SUSCEPTIBILITY AND IMMUNITY. 253 



occasions a slight and passing disturbance in susceptible members of the 

 same species. But here again we are dealing with fluids withdrawn from 

 the body, and so modified in various ways. Let us make an observation 

 more strictly to the point. Take, for instance, a rabbit vaccinated against 

 anthrax and inoculate it with anthrax bacilli, thus allowing these to exist 

 directly within the refractory organism. Such bacilli as are not destroyed 

 preserve their virulence for a sufficiently long period, and it is possible to 

 kill a guinea-pig with a drop of exudation, taken from the region of injection 

 thirty hours after subcutaneous inoculation, eight days after inoculation 

 into the anterior chamber of the eye. A sojourn of so long duration within 

 the vaccinated organism, then, has not deprived the microbes of their viru- 

 lence, although twenty-four hours suffice to completely attenuate the 

 bacilli cultivated in the removed blood of vaccinated sheep. 



Years ago it was established in M. Pasteur's laboratory that the refrac- 

 tory organism, instead of being an unfavorable soil for the preservation of 

 virulence, tends the rather to reinforce this property. To exalt the viru- 

 lence of an attenuated microorganism, one always employs, not animals 

 very susceptible to the specific disease, but those which are slightly suscep- 

 tible, or it may be, under many circumstances, refractory. In this manner 

 the most active anthrax virus has usually been obtained by passage through 

 birds, notably fowls ; the greatest virulence of chicken cholera was gained 

 by passage through the vaccinated cock ; and quite recently M. Malm has 

 shown that passage of the anthrax bacillus through the organisms of dogs, 

 which of all mammals are the* most refractory in this respect, increases its 

 virulence in a most remarkable manner, so that the general law may be laid 

 down that an organism which is but slightly susceptible or is refractory is 

 able not only to preserve, but even to exalt, the virulence of bacteria. The 

 principal argument in favor of the hypothesis that pathogenic microor- 

 ganisms become simple inoffensive saprophytes when they find themselves 

 in a refractory region, loses therefore its raison d'etre. 



M. Bouchard, in his objection to the theory of phagocytosis, may be re- 

 garded as introducing but a modification of this hypothesis. He holds that 

 pathogenic bacteria placed under favorable conditions give rise to substances 

 which hinder the inflammatory process, and that only when these inhibi- 

 tory substances are inadequately represented do the cells intervene. When, 

 therefore, the organism rendered refractory by vaccination becomes an un- 

 favorable soil for the production of these inhibitory bodies, the bacteria can 

 no longer prevent the inflammatory reaction ; free emigration of the leuco- 

 cytes ensues, these cells seize upon the impotent microbes and put a stop to 

 their further growth. In this theory the part played by the phagocytes is 

 again secondary, depending upon a dearth of anti-inflammatory substance. 



If the theory could be accepted in certain cases, it is nevertheless inap- 

 plicable as a general rule. In all those affections which are characterized by 

 the absence of leucocytes upon the field of battle there is certainly no lack 

 of inflammation. The very reverse obtains. In anthrax affecting small 

 mammals, just as in the vibrionic septicaemia of pigeons and guinea-pigs, and 

 other analogous diseases, we find that there is a very distinct dilatation of 

 the vessels, accompanied by great exudation ; the inflammatory reaction is 

 well marked ; nothing is wanting save the determination of the white coi'- 

 puscles. Or, employing yet further that affection which is, as it were, the 

 touchstone of the bacteriologist, a still clearer proof of our contention is to 

 be gained if we inoculate a rabbit on the one ear with a small quantity of 

 virulent, on the other with a like quantity of attenuated, anthrax virus. In 

 the course of a few hours the external signs of inflammation are far more 

 conspicuous in the former ; the vessels are greatly enlarged and there is 

 literally a huge exudation of clear serous fluid into the part ; in the latter 

 the external signs are less prominent, but examination of the seat of inocu- 

 lation shows it to be packed with leucocytes. Consequently, the phenome- 

 non we are discussing is to be explained, not by an absence of the inflamma- 

 tory process, but much more satisfactorily by a negative chemiotaxis of the 



