We are accustomed to regard the skin as mainly owing its existence to the need for the protection of the delicate vessels, nerves, viscera, and muscles underneath. Undoubtedly it performs, and by its tough and elastic texture is well fitted to perform, this extremely important service. But the skin is not merely a method of protection against the external world; it is also a method of bringing us into sensitive contact with the external world. It is thus, as the organ of touch, the seat of the most widely diffused sense we possess, and, moreover, the sense which is the most ancient and fundamental of all—the mother of the other senses.
It is scarcely necessary to insist that the primitive nature of the sensory function of the skin with the derivative nature of the other senses, is a well ascertained and demonstrable fact. The lower we descend in the animal scale, the more varied we find the functions of the skin to be, and if in the higher animals much of the complexity has disappeared, that is only because the specialization of the various skin regions into distinct organs has rendered this complexity unnecessary. Even yet, however, in man himself the skin still retains, in a more or less latent condition, much of its varied and primary power, and the analysis of pathological and even normal phenomena serves to bring these old powers into clear light.
Woods Hutchinson (Studies in Human and Comparative Pathology, 1901, Chapters VII and VIII) has admirably set forth the immense importance of the skin, as in the first place "a tissue which is silk to the touch, the most exquisitely beautiful surface in the universe to the eye, and yet a wall of adamant against hostile attack. Impervious alike, by virtue of its wonderful responsive vitality, to moisture and drought, cold and heat, electrical changes, hostile bacteria, the most virulent of poisons and the deadliest of gases, it is one of the real Wonders of the World. More beautiful than velvet, softer and more pliable than silk, more impervious than rubber, and more durable under exposure than steel, well-nigh as resistant to electric currents as glass, it is one of the toughest and most dangerproof substances in the three kingdoms of nature" (although, as this author adds, we "hardly dare permit it to see the sunlight or breathe the open air"). But it is more than this. It is, as Woods Hutchinson expresses it, the creator of the entire body; its embryonic infoldings form the alimentary canal, the brain, the spinal cord, while every sense is but a specialization of its general organic activity. It is furthermore a kind of "skin-heart," promoting the circulation by its own energy; it is the great heat-regulating organ of the body; it is an excretory organ only second to the kidneys, which descend from it, and finally it still remains the seat of touch.
It may be added that the extreme beauty of the skin as a surface is very clearly brought out by the inadequacy of the comparisons commonly used in order to express its beauty. Snow, marble, alabaster, ivory, milk, cream, silk, velvet, and all the other conventional similes furnish surfaces which from any point of view are incomparably inferior to the skin itself. (Cf. Stratz, Die Schönheit des Weiblichen Körpers, Chapter XII.)
With reference to the extraordinary vitality of the skin, emphasized by Woods Hutchinson, it may be added that, when experimenting on the skin with the electric current, Waller found that healthy skin showed signs of life ten days or more after excision. It has been found also that fragments of skin which have been preserved in sterile fluid for even as long as nine months may still be successfully transplanted on to the body. (British Medical Journal, July 19, 1902.)
Everything indicates, remark Stanley Hall and Donaldson ("Motor Sensations in the Skin," Mind, 1885), that the skin is "not only the primeval and most reliable source of our knowledge of the external world or the archæological field of psychology," but a field in which work may shed light on some of the most fundamental problems of psychic action. Groos (Spiele der Menschen, pp. 8-16) also deals with the primitive character of touch sensations.
Touch sensations are without doubt the first of all the sensory impressions to prove pleasurable. We should, indeed, expect this from the fact that the skin reflexes have already appeared before birth, while a pleasurable sensitiveness of the lips is doubtless a factor in the child's response to the contact of the maternal nipple. Very early memories of sensory pleasure seem to be frequently, perhaps most frequently, tactile in character, though this fact is often disguised in recollection, owing to tactile impression being vague and diffused; there is thus in Elizabeth Potwin's "Study of Early Memories" (Psychological Review, November, 1901) no separate group of tactile memories, and the more elaborate investigation by Colegrove ("Individual Memories," American Journal of Psychology, January, 1899) yields no decisive results under this head. See, however, Stanley Hall's valuable study, "Some Aspects of the Early Sense of Self," American Journal of Psychology, April, 1898. Külpe has a discussion of the psychology of cutaneous sensations (Outlines of Psychology [English translation], pp. 87 et seq.)
Harriet Martineau, at the beginning of her Autobiography, referring to the vivid character of tactile sensations in early childhood, remarks, concerning an early memory of touching a velvet button, that "the rapture of the sensation was really monstrous." And a lady tells me that one of her earliest memories at the age of 3 is of the exquisite sensation of the casual contact of a cool stone with the vulva in the act of urinating. Such sensations, of course, cannot be termed specifically sexual, though they help to furnish the tactile basis on which the specifically sexual sensations develop.
The elementary sensitiveness of the skin is shown by the fact that moderate excitation suffices to raise the temperature, while Heidenhain and others have shown that in animals cutaneous stimuli modify the sensibility of the brain cortex, slight stimulus increasing excitability and strong stimulus diminishing it. Féré has shown that the slight stimulus to the skin furnished by placing a piece of metal on the arm or elsewhere suffices to increase the output of work with the ergograph. (Féré, Comptes Rendus Société de Biologie, July 12, 1902; id., Pathologic des Emotions, pp. 40 et seq.)