148 TEXT-BOOK OP BACTERIOLOGY. 



another case, as Metschnikoff supposes, the cells are the active 

 agents, or that chemical properties of the blood or of the tissue 

 juices participate ; or, lastly, it is possible and even probable that 

 causes are in operation which are as yet unknown, and which 

 future investigators are destined to discover. 



In the second and third cases there would be an artificial in- 

 crease of the power given to the body by nature for resisting the 

 attacks of micro-organisms. With the phagocytes there would be 

 an augmentation of their peculiar function in the blood, an increase 

 of bacteria-killing power. The Jatter, too, would indeed have to be 

 referred also to a special action on the part of the cells. A re- 

 active change in the fixed tissue element under the influence of 

 definite bacterial products, which shows itself in a change of the 

 composition of the blood, would then have to be regarded as the 

 cause of the phenomena; the time elapsing between the protective 

 inoculation and the acquired immunity would be the time required 

 by the organism for the development of its mysterious powers, and 

 the strengthening of that degree of resisting power which has 

 been given to it by nature. 



I have considered these matters at such length, notwithstanding 

 their uncertainty and vagueness, because, as I have already said, 

 they are closely connected with the important question of the heal- 

 ing of infectious diseases. It is true that we cannot speak so much 

 of a healing as of a means of protection, but we see before us a pos- 

 sibility of mastering the most dangerous enemies of human life and 

 suppressing their destructive effects. A healing in the strict sense 

 of the term would indeed not be effected without our being able to 

 check and artificially remove the affection when it was already in 

 progress. This has also been attempted, and the present is per- 

 haps the best opportunity to consider the progress hitherto made 

 in this direction. The most natural way of proceeding is, of course, 

 to combat the further increase of the bacteria already in the body, 

 by the use of means known to possess qualities hostile to bacterial 

 life and calculated to kill bacterial germs. The natural powers of 

 the organism are in this case not called into requisition ; but an 

 artificial aid, to which they stood in no relation previously, is 

 offered to them. 



Unfortunately, the results obtained by this method have by no 

 means fulfilled the hopes with which it was first welcomed. All 

 the substances which, outside the body in the test-tube, have a 

 decided influence on the micro-organisms are powerless in the living 

 tissues unless they are employed in doses which would be directly 

 deleterious to the body, and would destroy it even more quickly 



