TEXT-BOOK OF BACTERIOLOGY. 209 



vessels, thus gaining- the opportunity to exert their terrible action 

 to its full extent. 



The animals on which we usually experiment are generally non- 

 susceptible to the intestinal or alimentary infection. It requires 

 very large quantities of spores, for instance, to infect a Guinea-pig 

 in this manner. When the experiment is successful, a hemorrhagi- 

 ^ally-infiltrated or even ulcerous spot in the intestinal mucous 

 membrane shows the place where the intestinal infection took 

 place. Sheep and cattle, on the other hand, are particularly sus- 

 ceptible to anthrax, and we shall shortly declare that this is the 

 most usual way in which the infection takes place under natural 

 circumstances. 



In our transmissions we must, of course, always take note of 

 these differences, and the susceptibilities of the various species of 

 animals to one and the same kind of infection are important parts 

 of the question. 



It has been found that dogs, the majority of birds, and the am- 

 phibia are almost entirely refractory. If we place even a large 

 quantity of anthrax culture or a portion of infected spleen under 

 the skin of a frog (for example, in the dorsal lymph-sac) it remains 

 unaffected by the attack, and at the place of inoculation will be 

 found a rich store of phagocytes, i.e., of white blood-corpuscles, all 

 filled with bacilli evidently on the high road to decay and dissolu- 

 tion. But if we keep our frog at a high temperature in the in- 

 cubator and employ warmth as a means of favoring the growth of 

 the bacteria, we may nevertheless succeed in bringing them to 

 develop, even on a soil to which they are naturally so averse. The 

 animal dies, and on examination is found to harbor numerous 

 bacilli, mostly arranged in long, curiously- winding, and intricately- 

 tangled threads. 



Of other frequently-employed animals, the white rat is usually 

 but little susceptible. The rabbit is very much more accessible; 

 Guinea-pigs, sheep, and cattle are sure to yield to the infection, and 

 at the top of the list stand the white mice, w r hich form a never-fail- 

 ing object of experiment. 



The pathogenic effects of bacteria, as stated, are referred to 

 their producing certain peculiar excretions which are the true 

 cause of disease, and are, therefore, the most important item in the 

 whole process. In the case of some micro-organisms we have suc- 

 ceeded, as already noted, in defining these substances in a tangible 

 form and fixing their nature. 



The attempts made to reach this point as regards the anthrax 

 bacilli have not as yet been entirely successful. Hoffa, indeed, has 

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