372 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS abruptly for a time, the so-called catagen stage, and finally the hair papilla withers and is largely destroyed. The hair then moves upwards and is shed. In this phase, a new epithelial shoot forms at the side of the hair follicle. This grows downwards and a new cycle begins. This interaction between the upper part of the hair follicle, the sebaceous gland, and the lower part of the follicle must proceed faultlessly if difficulties are to be avoided. Interruption of this interaction by internal or external factors might explain the occur- rence of comedones and pimples in infancy, in puberty, and in old age. The conception of this interaction can be extended to apply to the entire skin. This is a symbiosis of ectodermal and mesenchymal elements of various origin (9). In addition to the hair germ and all that is related to it, there are the sweat glands (acrosyringium), less closely connected, epithelial but not keratinised, which may keratinise, however, under the influence of certain stimuli. The melanocytes are foreign intruders originating from the neural crest, which produce melanin and inject it into the epithelial cells. Finally, there are the mesenchymal structures, such as blood vessels, lymphatic vessels and nerves. The whole is fed and co-ordinated through the connective tissue which encloses these organs as a nutrient medium. If changes in this connective tissue occur, in old age or under the influence of the sun on a skin with little or no pigment, the whole becomes dis- organized (10). VARIABILITY IN THE DIFFERENT PERIODS OF LIFE The skin displays distinct variations in the different periods of life. To a small extent these variations are the results of ageing in general they are also caused by the sensitix'ity of the skin to a number of factors, e.g. to hormone stimulation and climatic influences. The epidermis of the baby is thin and readily detaches itself from the underlayer. This epidermis has very thin lanugo hairs (average thickness of 0.03 mm) and a relatively large number of sweat glands per cm 2. The heat regulation, especially on the periphery, is not entirely in equilibrium. The connective tissue has a relatively high water content. Difficulties in the discharge from the adnexa, which may possibly lead to infection, occur chiefly in the sweat glands (peri- poritis), whereas diseases of the hair follicles are relatively rare. Clinically, the view is held that the skin of the baby is "more sensitive", although, in general, "tar" (e.g. pix liquida) in high concentrations can be tolerated very well. Marcussen (11) has recently established from experi- ments with 200 children, that patch tests with nickel sulphate and formalde- hyde, which cause no irritation to adults, give rise in children of 7-8 years

THE VARIABILITY OF THE SKIN 373 to follicular and eczematous reactions which might be wrongly interpreted as allergic reactions but which are in reality toxic. If 5% solutions are tested on adults, 1% solutions should be used on children. The difference pro- bably lies in the gradual formation of an effective barrier in the outer skin layers during the first eight years of life. Corresponding experience has been gained with tuberculin. When rubbed into the skin of very young children it has the same effect as an injection. But after a few years this treatment has no effect the tuberculin no longer passes through the dead skin layers. In puberty, the hair coverage attains its ultimate shape. The hairs of the scalp reach their final thickness of 0.15 mm. The puberal hair (pubic, arm-pit and beard hair) develops and forms, together with a final change in the body hair, the terminal hair. At the same time, both in boys and girls, there occurs an enlargement of the sebaceous glands on the face and the upper part of the trunk, mainly under the influence of androgenic hormones (12). This is a period for the occurrence of acne, eczematous and coccal infections, often localized around or in the follicles, and mostly originating from the nose and pharynx. During the climacteric, regulation disturbances of the blood vessels of the face may occur in the form of rosacea or couperose. Finally, in old age the productivity of the sebaceous glands decreases and growth of the skin becomes more sluggish. Increasing arteriosclerosis first becomes noticeable in the legs in the form of superficial vascular atrophy, as the result of minor infacts (blockage of small blood vessels). Different changes occur in the connective tissue. The normal, finely-branched thin elastic fibres of the connective tissue are replaced by coarser ones. The significance of this is not clear. Epstein thought it was mainly symbolic--"in the young person the finely-branched tissues symbolize hope and expectation in the skin of the old person we see the damaged remnants, bowed but unbroken, which have weathered the storms of time" (13). It is assurned that the so-called mucinous ground substance embedded between the collagen fibres becomes denser, and permeability, and consequently the possibility of regulation between the component parts, decreases. The first signs of disorganization within the skin then become visible. The decreased mutual influence of the component elements appears as a very localized protuberance at a few epithelial sites, which manifest themselves clinically as so-called warts (verrucae seborrhoicae). This process is seen in an intensified form on a skin exposed to climatic influences. Here the changes in the cutis are histologically far more pro- nounced. They display the picture of senile elastosis, in which the normal collagen is covered by an amorphous mass, which in many respects behaves. like elastin. It is highly probable that these changes run parallel with a decrease in the permeability of the connective tissue, which thus loses its