THE CONDITION OF OLD AGE 27 



of white hair has resulted from the activity of phago- 

 cytes which have eaten the pigment which should 

 have remained in the hair and kept its colour. 1 But 

 the pigment of the hair is not the only thing they 

 will attack ; they will make their aggressive inroads 

 upon any part of the body ; and Professor Metchnikoff 

 has advanced the theory that old age consists chiefly 

 in the damage which is done by phagocytes to pois- 

 oned parts of the body, the poisoning being due to 

 the fermentation in the large intestine. Now it has 

 been observed by some of the German investigators 2 

 of these matters that the presence of lactic acid inter- 

 feres with this fermentative process as it goes on in 

 the intestine. Lactic acid, as its name implies, is the 

 characteristic acid which occurs in milk when it be- 

 comes sour. An Italian investigator 3 tried drinking 

 some sour milk with the idea of stopping the fermenta- 

 tion in the intestine, and so putting an end to the dele- 

 terious change, and he believes in the short time that 

 he tried it that it did him good quite, you see, in 

 the way of a patent medicine. 4 Professor Metch- 

 nikoff, on this basis, has recommended, in his book 

 on the Nature of Man, the regular drinking of sour 



1 This interesting fact was discovered by Metchnikoff, Annales de Vlnstitut 

 Pasteur, 1901, p. 865. 



2 Compare Bienstock, Archiv fur Hygiene, xxxix., p. 390 (1902) ; also 

 Tissier et Martelly, Annales de I ' Instilut Pasteur, 1902, p. 865. 



3 Albert Rovighi, " Die Aetherschwefelsauren im Harn und die Darminfec- 

 tion," Zeitschr. fur Physiol. Cheinie, xvi., pp. 20-46 ; see especially p. 43. 



4 Rovighi used " kephyr," a fermented milk, in his experiments. For the 

 mode of preparation and for the use of " kephyr," see Fischer, Die neueren 

 Arzneimitteln, Berlin, 1887, p. 169. 



