CUTANEOUS SENSATIONS. 691 CUTANEOUS SENSATIONS. to time, to little Hashes of eoUl; over the rest of its course, it will arouse nothing but pressure sensations. If the poiul be warmed, and drawn in the same way over the closed eyelid, it will give rise, from time to time, to little dots of warm sensation; at other points, it c.eites noth- ing but pressure. There is, then, a mosaic of temperature organs as there is a mosaic of i)res- sure organs, iloreover, if a small square or circle of skin is accurately marked, and explored twice over, once with a cooled and once with a warmed point of metal, it will be found that the cold and warm sjjots do not coincide. A 'cold spot' never gives a warm sensation; a 'warm spot' never responds to stimulation by a cold sensation. While, therefore, we are justified, physically, in speaking of 'degrees' of tempera- tvire, and in arranging 'warms' and 'colds' upon a single thermometric scale, we must recognize the fact that, psychologically regarded, warmth and cold are distinct things; thei'e are two tem- jjerature senses, each with its own distribution and its peculiar terminal organs. The organs of cold are more numerous than those of warmth. They are to be found, probably, in the end-bulbs of Krause, while the organs of warmth are the cylinders of Rulfmi (von Frey). All these cu- taneous organs are of extremely simple struc- ture, consisting of little more than a skein or tangle of nerve-fibrils, twined about a cluster of connective-tissue cells. All alike are readily fatigued; and all show differences of responsive- ness of 'attunement,' some answering a given stimulus with an intensive, others with a weak sensation. (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 immistakable; even a novice will have no difficulty, after the first few trials, in dis- tinguishing it from the ticklish quality of fine pressures. JMoreover, 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 leathei', upon the elastic cutis. When, then, the skin is lightly touched, the resulting vibration passes through the epidermis to the underlying cutis; the epi- dermis, with its organs, is not stimulated at all. M'hen. 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 cutis 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 yet that, under ordinary circumstances, it takes less stimulation to call out pressure than to excite pain. (4) Heat. — We have seen that warm spots give only sensations of warmth, and cold spots only sensations of cold, it is rcniarkalile tluit, while the cold spots do not as a rule respond to warm stimuli — as, indeed, we should not exjjeet them to do — they respond, by a distinct cold sensation, to healed stimuli of some 45" C or over (von Frcy's 'j)aradoxical cold'). Xo ex- jdanation can at present be oll'ered. Stranger still is the experience that, if a piece of metal, heated to tliis lemi)eraturc, be laid upon a jior- tion of the skin that is furnished with both cold a. id warm spots, the result of the combined ex- citations is not warmth, or cold, or jiain, 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. Raise the temperature of the metal from 40° upward, by 1° steps. For the first few trials you get nothing but a mild warmth, from the warm spots. But as soon as you pass the critical tem- perature (the heat that, if the metal were a point, would evoke fron; a cold spot the para- doxical cold sensation), you gel 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- eliological and a psychophysical sensation. (See Elements, Conscious.) 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 be 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 nuicli larger than the area of original stimulation. Concomitant sensations of pressure (see Common Sensation) may also be aroused in remote re- gions of the skin. It is possible (b) 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-llesh, may contain sensory nerve-endings which functionate in the tickling complex. (c) 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 cutis. Weak pressure or blowing upon the skin is known to increase the arterial blood-pressure. (d) The movements of witlidrawal 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 wiiich, on the purely sensory side, subserve concomitant sensation. Moreover, laughter is the direct motor resjjonse to tickling, ami unpleasantness does not arise unless the stimulation be long continued, (e) For theories of the connection of laughter with tickling, see Lat-giiter. We 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.
