The elementary sensitiveness of the skin is shown by the fact that moderate excitation suffices to raise the temperature, while Heidenhain and others have shown that in animals cutaneous stimuli modify the sensibility of the brain cortex, slight stimulus increasing excitability and strong stimulus diminishing it. Féré has shown that the slight stimulus to the skin furnished by placing a piece of metal on the arm or elsewhere suffices to increase the output of work with the ergograph. (Féré, _Comptes Rendus Société de Biologie_, July 12, 1902; id., _Pathologic des Emotions_, pp. 40 et seq.) Féré found that the application of a mustard plaster to the skin, or an icebag, or a hot-water bottle, or even a light touch with a painter's brush, all exerted a powerful effect in increasing muscular work with the ergograph. "The tonic effect of cutaneous excitation," he remarks, "throws light on the psychology of the caress. It is always the most sensitive parts of the body which seek to give or to receive caresses. Many animals rub or lick each other. The mucous surfaces share in this irritability of the skin. The kiss is not only an expression of feeling; it is a means of provoking it. Cataglottism is by no means confined to pigeons. The tonic value of cutaneous stimulation is indeed a commonly accepted idea. Wrestlers rub their hands or limbs, and the hand-shake also is not without its physiological basis. "Cutaneous excitations may cause painful sensations to cease. Many massage practices which favor work act chiefly as sensorial stimulants; on this account many nervous persons cannot abandon them, and the Greeks and Romans found in massage not only health, but pleasure. Lauder Brunton regards many common manoeuvres, like scratching the head and pulling the mustache, as methods of dilating the bloodvessels of the brain by stimulating the facial nerve. The motor reactions of cutaneous excitations favor this hypothesis." (Féré, _Travail et Plaisir_, Chapter XV, "Influence des Excitations du Toucher sur le Travail.")