PROPHYLACTIC TREATMENT OF HYDROPHOBIA 517 



lengthens the period of incubation ; but, as we shall see 

 presently, this is an extremely important effect. 



The work of Pasteur has, however, revolutionised the whole 

 treatment of wounds inflicted by hydrophobic animals. Pasteur 

 started with the idea that, since the period of incubation in the 

 case of animals infected subdurally from the nervous systems of 

 mad dogs is constant in the dog, the virus has been from time 

 immemorial of constant strength. Such a virus, of what might 

 be called natural strength, is usually referred to in his works as 

 the virus of la rage des rues, in the writings of German authors 

 as the virus of die Strasswuth. Pasteur found on inoculating a 

 monkey subdurally with such a virus, and then inoculating 

 a second monkey from the first, and so on with a series of 

 monkeys, that it gradually lost its virulence, as evidenced by 

 lengthened periods of incubation on subdural inoculation of 

 dogs, until it wholly lost the power of producing rabies in dogs, 

 when introduced subcutaneously. When this point had been 

 attained, its virulence was not diminished by further passage 

 through the monkey. On the other hand, if the virus of la 

 rage des rues were similarly passed through a series of rabbits 

 or guinea-pigs, its virulence w T as increased till a constant strength 

 (the virus fixe) was attained. Pasteur had thus at command 

 three varieties of virus that of natural strength, that which had 

 been attenuated, and that which had been exalted. He further 

 found that, commencing with the subcutaneous injection of a 

 weak virus and following this up with the injection of the 

 stronger varieties, he could ultimately, in a very short time, 

 immunise dogs against subdural infection with a virus which, 

 under ordinary conditions, would certainly have caused a fatal 

 result. He also elucidated the fact that the exalted virus con- 

 tained in the spinal cords of rabbits such as those referred to, 

 could be attenuated so as no longer to produce rabies in dogs by 

 subcutaneous injection. This was done by drying the cords in 

 air over caustic potash (to absorb the moisture), the diminution 

 of virulence being proportional to the length of time during 

 which the cords were kept. Accordingly, by taking a series of 

 such spinal cords kept for various periods of time, he was 

 supplied with a series of vaccines of different strengths. Pasteur 

 at once applied himself to find whether the comparatively long 

 period of incubation in man could not be taken advantage of to 

 " vaccinate " him against the disease before its gravest manifesta- 

 tion took place. The following is the record of the first case 

 thus treated. The technique was to rub up in a little sterile 

 bouillon a small piece of the cord used, and inject it under the 



