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.

306 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The phylogenetic history of the human scalp is a fascinating one and we have begun to study it in other primates. It is much too early to make sweeping generalizations, but we have some clues. For example, the scalp of the few South American and African monkeys we have studied, has no eccrine sweat glands in the frontal and parietal regions, although there are many in the brow, and in the temporal and occipital regions. Of the higher apes we have studied only the chimpanzee, whose scalp does have some eccrine glands. The human scalp, being rich in eccrine sweat glands must be a relatively recent phylogenetic differentiation. These odd bits of information do not make the biology of the scalp completely intelligible and they do not make a definition of normality more plausible. They point out, however, that the scalp is a peculiarity of man, and that the process of balding is a norma/feature of ageing. What, then, is normal skin ? What is normal for the scalp is not normal for other regions and no two regions are exactly alike. This is not all what of ageing? The flabby skin of an aged person is as normal as the turgid skin of a youthful one, and norma/By must include the total biological life-history of skin. Skin is a kaleidoscopic organ. With the exception of the sweat glands, which are fairly stable, it has a turbulent rate of turnover. It is ever so sensitive to changes, internal or external, and makes constant adjustments to these changes. This instability is the wisdom of its survival skin stoutly resists attacks, but when it can no longer resist them it makes adjustments to them. There must be numberless regulatory mechanisms that guide the proper growth and differentiation of this system. The epidermis must prolif- erate exactly at the same pace that the surface dead layer is worn off. Some hair follicles must be stimulated to grow and others, perhaps adjacent to growing ones, must be suppressed or inhibited from growing. The eccrine sweat glands must be maintained in constant readiness to function in thermoregulation the apocrine glands must secrete in response to different stimuli, some of them psychic. The regulation of each of these systems is very precise and complicated. To mention just one example, it is not enough for the undifferentiated cells of the epidermis to divide in order to replace the cells lost at the surface the daughter cells must also synthesize specific kinds of complex fibrous proteins and lipids, and the cells must ascend in just a certain way and at a certain rate to the surface. A master unifying control system must guide the proper function of all of the subunits of skin. In spite of the very different functions that the units of the skin per- forms normally, these cells, or building blocks, are potentially all alike. l)octors Walter C. Lobitz, Jr. and Albert M. Kligman, and their colleagues, have made beautiful observations on the totipotentiality of epidermal