INFECTIOUS DISEASES. 631 



demonstrated in animals inoculated with various pathogenic bac- 

 teria. The writer observed this in his experiments, made in 1881, 

 in which rabbits were inoculated with cultures of his Micrococcus 

 Pasteuri ; and it was this observation which led him to suggest 

 the theory which has since been so vigorously supported by 

 Metschnikoff. But the presence of a certain number of bacteria 

 within the leucocytes does not prove the destructive power of 

 these cells for living pathogenic organisms. As urged by Weigert, 

 Bamngarten, and others, it may be that the bacteria were already 

 dead when they were picked up, having been destroyed by some 

 agency outside of the blood-cells. As heretofore stated, we have 

 now experimental evidence that blood-serum, quite independently 

 of the cellular elements contained in it in the circulation, has de- 

 cided germicidal power for certain pathogenic bacteria, and that 

 the blood-serum of the rat and other animals which have a natu- 

 ral immunity against anthrax is especially fatal to the anthrax 

 bacillus. 



Numerous experiments have been made during the past two 

 or three years with a view to determining whether pathogenic 

 bacteria are, in fact, destroyed within the leucocytes after being 

 picked up, and different experimenters have arrived at different 

 conclusions. But in certain infectious diseases, and especially in 

 anthrax, the bacilli included within the leucocytes often give evi- 

 dence of degenerative changes, which would support the view 

 that they are destroyed by the leucocytes, unless these changes 

 occurred before they were picked up, as is maintained by Nuttall 

 and others. 



Metschnikoff concludes an address delivered at the Pasteur 

 Institute in Paris, in December, 1890, as follows : " It is not pos- 

 sible at the present time to state fully and accurately all these 

 influences which are associated in aiding phagocytic action ; but 

 already we have the right to maintain that, in the property of its 

 amoeboid cells to include and to destroy micro-organisms, the ani- 

 mal body possesses a formidable means of resistance and defense 

 against these infectious agents" This statement, we think, is 

 justified by the experimental evidence relating to phagocytosis. 

 But in view of experimental evidence, to be referred to later we 

 can not accept the so-called Metschnikoff theory as a sufficient 

 explanation for the facts relating to acquired immunity in general, 

 and must regard phagocytosis simply as a factor which, in certain 

 infectious diseases, appears to play an important part in enabling 

 immune animals to resist invasion by pathogenic bacteria. 



Going back to the demonstrated fact that susceptible animals 

 may be made immune by inoculating them with the toxic products 

 produced during the growth of certain pathogenic bacteria, we 

 may suppose either that immunity results from the continued 



