(3) Pain.—If a small area of skin, say, upon the back of the hand, be shaved, moistened, stretched taut, and explored, point for point, by a fine horse-hair or needle, sensations of pain— more closely distributed than any of the three preceding sensations—will be obtained. The pain sensations are the most delicate, the smallest, so to speak, of all the skin sensations. The pain quality is unmistakable; even a novice will have DO difficulty, after the first few trials, in dis tinguishing it from the ticklish quality of fine pressures. Moreover, if a pressure spot be very accurately localized, and the needle-point thrust into its centre, no sensation of pain will be aroused. The temperature spots are similarly analgesic. Hence there can be no doubt that pain is a new, fourth sense, endowed with organs of its own.
These organs are, in all probability, the free nerve-endings in the epidermis. The epidermis lies, like a layer of stiff leather,. upon the elastic cutis. When, then, the skin is lightly touched, the resulting vibration passes through the epidermis to the underlying ends; the epi dermis, with its organs, is not stimulated at all. When, however, the skin is bruised, so that the epidermis is actually broken or crushed; or when the epidermis is itself explored, under ex perimental conditions that render it accessible to stimulation; then the pain quality is evoked. The mechanical character of eutis and epidermis thus enables us to explain the apparently para doxical fact that the pain organs are placed more superficially than the organs of pressure, and vet that, under ordinary circumstances, it takes less stimulation to call out pressure than to excite pain.
(4) Ileot.--We have seen that warm spots give only sensations of warmth, and cold spots only sensations of cold. It is remarkable that, while the cold spots do not as a rule respond to warm stininli—as, indeed, we should not expect them to do—they respond, by a distinct cold sensation, to heated stimuli of sonic 45' C. or Over (von l'rey's 'paradoxical No ex planation can at present he offered. Stranger still is the experience that, if a piece of metal, heated to this temperature, be laid upon a por tion of the skin that is furnished with both cold !t1 warm spots, the result of the combined ex citations is not warmth, or cold, or pain, but an altogether new quality, the quality of heat. A good place for experimentation is the median line of the forehead, close up to the hair. 11aise the temperature of the metal from 40'' upward. by I° steps. For the first few trials yon get nothing but a mild warmth, from the warm spots. flut as soon as you pass the critical tem perature (the heat that, if the metal were a point, would evoke from a cold spot the pant doxical cold sensation), you get a distinct sense of heat. Again, no explanation can be given.
It is noteworthy, however, that heat affords a good instance of the difference between a psy chological and a psychophysical sensation. (sec ELEMENTS, CoNsciots.) To introspection. heat is a sensation, entirely unanalyzable; but when we take account of its bodily conditions, it ap pears as a fusion, a mixture of the stimulus qualities warm and cold.
(5) Tickling.—The psychophysics of this sen sation complex is still obscure. Tickling may he set up, at certain parts of the cutaneous sur face, by light intermittent pressure, or even by a single light touch. The resulting pressure sensations are (a) in some way diffused, so that the area of sensation presently becomes much larger than the area of original stimulation. Concomitant sensations of pressure (see COM MON SENSATION) may also be aroused in remote re gions of the skin. It is possible (h) that the smooth muscle-fibres at the roots of the hairs, the muscles that cause the hair to 'stand on end' and whose contraction produces goose-flesh, may contain sensory nerve-endings which funetionate in the tickling complex. (e) The occasional thrills of warmth which are characteristic of tickling are due, apparently, not to mechanical stimulation of the warm spots, but to a change of blood-supply in the vessels of the eutis. Weak pressure or blowing upon the skin is known to increase the arterial blood-pressure. (d) The movements of withdrawal have been ascribed to the unpleasantness of the intermittent stimula tion; a flickering light, a beating tone, an in terrupted pressure, are all disagreeable. While this statement may contain a part of the truth, it seems probable that the movements are re ferable, in part at least, to reflex connections between the sensory and motor nerves. akin to the connections which, on the purely sensory side, subserve concomitant sensation. Moreover, laughter is the direct motor response to tickling., and unpleasantness does not arise unless the stimulation be long continued. (e) For theories of the connection of laughter with tickling, see LAUGHTER.
'e have spoken. so far, only of the 'external' skin. The 'internal skin' of the body. or mucous membrane, is sensitive to pressure and pain over most. if not all, of its extent. It is, however, very weakly if at all sensitive to temperature from the pharynx downward.
BIBLIOGRAPHY. For a general account of exBibliography. For a general account of ex- perimental work upon the cutaneous *spots,' consult: Titchener, Experimental Psychology (New York, 1901) ; Donaldson, Mind (London, 1883) ; for the quality of heat, Alrutz, Mind, vi., vii. (London. 1897-95) ; for tickling, Outlines of Psychology (London, 1895). wee PAIN.