166 LIFE OF ELIE METCHNIKOFF 



and digested the injected red corpuscles and no 

 phenomenon similar to Pfeiffer's took place. 



These observations led Metchnikoff to a thorough 

 study of the destruction of cellular elements by the 

 phagocytes. He had already observed that, whilst the 

 struggle with microbes is chiefly undertaken by small 

 leucocytes with a lobed nucleus the microphages 

 it is the great leucocytes with a single large nucleus 

 the so-called macrophages which undertake the 

 destruction of cells, "figured" elements, as well as that 

 of toxins. The macrophages are to be found not only 

 in the blood but also in different organs such as the 

 liver, spleen, kidneys, etc. ; they seize upon living 

 cells by means of mobile protoplasmic prolongations 

 with which they draw them in and end by ingesting 

 them completely. Not only do they thus absorb 

 foreign cellular elements such as red corpuscles, 

 spermatozoa, etc., but also all the weakened cells 

 of the organism itself. 



This weakening may be due to normal phenomena 

 such as the metamorphosis of insects or tadpoles, 

 when certain organs, as they weaken, become useless 

 or inactive. But, oftener, this weakening is due to 

 pathological causes, as in morbid atrophies or poisoning 

 by microbian toxins. In any case, the enfeeblement 

 of cells exposes them to be devoured by macro- 

 phages, which brings about the atrophy of the cells 

 or even of the organs which contain them. 



These observations suggested to Metchnikoff the 

 idea that senile atrophy might be due to the same 

 mechanism, and his thoughts turned towards the 

 problem of the causes of old age. 



But, before undertaking researches in a new 

 direction, he wished to conclude those he had been 



