112 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS of lower species, is negligible in man. It has been calculated that a human foot immersed in water "absorbs" 1-2 gm. of water per hour much of this is taken up by the horny layer (23). Water can be taken up even from hypertonic solutions. Oxygen uptake, although extolled by commercial interests as indispensable for skin beauty, is negligible. The skin does not "breathe" in any active sense of the word. Oxygen is absorbed passively the amounts passing through the skin are not more than 1.5 per cent of the total pulmonary oxygen uptake (24). Reports of "choking" by presumed interference with skin respiration in people painted with some occlusive substance, are invariably cases of systemic poisoning from percutaneously absorbed solvents or toxic agents. The only physiologically important substance absorbed by the skin is vitamin D. Its precursor occurs in the epidermal cells ultraviolet light converts this provitamin to its active form (25). In an interesting atavistic way, human skin produces more vitamin D on the back than on the belly, just like four-legged animals (26). Apparently our skin has not become aware that we have assumed an erect posture ages ago and receive equal amounts of sunlight on all sides of our body. Vitamin D is the only percutaneously absorbed compound of physiologic importance. All the other so-called skin foods, marketed by the cosmetic industry, have purely surface effects and have never fed anybody except their manufacturers. EXCRETION As an organ of excretion, the role of the skin has often been vastly over- rated. Loss of water and electrolytes through sweating can never com- pensate for kidney function. It has been estimated that the body would have to produce 80 L. of sweat to take over the function of a single kidney (12). Nevertheless, epidermis and hair follicles sometimes perform the task of detoxifying substances which are excreted through the skin. For example, arsenic and some heavy metals have a predilection for horny structures (1). They may be retained in the skin for much longer periods than is often realized. This chronic retention may account for the late appearance of arsenical keratoses and malignancies. EXPRESSION As an organ of expression, human skin has lost the defensive importance that such phenomena play in animals. Piloerection, changes in skin color, in pigment distribution, in skin turgor, etc., were all protective devices, cal- culated to deceive or scare off potential enemies. In man, some of these mechanisms are still brought into action by axon reflexes, which apparently do not travel in a centripetal direction at any time (27, 28) their pro- tective significance, however, has been lost. The ways in which human

FUNCTIONS OF THE SKIN 113 skin can express emotions is limited. Vasodilation occurs when we blush in shame or become red-faced in anger we respond with vasoconstriction, sweating or gooseflesh when we are frightened under stress we sweat, dilate or constrict our cutaneous vessels or release the contents of our apocrine glands, and so on. It is possible to find some protective purpose in these autonomic reactions, but their practical value is next to nothing (29). In conclusion, it is safe to say that an understanding of human skin physiology is incomplete without knowledge of skin functions in lower ani- mal species. The morphology and physiology of our skin result from an evolutionary trend. To understand this trend, it will be essential to con- duct additional comparative anatomical and physiological studies of the skin of other vertebrates. It is good to know that such studies are now in full progress (12, 30-32) and that we may look forward confidently to a broadened and deepened understanding of our skin and its functions. ztcknowledgment: The assistance and cooperation of Chesebrough- Pond's, Inc., is gratefully acknowledged. REFERENCES (1) Rothman, S., "Physiology and Biochemistry of the Skin," Chicago, Univ. of Chicago Press (1954). (2) Zawisch, C., Acta Anat., 29, 143 (1957). (3) Gersh, I., and Catchpole, H. R., Perspectives in Biol. Med., 3, 282 (1960). (4) Hall, D. A., Intern. Rev. Cytol., 8, 211 (1959). (5) Flesch, P., y. Invest. Dermatol., 31, 63 (1958). (6) Rubin, L., Ibid., 13, 313 (1949). (7) Flesch, P., and Esoda, E. C. J., Ibid., 35, 43 (1960). (8) Vasileva, L. I., Abst. Soviet ivied. Parl B, 2, 1213 (1958). (9) Hale, A. R., and Burch, G. E., Medicine, 39, 191 (1960). (10) LeBlanc, J., Am. y. Physiol., 196, 1042 (1959). (11) Barnett, S. A., •ttarl. y. Exptl. Physiol., 44, 35 (1959). (12) Weiner, J. S., and Hellmann, K., Biol. Revs. Cambridge Phil. Soc., 35, 141 (1960). (13) Barcroft, H., Physiol. Revs., 40 (Suppl. 4), 81 (1960). (14) Sperling, F., and Koppanyi, I., Am. y. Anal., 84, 335 (1949). (15) Billingham, R. E., and Silvers, W. K., •uart. Rev. Biol., 35, 1 (1960). (16) Lorincz, A. L., .% Am. Med. Assoc., 173, 1227 (1960). (17) Hals, I. M., and Zenisek, A., Am. Perfumer Aromat., 74, 26 (1959). (18) St/ipel, H., and Szakall, A., "Die Wirkung yon Waschmitteln auf die Haut," Heidelberg, Dr. Alfred Hiltbig Verlag (1957). (19) Malkinson, F. D., A.M.A. Arch. Ind. Health, 21, 87 (1960). (20) Scott, A., Brit. y. Dermatol., 69, 39 (1957). (21) Wall, P. D., and Cronly-Dillon, J. R., A.M.A. Arch. Neurol., 2, 365 (1960). (22) Houssay, B. A., "Human Physiology," New York, McGraw-Hill Co. (1951). (23) Buettner, K. J. K., 5 t. Appl. Physiol., 4, 261 (1959). (24) Petrun, N.M., Abst. Soviet Med. Part B, 2, 231 (1958). (25) Reinertson, R. P., and Wheatly, V. D., y. Invest. Dermalol., 32, 49 (1959). (26) Bekemeier, H., and Pfordte, K., Z. Physiol. Chem., 318, 159 (1960). (27) Kisch, B., Exoetl., Med. Surg., 16, 221 (1958). (28) Auerbach, R., Pearson, R. W., and Lorincz, A. L., y. [nvesl. Dermatol., 35, 343 (1960). (29) Whitlock, F. A., in "Progress in the Biological Sciences in Relation to Dermatology," New York, Cambridge Univ. Press (1960), pp. 207-216. (30) Montagna, W., and Ellis, R. A., Am. y. Phys. Anthropol., 17, 137 (1960). (3l) Montagna, W., and Ellis, R. A., Ibid., 18, 19 (1960). (32) Montagna, W., and Ellis, R. A., Anat. Record, 137, 271 (1960).