AGE-INCIDENCE 361 



dead tissues (the products of katabolism) and the age- 

 incidence of cancer. 



The body is mainly composed of living cells, and 

 they constitute an elaborate combination of living 

 factors. We know that in certain tissues these cells 

 are continually dying and being replaced, so that it 

 is evident that birth and death must be going on 

 incessantly in the body. What happens to the dead 

 cells ? They of course liquefy and become dis- 

 organised, and their constituents are presumably ex- 

 creted or converted into other compounds. While 

 this is happening it seems probable that some of the 

 products of the remains of dead cells may be absorbed 

 by their neighbours, for it must be remembered that 

 the diffusion of substances into living cells appears to 

 be a physical process over which they exercise no 

 control. There are doubtless some cells which remain 

 alive for long periods; for instance, it has been 

 estimated (and we are informed that it is practically 

 certain) that some cells of the central nervous system 

 live throughout the life of a man. Many cells, how- 

 ever, only live a very short time, the length of their 

 lives perhaps varying in different parts of the body, so 

 that the remains of dead cells are probably always 

 present in the body fluids. In this connection, how- 

 ever, we have to keep in mind the physiological curve 

 expressive of the relationship between anabolism and 

 katabolism. There are only three stages of life if "it is 

 viewed from this point of view, the first terminating at 

 about the thirtieth year, when a man reaches his prime, 

 and up to which period cellular birth must preponderate 

 over its death-rate. For some years it may be sug- 

 gested that anabolism and katabolism remain balanced ; 



