28 BACTERIOLOGY. 



of comparatively few bacilli in the blood ; and, again, the 

 blood may teem with parasites such as the flagellated monads in 

 well-nourished, healthy- looking rats, without apparently causing 

 any symptoms whatever. In the same category may be placed the 

 theory that eminently aerobic organisms seize upon the oxygen of 

 the blood and produce death by asphyxia. Another explanation is 

 afforded by the suggestion of interference with the functions of the 

 lung and kidney by mechanical blocking of the capillaries. Here 

 the same objection is met with in the case of anthrax, the same 

 fatal result may occur with only a few bacilli, while other cases 

 yield very beautiful sections, looking like injected preparations from 

 the mapping out of the capillaries with the countless crowds of 

 bacilli. 



Putrefactive bacteria derive their necessary elements from com- 

 plex organic substances, and accompanying the residue we find the 

 presence of poisonous substances. Pathogenic bacteria, in a similar 

 way, give rise to virulent poisons. Anthrax bacilli produce poisonous 

 principles in the blood which cause death, independently of the 

 number of bacilli, provided there are sufficient present to develop a 

 fatal dose. 



It has been also suggested that possibly a special ferment is 

 .secreted by some organisms, and that by the changes ultimately 

 wrought by the action of this ferment the symptoms and phe- 

 nomena of disease arise. We have an analogy with this theory 

 in the alkaline fermentation of urine by means of the torula urese. 

 By the researches of Musculus, and later of Sheridan Lea, it has 

 been shown that a ferment is secreted by the cells which can be* 

 isolated in aqueous solution, and is capable of rapidly inducing an 

 active fermentation of urea. 



We can now understand how it is that in anthrax or in tuber- 

 culosis we may find the presence of only a few bacilli, or that in 

 tetanus we can have such a violent disturbance of the system 

 produced by the presence of very few micro-organisms. We may 

 conceive that different species of bacilli may vary greatly in their 

 power of producing a toxin or secreting a ferment, just as the 

 elaboration of pigment is much more marked in some species than 

 in others ; thus it need not follow that the number of micro- 

 organisms bears any relation to the virulence or activity of the 

 .substance they produce. There is, however, yet another factor in 

 the production of disease. We know that in health we are proof 

 against most of these micro-organisms ; if it were not so, we should 

 all rapidly fall victims to the tubercle bacillus or others, which in 



