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

374 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS connective function. On the skin of these people, a complete disorganization (14) therefore becmnes apparent, in which each component--hair germ, hair follicle, sweat gland, sebaceous gland, covering epithelium, melanocytes, and blood vessels--separately begin to display limited growth, and finally again adjust themselves to one another but to a limited extent. INDIVIDUAL DIFFERENCES These differences manifest themselves in the type of skin--dry, greasy and normal. Analogous to medico-psychological typology, we might also speak of an asthenic and pycnic skin. The asthenic skin grows and recovers slowly and the activity of the adnexa is slight. Because of a typical peculi- arity the keratinization process (15) does not proceed in the normal way. Fish-like scales are formed to which the disease owes its name of ichthyosis. The metabolic disturbance in the keratinization process may be congenital or acquired. It is very interesting that a syndrome corresponding comple- tely with ichthyosis vulgaris can be encountered in some test subjects with a normal skin, to which triparanol (a substance which blocks the synthesis of plasmacholesterol in the liver) has been administered. Ichthyotic skin may also develop in the course of serious general diseases. The opposite of this, the thick, rapidly growing skin, is known in its most extreme form as ichthyosis congenita. The common seborrhoeic skin is formed functionally in puberty under influence of androgenic hormones, but nevertheless is very probably engendered in these people in their young years. DIFFERENCES BETWEEN THE SEXES Although many psychologists would like to see the difference between the sexes ascribed purely to cultural causes (a farmer's wife is a farmer who bears children), for the dermatologist there are striking differences, even though these are most pronounced at the time of sexual maturity, and in old age not only the hair coverage tends to be similar for both sexes. There is also a distinct difference in peripheral blood flow, which in women, at least in the Netherlands, is considerably more limited than in the male. This also applies to sweat secretion. SEASONS By and large it is possible to differentiate "summer" and "winter" skin. In this way the fully functioning, rapidly growing, profusely sweating and metabolically active summer skin--fully supplied with blood--is contrasted with the non-sweating, inactive winter skin, only poorly supplied with blood. The effect on the stratum corneum of a summer skin, is swelling and possible plugging of the sweat gland pores. The effect on winter skin is drying out and hardening followed by chapping and eczematic symptoms.