis.  While we are not here concerned with laughter and the comic sense,—­a subject which has lately attracted considerable attention,—­it may be instructive to point out that there is more than an analogy between laughter and the phenomena of sexual tumescence and detumescence.  The process whereby prolonged tickling, with its nervous summation and irradiation and accompanying hyperaemia, finds sudden relief in an explosion of laughter is a real example of tumescence—­as it has been defined in the study in another volume entitled “An Analysis of the Sexual Impulse”—­resulting finally in the orgasm of detumescence.  The reality