Another type of laughter is that associated with the ticklish points of certain parts of the body, the soles of the feet and certain parts of the trunk and of the abdomen. The excitation of the ticklish receptors, like pain, compels self-defensive motor acts. This response is of phylogenetic origin, and may be awakened only by stimuli which are too light to be painful. In this connection it is of interest to note that a superficial, insect-like contact with the skin rarely provokes laughter, and that the tickling of the nasal, oral, and pulmonary tracts does not produce laughter. The ticklish points that cause laughter are rather deeply placed, and a certain type of physical contact is required to constitute an adequate stimulus. That is, the contact must arouse a phylogenetic association with a physical struggle or with physical exertion. In the foot, the adequate stimuli for laughter are such contacts as resemble or suggest piercing by stones or rough objects.. The intention of the one who tickles must be known; if his intention be playful, laughter results, whereas if injury be intended, then an effort toward escape or defense is excited, but no laughter. If deep tickling of the ribs is known to be malicious, it will excite physical resentment and not laughter. Self-tickling rarely causes laughter for the reason that auto-tickling can cause only a known degree of stimulation, so that there results no excessive integration which requires relief by the neutral muscular activity of laughter. In fact, one never sees purposeful acts and laughter associated. According to its severity, an isolated stimulus causes either an action or laughter. The ticklish points in our bodies were probably developed as a means of defense against serious attacks and of escape from injurious contacts. Anger, fear, and grief are also strong excitants and, therefore, are stimuli to motor activity. It is obvious that whatever the excitant the physico-chemical action of the brain and the ductless glands cannot be reversed--the effect of the stimulus cannot be recalled, therefore either a purposeful muscular act or a neutralizing act must be performed or else the liberated energy must smoulder in the various parts of the body. It is on this hypothesis that the origin and the purpose of laughter and crying may be understood. Even a superficial analysis of the phenomena of both laughter and crying show them to be without any external motor purpose; the respiratory mechanism is intermittently stimulated and inhibited; and the shoulder and arm muscles, indeed, many muscles of the trunk and the extremities are, as far as any external design is concerned, purposelessly contracted and released until the kinetic energy mobilized by excitation is utilized. During this time the facial expression gives the index to the mental state.