580 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 3.--Ichthyosis simplex (inhibition of shedding). of the epidermal cells. The Malpighian layer--as contrasted to psoriasis-- is not thickened (Fig. 4). I mentioned that keratinization is connected with a considerable de- hydration. Long ago I advanced the hypothesis that the water vapox loss of the skin to the outside, the so-called insensible perspiration, is in part due to this dehydration. In other words, the water lost by the kera- tinizing cells evaporates to the outside. If this is so, the insensible per- spiration should be increased if keratinization is accelerated. This is in- deed the case. Several years ago we (8) measured the insensible perspiration from normal skin areas and from adjacent or symmetrically situated psoriatic patches. We found an average of fivefold greater water vapor loss from psoriatic areas. Even more marked is the increase of water vapor loss in exfoliative dermatitis. Patients suffering from this disease feel cold all the time because of the great evaporative heat 1osk and they drink large volumes of water without noticeable increase of the urinary volume. In this disease we found a tenfold increase of the insensible perspiration and return to normal values after the skin cleared. In psoriasis and in exfoliative dermatitis the increase of insensible water loss is so great that, as shown by some calculations, accelerated kera- tinization alone cannot account for it. An additional factor probably is a break in the subcorneal membrane whereby the permeability of the epi-

PHYSIOLOGY AND PATHOLOGY Oi: KERATINIZATION 581 Figure 4.--Thickening of the entire epidermis in psoriasis with increzsed I:roliferation of the living cells. Thickening of the horny layer only in ichthyosis. dermis for water increases. However, this increased permeability factor is minor, if it exists at all, in postinflammatory scaling. The inflammation in itself, for instance in the case of a sunburn reaction, does not enhance or enhances only slightly the insensible perspiration. But as soon as the in- flammation subsides and visible scaling sets in the water vapor loss goes considerably higher. In contrast, in ichthyosis, where there is no ac- celerated keratinization, only a piling-up of horny material, no increase in insensible perspiration was observed. Ichthyosis is only a very special example of inhibition of shedding. A much more common and more important event is when the horny layer thickens after chronic, mechanical, chemical or actinic irritation. The simplest examples for this phenomenon are corns and calluses which de- velop under the influence of chronic mechanical pressure. They represent formidable thick, hard keratin piles on top of a normal or atrophic epi- dermis.