NATURAL IMMUNITY 71 



In my chapter on " Bacteria in Infectious Diseases," 

 in Bacteria, published in the spring of 1884, but 

 placed in the hands of the publishers in 1883, I say : 



"Jf we add a small quantity of culture fluid containing the 

 bacteria of putrefaction to the blood of an animal, withdrawn 

 from the circulation into a proper receptacle, and maintained in 

 a culture oven at blood-heat, we shall find that these bacteria 

 multiply abundantly, and evidence of putrefactive decomposition 

 will soon be perceived. But if we inject a like quantity of the 

 culture fluid, with its contained bacteria, into the circulation of 

 a living animal, not only does no increase and no putrefactive 

 change occur, but the bacteria introduced quickly disappear, 

 and at the end of an hour or two the most careful microscopical 

 examination will not reveal the presence of a single bacterium. 

 This difference we ascribe to the vital properties of the fluid as 

 contained in the vessels of a living animal, and it seems probable 

 that the little masses of protoplasm known as white blood-cor- 

 puscles are the essential histological elements of the blood, so 

 far as any manifestation of vitality is concerned. The writer 

 has elsewhere (1881) suggested that the disappearance of the bacteria 

 from the circulation, in the experiments referred to, may be effected 

 by the white corpuscles, which, it is well known, pick up, after the 

 manner of amoebae, any particles, organic or inorganic, which 

 come in their way. And it requires no great stretch of credulity 

 to believe that they may, like an am&ba, digest and assimilate the pro- 

 toplasm of the captured bacterium, thus putting an end to the possi- 

 bility of its doing any harm. 



" In the case of a pathogenic organism we may imagine that, 

 when captured in this way, it may share a like fate if the captor 

 is not paralysed by some potent poison evolved by it, or over- 

 whelmed by its superior vigour and rapid multiplication. In the 

 latter event the active career of our conservative white corpuscles 

 would be quickly terminated, and their protoplasm would serve 

 as food for the enemy. It is evident that in a contest of this kind 

 the balance of power would depend upon circumstances relating 



