KERATIN REPLACEMENT AS AN AGING PARAMETER 339 Figure 3. Method for nail growth measurement alization as duNouy measured it) of a wound, with size of the wound and age of the individual as the only variables. The subjects were soldiers, healthy men between the ages of 20 and 40. This index of ½icatrization formula enabled duNouy to calculate the time a wound would take to epithelialize, given only its dimensions and the age of the patient. Shock (2) measured the decline in various physiologic functions with age in a large population segment. The graph in Fig. 2 represents a cross-sectional average of the per cent decline in performance of six functions which Shock measured in subjects of all ages. Although some functions, such as conduction velocity, decline only about 15% from age 30 to age 90 others (such as maximal breathing capacity) were found to retain only about 40% of their original capacity in 90-year-old individuals. Averaging all nine parameters, it appears that the decline in function of an individual is about 40% between the ages of 30 and 90, or that a 90-year-old retains, on the whole, about 60% of the reserve capacity of his youth. Studies have been performed on the technique of measuring mitosis in the human epidermis, and the relation of keratin replacement to age (3-7). Barman (8, 9) and Rook (10) have studied changes in human

54O JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS o, , . ß ß . Figure 4. Scattergram of Nail Growth Index as a function of age hair with age. Their experiments have shown that definite changes with age include a decrease in the density of hairs, a decrease in the percentage of coarse hairs with an increase in the percentage of fine hairs, and a decrease in the percentage of anagen (growing) hairs as com- pared to an increase in telogen (resting) hairs,