HOW WE FEEL

The whole surface of the human skin is not equally sensitive to stimulation. In fact, a very large proportion of it is completely insensitive. But dotted throughout this insensitive area are hundreds of thousands of tiny sense organs. They are clustered together much thicker in some regions of the body than in others, and wherever they are more numerous the skin is, of course, more sensitive; yet everywhere there is a sufficient number of them to make us unaware, unless we investigate carefully, that there are any portions which are insensate. These organs are of three different kinds and each kind is responsible for a special type of sensation. The most numerous, the corpuscles of Meissner, give us our experiences of pressure, the bulbs of Kraus provide our sensations of cold and the corpuscles of Ruffini those of warmth. In a square centimeter of the skin there are approximately twenty pressure spots, thirteen cold spots and two warm spots.

When a pressure spot is touched very lightly by a small narrow object it reacts with a sensation which may be described as a tickle. The same sensation can be got by gently moving a hair on the forearm or the back of the hand for the corpuscles of Meissner are situated directly beneath and to the windward of the hair bulbs and a movement of the hairs stimulates them adequately. If the pressure is made a little stronger, the tickle becomes a sensation of contact. Both the tickle and the contact are definitely experiences of pressure. If the same pressure spot is stimulated very carefully by either a cold or a warm object, the experience will still be one of pressure. The pressure will be neither warm nor cold for the pressure spots can respond only with tactual sensations. They are as insensitive to thermal stimulations as are the many other portions of the skin where there are no temperature spots. The warm and cold spots, alone, give us all our experience of temperature.

When a cold spot is stimulated by an object whose temperature is between 12 and 15 degrees C., a cold sensation is felt. When a stimulus whose temperature is between 37 and 40 degrees C. is brought in contact with a warm spot, the typical sensation of warmth results. If, however, the cold stimulus is applied to the warm spot, nothing at all is felt. The warm spots do not respond at all until the temperature reaches 37 degrees. Similarly the cold spots do not react to warm stimuli, warmth can be realized only from a warm spot. Yet when the temperature reaches 45 degrees C. (113° F.) the cold spots do respond, but with neither warmth nor heat. They are cold spots, not warm spots, therefore they can react in only one way—with a cold sensation. So we find that due to the physiological functioning of an anatomical sense organ, a physically hot stimulus really feels cold.

These three groups of sense organs, the pressure, cold and warm spots, are the only sense organs on the whole surface of the body. Even the pain which we feel when our skin is pricked or lacerated is not represented by a special organ. Pain is experienced whenever a nerve is stimulated directly and cutting the skin immediately exposes a nerve and pain results. The only other organs from which we receive any tactual sensations are the muscle spindles which produce aches and soreness, the spindles of the tendons from which come our sensations of strain, and the corpuscles of the joints which give us subcutaneous pressure. Yet think of the wealth of tactual experiences which we have every day, in fact, every few minutes. Not only such gross experiences as hunger and thirst and fatigue and dizziness, but simple perceptions of wetness and oiliness and smoothness and roughness and heaviness and lightness. There are no specific sense organs for any of these experiences. They are brought about by the six sense organs already mentioned and the free nerve endings of pain functioning singly or in combination, in greater or less degree.

There are not even any special sense organs of heat. Yet heat appears as a unique sensation. It is different from warmth because warmth is essentially comfortable while heat has a distinct and disagreeable sting. The characteristic sting of heat is recognized whenever we refer to pepper and other spices as hot. Psychologically, of course, the sensation of heat is unique and simple, but on its biological side it is slightly complicated. It is occasioned by the simultaneous response of a warm spot and a cold spot, or several warm and cold spots, to a stimulus of more than 45 degrees C. And it can be produced in no other way. The warm spots, of course respond normally, the cold spots paradoxically. The normal response is more vigorous than the paradoxical, consequently we do not often realize the sensory quality of the cold, while the warmth comes into consciousness. Ordinarily, then, heat is warm. With particular stimuli and in certain regions of the body where cold spots are particularly numerous, however, trained psychologists are able to detect a heat which is cold. Also it is not an especially rare occurrence to feel cold when we plunge our hands unknowingly into very hot water.

Wetness, which seems almost as simple as heat, is produced by the simultaneous stimulation of a cold spot and a pressure spot. It is really nothing more than a cold pressure. It is true that we speak of being wet when we are in a warm bath, but it is only when the water begins to cool that we get the typical wet perception. This explains why cold objects so often feel damp when there is actually no moisture on them and why it is very difficult to determine whether cloth is dry or not when it is held close to a hot fire, also why we are so generally unconscious of the continual moisture of our own bodies.