Summary 567 



cerned in the absorption of micro-organisms. This absorption being 

 carried out by an act of intracellular digestion, the study of immunity 

 comes into the chapter on digestion regarded from the general point 

 of view. 



As in the struggle of the body of the animal against infective 

 agents the phagocytes play the principal part, it happens that in 

 certain diseases the micro-organisms in order to manifest their 

 morbific effect must be protected from the attacks of these 

 defensive cells. It is for this reason that the cholera vibrio, 

 which is not very injurious when introduced below the skin of the 

 human subject, becomes very formidable when it succeeds in gaining 

 access to the digestive canal. Incapable of maintaining a struggle 

 against the phagocytes, the vibrio is able to overcome in the stomach 

 and in the intestines without difficulty the obstacles which.it here 

 meets with. It is for this reason that the channel of entrance of the 

 micro-organisms at times plays such a prominent role in immunity 

 against infective diseases. 



The question is often asked whether a theoretical study of 

 immunity is capable of rendering service in the search for means 

 of conferring immunity on the animal. It must not be forgotten that 

 theory and practice frequently march side by side, but that sometimes 

 they advance without very much regard for each other. Thus the 

 first preventive inoculations against snake-bite, small-pox, and pleuro- 

 pneumonia, attempted by laymen were evidently made independently 

 of any theoretical ideas of any kind, but were guided by the purest 

 empiricism. On the other hand, the theoretical researches on the 

 nature and origin of ferments led to the discovery of vaccinations 

 by means of micro-organisms and microbic products which have 

 rendered immense services to practical medicine. 



The discovery of antitoxins, so rich in practical applications, was 

 influenced by theoretical researches on the mechanism of immunity. 

 Yon Behring began his important series of investigations on this 

 subject with the study of the immunity of rats against the anthrax 

 bacillus. It did not suggest itself to anyone to suppose that this [592] 

 question could have the slightest immediate practical interest ; never- 

 theless, starting from this investigation, von Behring, after giving up 

 the theory of the bactericidal property of the body fluids as a cause of 

 immunity, advanced, step by step, to the discovery of the antitoxic 

 power of the serums. When a study of the properties of the blood of 



