Soon after this distinction had been recognized, the tactual, or rather cutaneous, sense was found to consist of several senses. The impressions of touch, that is, of pressure on the skin, of temperature, and of pain had always been distinguished; but it had not been known that the areas of greatest sensitivity for touch are not identical with those for temperature, and that the sensitivity for pain may be greatly diminished without a corresponding change in the sensitivity for touch. It was only about 1880 that these observations were explained, when an anatomical separation of the neurons serving these different sensations was demonstrated. If we test the sensitivity of the skin by carefully stimulating single points, it is found that not every point of the skin is sensitive, but that the sensitive points are isolated by larger or smaller insensitive areas. It is further found that the points sensitive to warmth are different from those sensitive to cold or to pressure or to pain. This can easily be demonstrated for the cold points by touching the skin in a number of successive points with a steel pen or a lead pencil. Generally only the touch is perceived, but now and then an intense sensation of cold is felt on definite points, always recurring when these points are touched. It is somewhat more difficult to demonstrate the points sensitive to warmth. The sensation is in this case much less noticeable. The points sensitive to touch are on hairy parts of the skin always close to a hair; on other parts, for instance the palm of the hand and particularly the finger tips, they are located so close together that their separateness can be proved only by the use of very delicate instruments. The same is to be said of the pain points of the skin. We cannot, therefore, regard the skin as one organ of sense, but must regard it as containing four classes of organs serving the senses of warmth, cold, pressure, and pain. We must be sure, of course, to distinguish between pain, as a sensation, and the feeling of unpleasantness which almost without exception accompanies pain. We must further distinguish the sensation of pain from intense cold, intense heat, strong pressure, dazzling light, all of which may produce pain as a secondary effect. But the sensation of pain is quite dissimilar from the sensations of cold, heat, pressure, and light, to which it is added in consequence of physiological conditions. The independence of the sensation of pain can easily be demonstrated by touching the cornea of the eye with a hair. Pain is then perceived without any touch or temperature sensation. The pricking sensation in our nose resulting from the breathing of chlorine or ammonia may also be mentioned as an illustration of the same point. Let us further understand that pain is not only a cutaneous sensation, but also a sensation localized in internal organs; for instance, headache, toothache, colic.