124 TEXT-BOOK OF BACTERIOLOGY. 



Under natural conditions, their importance, especially for man- 

 kind, is not great. It seldom occurs that large quantities of such 

 substances formed by bacteria outside the body are taken into it 

 on any one occasion. Such a possibility comes seriously in ques- 

 tion only in exceptional cases — for instance, in cases of meat-poison- 

 ing — and it is chiefly from a therapeutical point of view that the 

 purely toxic micro-organisms are interesting to us. 



The great majority of all known bacteria, particularly the par- 

 asitic kinds, can in this manner, as experiments on animals have 

 proved, be made to exercise a pathogenic action. But taken in its 

 more restricted, proper sense, the word " pathogenic " is applied 

 only to a comparatively small definite class of micro-organisms, 

 which form a very striking contrast to those just treated of. 



They possess, in fact, the capacity to multiply' indefinitely within 

 the organism which they invade. From the first original cell fresh 

 members are continually proceeding, and in this manner are pro- 

 duced such enormous accumulations of bacteria in the living 

 organs as in the case of the anthrax preparation alluded to. Here 

 the excretions, the bacterial poisons, originate w^ithin the body, and 

 in ever-increasing proportions, so that the quantity of germs which 

 invaded that body at its first infection is a matter of indifference. 



The difference which separates these infectious species from the 

 toxic ones is very essential. The first are transmissible in the 

 smallest quantities — i.e., they are always able to multiply within a 

 susceptible organism. With the others that is not the case. If 

 they find entrance into a body in such large quantities as to poison 

 it, they probably find their way through the circulating blood into 

 its different organs, and may be discovered there. But it would be 

 a great error to confound such a case, in which the individual bac- 

 teria only play a passive part, being merely floated away and 

 stranded again, with the other case in which the infectious species 

 penetrate actively into the tissues and grow within the body. 



To what must we refer the capability of certain micro-organisms 

 to live in warm-blooded organisms, to multiply and produce their 

 poisonous excretions in them ? Is this a constant, invariable prop- 

 erty of certain bacteria, or is this particular expression of patho- 

 genic agency subject to variations which influence or obliterate 

 the distinction between them and the harmless species ? 



We are still far from a satisfactory solution of these questions, 

 but the last few years have yielded us so many valuable facts bear- 

 ing upon them that we already have — or believe we have — firm 

 ground under our feet as regards some few points. 



We will allow the facts to speak for themselves. The very first 



