386 PATHOGENIC BACTERIA. 



infection, and are more appropriate for experimental pur- 

 poses than the saliva, which is invariably contaminated 

 with accidental pathogenic bacteria. 



The introduction of a fragment of the medulla ob- 

 longata of a dog dead of rabies beneath the dura mater 

 of a rabbit causes the development of rabies in the 

 rabbit in about six days. The medulla of this rabbit 

 introduced beneath the dura mater of a second rabbit 

 produced a more violent form of the disease in a shorter 

 time, and by frequently repeated implantations Pasteur 

 found that an extremely virulent material could be ob- 

 tained. 



Inasmuch as the toxins of diphtheria and tetanus 

 circulate in the blood, and not infrequently saturate 

 the nervous systems of animals affected, it might be 

 concluded that the material with which Pasteur worked 

 was a toxin. This is readily disproven, however, not 

 only by the fact that the toxin would weaken instead of 

 strengthen by the method of transfer from animal to 

 animal, it not being a vital entity, but also by the dis- 

 covery that when an emulsion of the nervous system of 

 an affected animal is filtered through porcelain, or when 

 it is heated for a few moments to ioo° C, or exposed 

 for a considerable time to a temperature of 75 or 8o° C, 

 its virulence is entirely lost. This would seem to prove 

 that that which is in the nervous system and communi- 

 cates the disease is a living, active body — a parasite, and 

 in all probability a bacterium. However, all endeavors 

 to discover, isolate, or cultivate this organism have failed. 



Pasteur noted that the virulence of the poison was less 

 in animals that had been dead for some time than in 

 the nervous systems of those just killed, and by experi- 

 mentation showed that when the nervous system was 

 dried in a sterile atmosphere the virulence was attenu- 

 ated in proportion to the length of time it had been dry. 

 This attenuation of virulence of course suggested to 

 Pasteur the idea of a protective vaccination, and by in- 

 oculating a dog with much attenuated, then with less 



