266 president's address — section l. 



syphilis. As Theobald Smith well puts it, '^ There is another type 

 o*^ parasite which may dispense largely with both offensive and 

 defensive processes. We can conceive of this type as exerting a 

 metabolic activity approximating so closely to that of the host that 

 the latter reacts but slightly, and then only after a long period 

 of stimulation." At the same time, the danger of spread to other 

 organs or of generalization is always existent. 



Sometimes, on the other hand, the insidious nature of the infec- 

 tion, and its avoidance of serious irritation, produces a state almost 

 of commensalism. The germ is not destroyed, but is practically 

 impotent, antibodies continue to be produced, and so protect the 

 body generally. Something like this appears to be the case with 

 the protozoan parasite of " tick fever," and the same may be said 

 of Bang's aboatioii bacillus located in the cow's mammary gland. 



But we must pass on to consider some of the mechanisms of 

 defence, and first phagocvtosis. 



Phagocytosis. 



The importance of phagocytosis as a defensive mechanism aud 

 the vxnderstanding of the conditions surrounding it must always be, 

 associated with the genius of Metchnikoff. A classic observation 

 of his introduces us to the heart of the subject. A normal rabbit 

 was inoculated subcutaneously in one ear with a fresh virulent 

 culture of anthrax, in the other ear with an attenuated culture of 

 the same organism. In the latter case diapedesis of leucocytes was 

 rapid, and phagocytosis of the organisms was soon in full pro^- 

 gress with the formation of a small purulent focus. In the former 

 a gelatinous blood-stained oedema occurred with practically no 

 leucocytes and no phagocytosis, whilst the organism multiplied 

 rapidly, became generalized, and killed the animal. But now two 

 other rabbits were taken, the one normal, the other previously 

 vaccinated by Pasteur's method against anthrax, and both were 

 inoculated with a virulent culture of anthrax. In the vaccinated 

 animal therei was observed marked diapedesis and rapid and com- 

 plete phagocytosis of the virulent organism, whilst the unprotected 

 rabbit died oi acute anthrax infection. Thus was demonstrated 

 the efficacy of phagocytosis, and Metchnikoff asserted that the ulti- 

 mate result of the infection of an animal with bacteria depended 

 essentially on the efficiency of its phagocytary powers. That this* 

 is a complete statement of the position cannot now be accepted, 

 but, despite numerous attempts to belittle its significance, phago- 

 cytosis still stands in the forefront of protective measures . 



Two of the main methods which have been used to investigate 

 the phenomena associated with phagocytosis may be mentioned. 

 The first is by the injection of such substances as aleurone grains 

 or dead bacteria, or, what answers quite well, a few c.c. of 

 bouillon, into the peritoneal cavity of a guinea pig or rabbit. 

 Such an injection causes the pouring out of an inflammatory 



