AND THE HEALING ART 507 



have seen that Pasteur discovered that microbes might under some circumstances 

 undergo mitigation of their virulence. He afterwards found that under different 

 conditions they might have it exalted, or, as he expressed it, there might be a 

 renforcement du virus. Such proved to be the case with rabies in the rabbit ; 

 so that the spinal cords of animals which had died of it contained the poison in 

 a highly intensified condition. But lie also found that if such a highly virulent 

 cord was suspended under strict antiseptic precautions in a dry atmosphere at 

 a certain temperature, it gradually from day to day lost in potency, till in course 

 of time it became absolutely inert. If now an emulsion of such a harmless cord 

 was introduced under the skin of an animal, as in the subcutaneous administra- 

 tion of morphia, it might be followed without harm another day by a similar dose 

 of a cord still rather poisonous ; and so from day to day stronger and stronger 

 injections might be used, the system becoming gradually accustomed to the 

 poison, till a degree of virulence had been reached far exceeding that of the bite of 

 a mad dog. When this had been attained, the animal proved incapable of taking 

 the disease in the ordinary way ; and more than that, if such treatment was 

 adopted after an animal had already received the poison, provided that too long 

 a time had not elapsed, the outbreak of the disease was prevented. It was 

 only after great searching of heart that Pasteur, after consultation with some 

 trusted medical friends, ventured upon trying this practice upon man. It has 

 since been extensively adopted in various parts of the world with increasing 

 success as the details of the method were improved. It is not of course the 

 case that every one bitten by a really rabid animal takes the disease ; but the 

 percentage of those who do so, which was formerly large, has been reduced 

 almost to zero by this treatment, if not too long delayed. 



While the intensity of rabies in the rabbit is undoubtedly due to a peculiarly 

 virulent form of the microbe concerned, we cannot suppose that the daily dimi- 

 nishing potency of the cord suspended in dr}' warm air is an instance of attenua- 

 tion of virus, using the term ' virus ' as synonymous with the microbe concerned. 

 In other words, we have no reason to believe that the special micro-organism of 

 hydrophobia continues to develop in the dead cord and produce successi\-ely a 

 milder and milder progeny ; since rabies cannot be cultivated in the nervous 

 system of a dead animal. We must rather conclude that there must be some 

 chemical poison present which gradually loses its potency as time passes. And 

 this leads me to refer to another most important branch of this large subject of 

 bacteriology, that of the poisonous products of microbes. 



It was shown several years ago by Roux and Yersin. working in the Institut 

 Pasteur, that the crust or false membrane which forms upon the throats of 

 patients affected with dij^htheria contains bacteria whicli can boculti\ated outside 



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