THE CELLULAR CHANGES OF AGE 71 



In other cases we find that there is a change going 

 on which we call necrobiosis, which means that the 

 cells continue to live, but change their chemical 

 organisation so that their substance passes from a 

 living to a dead state. No more perfect illustration 

 of this sort of change can be found than that which 

 is afforded by the skin. In the deep layer of the 

 outer skin are the living and growing parts, which we 

 all know from experience are sensitive. As these 

 multiply some of them move up towards the surface ; 

 and they are continually shoved nearer and nearer 

 the surface by the growth of the cells underneath. 

 They finally become exposed at the surface by the 

 loss of the superficial cells which preceded them. 

 During this migration the protoplasm of each cell, 

 which was alive, is changed chemically into a new 

 substance which we call keratin, or in common lan- 

 guage, horny substance. Ultimately the cell proto- 

 plasm becomes nothing but horny substance and is 

 absolutely dead. Here life and death play together 

 and go hand in hand. Hence the term necrobiosis, 

 death and life in one. 



Another form of degeneration which occurs in 

 many cases is of great interest because it seems as if 

 the cells were making a last great effort ; and their final 

 performance is one of enlargement. They become 

 greater in size than before ; but there will follow a disin- 

 tegration of these cells also ; and they break down and 

 are lost. This form of degeneration is termed hyper- 

 trophy, and represents a third type, as I have stated. 



