SUSCEPTIBILITY AND IMMUNITY. 263 



passing from one medium to another by successive slight modifications in 

 the fluid of growth, it is easy to make bacteria live in fluids which, when 

 the change of environment has been abrupt, swiftly lead to their destruc- 

 tion. 



In order to gain an idea as to the part played in the refractory animal by 

 the fluids and the phagocytes respectively, the endeavor has been made to 

 separate the two by placing under the skin of frogs (which are naturally 

 immune to anthrax) minute packets formed of filter paper or of animal 

 membrane, and containing the bacilli. The paper, while permitting the 

 passage of fluid, wards off the wandering amoeboid cells for a certain time. 

 Shielded in this way from the phagocytes, though exposed to the action of 

 the juices, the bacilli grow well and produce the characteristic felted mass 

 of anthrax filaments. Baumgarten has not been able to confirm this experi- 

 ment, but Hueppe and Lubarsch have repeatedly verified it. 



But it is not even necessary to take these precautions in order to assure 

 one's self that anthrax spores germinate in the juices of refractory animals. 

 Recently, for instance, M. Trapeznikoff has found that, when these spores 

 are injected into the dorsal lymph sac of the frog, they constantly tend to 

 develop into bacilli, whose further growth is stopped by the phagocytes, 

 which include them, along with such spores as have not had time to germi- 

 nate. Eventually the bacilli so absorbed are digested by their hosts, while 

 the included spores remain intact, although incapable of giving birth to 

 bacilli for so long a time as the phagocytes remain alive. And I might ad- 

 duce other similar cases. Such a comparative examination proves that in 

 the living body the bactericidal property resides in the phagocytes and not 

 in the fluids. 



Still, it may be urged that possibly these cells, which can thus devour and 

 destroy the living microbes, are only in a position to attack bacteria whose 

 virulence has already been lessened by other means Were this so, the mi- 

 crobes present in a refractory organism should behave, not like parasites, 

 but as simple, inoffensive saprophytes. Hence these microbes powerless 

 to produce upon a refractory soil the toxic substances which render them 

 pathogenic and dangerous should easily be included and destroyed; so 

 that, according to this hypothesis, which "has frequently been brought for- 

 ward, the phagocytes play a purely secondary and dependent part, waiting 

 until the microbes are weakened before they seize upon them. In favor of 

 this view the fact has been cited that certain microorganisms cultivated in 

 the blood, or serum, of vaccinated animals become attenuated, so that they 

 no longer induce a fatal disease. The Bacillus anthracis grown in the blood 

 of vaccinated sheep no longer kills rabbits, and, according to Roger, the 

 Streptococcus erysipelatos grown in the blood of vaccinated rabbits only 

 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 



