INFECTIOUS DISEASES AND MICROBES, ETC. 187 



system, where the virus is obtained in a pure state. 

 This pure virus is continually being inoculated on 

 the surface of the brain of healthy animals; the 

 object of this is to keep up the supply of the virus, 

 in question. The virus can be intensified or modi- 

 fied by passing it through various animals. For 

 instance, by passing it from the dog to the monkey, 

 and subsequently from monkey to monkey, the 

 virus grows weaker at each passage, until its 

 virulence entirely disappears. Successive passages 

 from rabbit to rabbit, and from guinea-pig to 

 guinea-pig, increase the virulence of rabies virus. 

 The intensified virus comes to a fixed maximum 

 in the rabbit. If now transferred to the dog it 

 remains intensified, and shows itself to be much 

 more virulent than the virus of ordinary street 

 rabies. So great is this acquired virulence, that 

 the intensified virus injected into the blood- 

 system of a dog unfailingly gives rise to mortal 

 madness. These facts suggested to Pasteur that, 

 by keeping a set of attenuated viruses of different 

 strength, some not mortal, he could preserve the 

 animal economy against the ill effects of more 

 active ones, and these latter against the effects of 

 mortal ones. 



The sets of attenuated viruses are not obtained by 

 the passage of the virus through different animals, 

 for the method now in use at the Pasteur Institute 

 consists in suspending portions (a few centimetres 

 in length) of the spinal cords of inoculated rabbits 

 in a dry atmosphere (i.e. the marrows are desiccated 

 in sterilised bottles of one litre capacity by means 



