304 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the many vicissitudes of the epidermis, and not before. We do not even know what agencies govern the differentiation of cutaneous appendages from the simple, undifferentiated epidermis during the early epochs of organogenesis. The early pattern of differentiation of cutaneous append- ages is orderly and precise. The hair follicles grow. first, and from them grow their secondary appendages, the sebaceous glands and apocrine sweat glands. The eccrine sweat glands develop last, their topographic distribution often determined by the distribution of the hair follicles. Each of these appendages differentiates into an organ that produces a different end-product. The surface epidermis is concerned with manu- facturing keratin, a mantle of dead cells that protects the body the hair follicles produce cylinders of keratin of varying thickness, length and color. Other than being ornamental, pelage serves little purpose in modern man. Sebaceous glands secrete on the surface of the skin oily, waxy substances that keep the corneal mantle supple. Eccrine sweat glands, the thermoregulatory organs of the body, are structurally and functionally beautiful organs that must be indefatigable in eliminating sweat. Apo- crine sweat glands, in spite of their exuberant development, in the axilla of man, secrete a small amount of substance, which when degraded by bacteria becomes fetid, a function of little use in modern man. These glands develop slowly during infancy, attain some activity in late child- hood and blossom just before adolescence. Regardless of what usefulness we can ascribe to apocrine glands, it is incorrect to consider them vestigial organs in man, since in no other mammal are they as well developed as they are in the human axilla, around the nipple of the breast and in the external auditory meatus. Skin shows striking topographic differences over the body. It is rough in some places, smooth in others it is taut or lax, highly colored or pale it is naked or hairy, and thick or thin. Skin is always thicker on the dorsal than on the ventral part of the body, and it is thicker on the ex- tensor than on the flexor side of the limbs. Satisfy yourselves of this by pinching first the skin on the lateral side of your forearm and then the skin on the roedial face of it. Differences in thickness, in tautness, in color, in smoothness and in the number of hairs and glands present are accompanied by differences in the quantity and quality of the connective tissue ele- ments present in the dermis, and most important, by the pattern and the richness of the blood-vascular system. An extensive study is now being carried out in our laboratory by Doctors Richard A. Ellis and Giuseppe Moretti on the differences in vascularity of the skin over the different regions of the body, and the results obtained thus far emphasize the enor- mous differences that occur. When the catalogue of the surface blood vessels of the skin is completed, we will have a better understanding of the gross and the minute topographic differences that exist in the skin.

PARTICULARITY OF HUMAN SKIN 305 The skin of the palms and soles is enormously different from that any- where else in the body. This skin never contains hair follicles and is the richest field of eccrine sweat glands in the body. The dead outer layer of the palms and soles is thicker than that anywhere else, and its physical and chemical composition must also be different. Nature has been lavishly ingenious in fashioning these adaptations the thickness of the corneal layer, the surface sculpturings and the numerous sweat glands in these areas provide a mechanism for sure gripping. The many sensory cor- puscles in this skin give one discrimination of what is being gripped. The prehensile part of the tail of some South American monkeys is tailored ex- actly like a palm, it being in effect a fifth palm or sole of these animals. Histological preparations of this skin look so much like palm or sole that even the seasoned histologist might be mistaken. The scalp has many features that set it apart from the rest of the skin. When it first differentiates in the embryo, the scalp starts just above the eye-brows, there being no forehead. The "hair line" then begins to re- cede, in both male and female fetus, and continues after birth it is ar- rested when the familial pattern of the hair line is established. This precocious process of balding deserves some comment. Although the forehead is considered to be glabrous, or hairless, it is nonetheless popu- lated with hairs, so minute and colorless that they escape casual observa- tion. During the attainment of an apparently naked forehead, the hair follicles undergo a retrograde metamorphosis, and become so small that the hairs they produce are almost invisible. In adult male baldness, the involutionary process which occurred in the forehead of the fetus and in- fants starts again, and the hair follicles are transformed into tiny ones. The bald scalp may seem to be naked, but its population of hairs is not appreciably decimated. Baldness, then, can be looked upon as the con- tinuation of the ageing process which begins during embryonic life. Al- though less precipitous and less dramatic, such involutionary changes take place also in the scalp of ageing women. Many of the hair follicles in the scalp of old women are also metamorphosed to tiny ones. This could be called insensitive baldness. Not only the hair follicles but the entire skin of the scalp undergoes profound changes in baldness. When the hair follicles become involuted, the sebaceous glands grow to gigantic sizes, the underside of the surface epidermis is flattened out and the subpapillary plexus of capillaries atrophies. Is it not remarkable that the agencies which are deleterious to hair follicles, epidermis and subpapillary plexus should actually be favorab}e to the sebaceous glands, which are never better developed! Topographic differences exist even in the restricted region of the scalp. The temporal and occipita} regions, for example, are resist- ant to pronounced ageing changes. The upper frontal and parietal regions, which are prone to balding in the male, must be different ecological fields.