PROPHYLAXIS OF HYDROPHOBIA. 471 



duced 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 was 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 ulti- 

 mately, 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 contained 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 injec- 

 tion. 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 com- 

 paratively long period of incubation in man could not be 

 taken advantage of to " vaccinate " him against the disease 

 before its gravest manifestation 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 hypodermically by means of a Pravaz's 

 syringe. The first injection was made with a very attenu- 

 ated virus, i.e., a cord fourteen days old. In subsequent 

 injections the strength of the virus was gradually increased, 

 as shown in the table : 



July 7, 1885, 9 A.M., cord of June 23, i.e. 14 days old. 



7 6 P.M. 25 ,, 12 



,, 8 ,, 9A.M. ,, 27 ,, ii ,, 



8 6 P.M. 29 9 



