26^ ROYAL SOCIETY OF CANADA 



the phagocytes take np into their substance bacteria, much as they would 

 ordinary food-stuffs, digesting them, assimilating portions of their 

 material, and rejecting others. In the opinion of this eminent investi- 

 gator and his pupils, phagocytosis is the all important factor in the 

 struggle of the body against infective disease. Opposed to this view 

 is that of the " Humoralists." The work of Traube, Von Fodor, 

 Pfeiffer, and Nuttall, to mention only a few, goes to show that certain 

 protective substances are contained in the serum and plasma of the 

 blood, which have the power of destroying defective micro-organisms. 

 From this point -of view, the struggle against infection may be regarded 

 as a sort of scavenging process, the phagocytes taking up and digesting 

 micro-organisms that have been destroyed by other means. Flugge 

 has graphically illustrated this conception by comparing the phagocytes 

 to the trenches made ready behind the fighting line to receive the con- 

 quered dead. 



It might at first sight be thought that these views are incompatible, 

 but a little reflection will show that this is by no means so. It ,is 

 conceivable that the bacterial substances in the plasma or serum are 

 derived from certain cells, and that, therefore, the cells are the impor- 

 tant factor after all. More searching enquiry would seem to bear this 

 out. Long ago, Hankin proved that the leucocytes of immune animals 

 contain bactericidal substances, and the more recent researches of Buch- 

 ner, Vaughan, Denys and Havel, and Eibbert, go to prove that the 

 bactericidal power of blood serum is due to substances derived from 

 the leucocytes. As the matter now stands it may be said that prac- 

 tically all pathologists admit the great importance of phagocytosis in 

 the reaction of ^the organism against infective disease. Metschnikoff 

 on his part has been constrained to modify the original position ^and 

 now accepts the view that there is an extracellular as well as an intra- 

 cellular activity brought into play. The points in debate at the present 

 time appear to be the relative importance of the intra- and extra-cellular 

 activity as well as certain questions in regard to the details of the 

 process by which immunity is brought about. 



The process of immunity has been aptly compared to a conflict, 

 the micro-organisms damaging the tissues by means of the toxic pro- 

 ducts of their metabolism, the invaded organism in its turn endeavour- 

 ing to protect itself against the microbe by the elaboration of a certain 

 defensive mechanism. "When we consider the vital processes of the 

 invader and the invaded we can readily understand that the phenomena 

 of infection and the reaction against infection must be highly compli- 

 cated. From all we know of metabolism in general we may infer with 

 considerable probability that the mechanism of defence consists in the 



