TEXT-BOOK OF BACTERIOLOGY. 



poison and prepare the way for resisting more virulent successors. 

 To overcome this difficulty, it would be necessary to . suppose that 

 the reception of attenuated germs acts upon the cells only as a 

 specific stimulant, to which they answer by a functional reaction, 

 and that this stimulating power exists in the same degree and 

 works in the same manner also in the bacterial products. 



Another objection which we have already alluded to is perhaps 

 still more serious. We know that a number of very celebrated in- 

 vestigators are of opinion that the leucocytes are only able to re- 

 ceive bacteria which have been killed, or at least weakened, by 

 some influence or other, and that they cannot master a healthy 

 living enemy. In fact, an agency outside the cells whose existence 

 was formerly only a conjecture has lately become known to us in 

 the power of the serum of the blood to destroy bacteria. How far 

 this fact may be able to explain the phenomenon of acquired im- 

 munity cannot yet be stated with certainty. Lubarsch justly re- 

 marks that the experiments hitherto made have shown no differ- 

 ence between the power of the blood in susceptible animals and in 

 those artificially rendered insusceptible, whereas we might expect 

 such a difference if we regard this power in the blood as the chief 

 agent in removing the micro-organisms from bodies protected by 

 inoculation. 



It is possible that the future may throw more light on this dark 

 question. But even if we do place peculiarities of the serum, in- 

 stead of direct cellular influence, in the foreground of the picture, 

 we only touch those diseases whose exciters increase and exert 

 their influence within the vascular system, as the strictly septicce- 

 mic diseases, such as inoculated anthrax, chicken-cholera, and swine- 

 erysipelas. In other diseases the blood holds a secondary place. 

 For the toxic species of bacteria, for instance, we should be com- 

 pelled to admit a possibility which lies entirely outside the region 

 to which we have hitherto confined our observation; we should 

 have to admit that in this case the body gradually accustoms 

 itself to the poison. The adaptability of our organism to bear the 

 action of several kinds of poison, provided they be administered in 

 slowly-increasing doses, is great, and the influence is therefore justi- 

 fiable that a similar adaptability may here also play an important 

 part. One indubitable fact may certainly be gathered from this 

 mass of contradictory views and observations, namely, that arti- 

 ficial immunity is acquired not by one regular process, but some- 

 times in this manner and sometimes in that. It is possible that 

 chemical substances retained in the body (as supposed by the hypoth- 

 esis of retention) are often the cause of the phenomenon ; that in 



