ANATOMY AND HISTOLOGY OF AGING SKIN 307 there is some indication of an increase in thickness of the epidermis in these regions. There is also apparently a correlation between the thickness of the epidermis in any one age group and the degree of development of the stratum granulosum, such that it is best developed in those animals where the epidermis is thickest. Therefore, from our studies on the rat we were not able to see anything which we could call atrophy of the epidermis as such. We did find pockets and groups of somewhat abnormal cell types at certain places and some of these may well represent the beginnings of pre- cancerous lesions. We turn now to the human material. In the human skin which we have examined up to the present time there seem to be certain features of the aging process which are similar to those found in our laboratory animal material and others which are different. In the dermis, for instance, we find the greater degree of cellularity, greater number of cells, particularly in the areas close beneath the epidermis, to be present in the younger individuals just as among the animals and a relative paucity of cells in older persons. The elastic tissue so far as we have studied it appears to be less in amount in senile human beings than in younger, but we do not have means at pres- ent for obtaining good quantitative data on this subject. It is hoped that such a method will be forthcoming. It is to the epidermis, however, that we have paid most attention and it is here that the findings are of most interest because many of them have not been described by other authors. First, we do readily confirm the presence of the abundant rete pegs or shelf-like extensions of the epidermis down into the dermis in the younger individuals. In our senile persons, on the other hand, these rete pegs are actually ironed out so as to be in many cases entirely obliterated, leaving a very smooth and also a very sharp line of demarcation between the epidermis and the dermis. In spite of this difference in the epidermis of the young and of the old individuals, I would hesitate to speak of the epidermis of the senile individ- ual as being in an atrophic condition or as having atrophied. It is still a good many cell layers thick and the cells themselves seem to be in good condition. There are, however, very interesting differences between the epidermis as such of the young and of the old individuals. It will be remembered that the epidermis is generally divided into two or three layers. The two chief layers are the stratum germinativum, composed of living cells some of which usually are found in division, and the stratum corneum, formed of dead cells which are scale-like and which "scuff off" and are lost, being carried away from the body. Interposed between these two layers there frequently is a narrower granular layer, a stratum granulosum. The

308 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS stratum corneum is a very important layer since it is actually the dry pro- tective covering of the entire body. According to our findings in the human material this stratum corneum is quite different in its appearance and to some extent in its staining properties in young and old persons. In the young individuals it is made up of lamellae or plate-like structures which seem to be loosely adherent and which come apart rather readily from each other and fairly readily from the surface of the living portion of the epidermis. In the senile individuals, on the other hand, this stratum corneum appears as a very densely adherent group of plates looking as though it would be extremely difficult for them to be separated one from the other. In regard to staining properties, the histologists will remember that we use two stains, that is, the hematoxylin, which gives a purplish color, and the eosin, which gives a pink color. The lamellae of the stratum corneum in the younger individuals tend to take either a basophilic or a lightly eosino- philic stain whereas in the senile individuals the lamellae often take a very brightly eosinophilic, bright pink or red, stain. These findings, it will be recalled, agree well with those made on the skin of the rat. In the stratum germinativum the layers of living cells also seem to show definite but rather subtle differences, such that careful microscopic observa- tion and concentration is needed in order to notice them. In the human skin, then, we find that in the young individuals the nuclei are quite promi- nent and have to some extent the rather crowded appearance that they had in the younger individuals among the laboratory animals. Particularly striking is the difference in the degree of definition of the individual cells in the young and in the senile persons. In the young persons the cell out- lines are fairly vague, somewhat difficult to make out, whereas in the senile individuals even with varying stains the cell outlines are extremely sharp and the cells themselves very well individualized. From our studies ini- tially on laboratory animals, and now followed up by a rather large series of human beings, we would say that the epidermis of the senile individual is quite a different type of tissue in its microscopic anatomy from that of the young individual. We would like now to show some slides of the skin of human beings of different ages. The first slide (Fig. 1) presents at low power the appear- ance of the skin from the abdomen of an eighteen-year-old female subject. It will be noticed immediately that the dermis is quite cellular, that the epidermis is thrown into a number of folds and that there are shelf-like extensions or masses of epidermal cells protruding downward into the der- mis. In the next slide we see the abdominal skin of a seventy-six-year-old female subject (Fig. 2). Note the remarkable smoothness of the line of demarcation between the epidermis and the dermis. The epidermis, while thinner than in the younaer subject, hardly seems to be what we would call