SKIN, SENSORY FUNCTIONS OF. It is the function of our senses to keep us in relation with our surroundings and to enable us to move with impunity about a world full of hard angular and dangerous objects. In this task the senses of the skin take an important and characteristic part. They occupy a position between what we may call the intimate senses of taste and smell on the one hand and the distant senses of sight and hearing on the other. In the former group the stimulating sub stance has to be brought into very close contact with the body and the mechanism of stimulation is probably chemical; in the latter group the stimulating agent may act from great distances. Unlike both these the skin gives us news chiefly of nearby events; it has nothing to tell us of the intimate chemical nature of objects and very little directly of distant things.
The external events which by acting on the skin arouse sensa ti8n are found to be for the most part the simple and obvious physical processes, mechanical and thermal.
The feeling of contact or touch in the non-technical sense comprises two sensations. One of these, which is sharply localized in time and place and is in its essence delicate and exact, is touch in the strict technical sense. Its characteristic stimulus is movement at the surface of the skin. The other sensation is pressure; it is in its essence less precise and although well localized in place, is temporally indefinite so that the subject of it finds it impossible to decide by the sensation alone when a pressure stimulus has ceased. The characteristic stimulus of this sensation is simple static pressure.
True touch is probably the most elaborate product of evolution among the skin senses. It is interesting to notice that the essential neurological fact behind the tactile sense—that mere mechanical movement can be transmuted by the body into sensation—is also the fact on which the not more useful but far more imposing sense of hearing is based.
Thermal sensations do not depend directly on the absolute temperature of the object that causes them but on its temperature relative to that of the skin. In other words the characteristic stimulus of the thermal sense is the interchange of heat between the stimulating object and the body. Sensations of warmth and heat are caused by heat passing into the skin, and sensations of coolness and cold by heat passing out of it. The intensity of the sensation varies with the rapidity of heat trans ference and therefore not only with the temperature difference but also with the conductivity for heat of the object. A good
conductor therefore feels hotter or colder as the case may be than a bad conductor at the same temperature.
Most of our thermal sensations come to us by conduction through things actually touching the skin, including of course the air. The thermal sense is then usually, like the tactile sense, a short range process involving contact. Thermal sensations can however also be set up by radiated heat. Such radiation probably acts by warming the skin from which heat is then conducted in the ordinary way to the thermal nerve endings. It is conceivable however that these nerve endings are capable of direct response to the radiation. If this were so it would show the skin to possess the rudiment of a true long range sense, and perhaps a rudiment that by adaptation to higher frequencies of radiation has given us sight—the greatest of all the senses.
The pain sense does not show the same simple and orderly relation between stimulus and sensation that is found with the tactile and thermal senses. Sensations of pain are set up by a heterogeneous group of stimuli which do not permit of any in clusive general physical or physiological description. It is of course obvious that any process that actually damages the skin is likely to cause pain at the moment of infliction though even this, as we shall see, is not an invariable rule. Many stimuli however, though at intensities giving clear sensation of pain, do not cause any perceptible injury—such for example are brisk heat and cold and the faradic current. The anomalous relation of stimu lus and sensation to the possibility of injury is well seen in con nection with radiant stimuli. Heat rays easily excite pain when too weak to cause injury as is shown in the sensitiveness of the ocular cornea to the radiation of a dull red fire. On the other hand the far more injurious ultra-violet and X-rays excite no sensa tion whatever even at the moment when they may be seriously damaging the skin. In general however it may be said that most of the stimuli capable of causing pain are such that if they were of but little increased intensity they would actually cause injury. From this it follows that the characteristic pain stimulus is of considerable energy—far greater in fact than that of the tactile or thermal stimulus—and that the pain sense in comparison with all others is a dull sense or has, in physiological terms, a high threshold.