PHASES IN THE LIFE OF THE INDIVIDUAL 685 



This biologist finds it impossible to accept the view that the 

 high mortaUty observable between the ages of seventy and 

 seventy-five indicates a natural limit to human life at about 

 this period. Centenarians, he points out, are not really very 

 rare, and he cites many cases of extreme old age, including 

 that of Thomas Parr referred to above. Eeal old age, we are 

 told, is associated with an instinct for death which is as natural 

 as is the instinct for sleep. Metchnikofi therefore answers in 

 an emphatic negative the question asked by Admetus in 

 Euripides' Alccstis, " Is it the same thing for an old man as 

 for a young man to die ? " The fact that the instinct for death 

 seems so rarely to exist is regarded as evidence that true senility 

 is a comparatively infrequent phenomenon. 



According to MetchnikofE, senescence is not brought about 

 simply as the result of arrest of the reproductive powers of the 

 cells. The whitening of hair in old age is due to the destructive 

 action of phagocytes which remove the pigment. Moreover, 

 hairs become old and white without ceasing to grow. MetchnikofE 

 believes also that atrophy of the brain is due to the destruction 

 of the higher nerve cells by neuronophags, and that there are 

 many other devouring cells which are adrift in the tissues of 

 aged men and animals and cause the destruction of other ceUs 

 of the higher type. The testes, however, appear to have the 

 power to resist these phagocytes, and with this power is corre- 

 lated the fact that spermatozoa are often produced even in ad- 

 vanced old age. Metchnikoff's theory as to the cause of death 

 is that it is due to the poisoning of the tissues and to the damage 

 •done by phagocytes to those parts of the body affected by the 

 toxic action. He believed further that in Man and certain of 

 the animals this process of poisoning is brought about by fer- 

 mentation set up by microbial action in the large intestine. 

 The toxic substances produced by the intestinal fermentation 

 are supposed to enter the system and poison it, the result being 

 that the vitality of the tissues is lowered, so that they are less 

 able to resist the action of devouring phagocytes. The presence 

 of lactic acid in the intestine is believed to arrest the process of 

 fermentation. Metchnikofi recommends, therefore, the regular 

 drinking of sour milk as a means of destroying the microbes in 

 the intestine in the hope of prolonging life. 



