666 APPENDIX. 



had actually been bitten by a rabid, or presumably 

 rabid, animal, Pasteur continued his experiments, in 

 order to see whether it would not be possible to cure a 

 patient already bitten. He carried on, therefore, a 

 series of experiments which led to the discovery that if 

 the process of inoculation be begun within five days of 

 the bite in animals in which the incubation period was 

 at least fourteen days, almost every animal bitten can 

 be saved; and that even if the treatment be com- 

 menced at a longer interval after the bite a certain 

 proportion of recoveries can be obtained. Thus the 

 application of this method of treatment to the human 

 subject was not tried until it had been proved in 

 animals that such protection could be obtained, and 

 that such protection would last for at least two years, 

 and probably longer. 



The chance of success in the human subject appears 

 to be even greater than in the dog or rabbit, seeing that 

 on account of the resistance offered by the human tissues 

 to the virus the period of incubation is comparatively 

 prolonged; very rarely, if ever, does an outbreak of the 

 disease in man occur before an interval of at least fifteen 

 days. The first symptoms usually appear in the fifth 

 or sixth week, sometimes not until the third month ; 

 exceptionally the incubation period has lasted for a 

 year. Thus there is an opportunity of obtaining 

 immunity by beginning the process of vaccination 

 soon after the bite has been inflicted, the protection 

 being complete before the incubation period has passed. 

 In his earlier experiments Pasteur injected on each 

 succeeding day emulsions from a cord dried for one 

 day less until cords dried five days were reached; but 

 later he used those dried for only three days. This 



