m PASTEUR AND HYDROPHOBIA 141 



This discovery is the starting-point of all Pasteur's 

 further work. It enabled him to experiment with 

 sufficient certainty as to results. It has rendered it 

 possible for him to determine whether a dog is really 

 affected with rabies or not, by killing it and inoculat- 

 ing the brain of a second dog with the spinal cord of 

 the dead dog, and similarly to determine whether a 

 human being has really died of hydrophobia {ixtbies 

 hominis) or not. It has also enabled him to propa- 

 gate with certainty the disease from rabbit to rabbit 

 through ninety successive individuals — extending over 

 a period of three years — and to experiment on the 

 result of varying the quantity of virus introduced as 

 well as on the result of passing the virus from one 

 species of animal to another, and back again to the 

 first species [e.g. rabbit as the first and monkey as the 

 second species). Before Pasteur's time Eossi, con- 

 firmed by Hertwig, had used nerve-tissue for inocula- 

 tion with less definite results. Pasteur has the merit 

 of establishing this method as the really efficient one 

 in experimenting on the transmission of rabies. 



Using the nerve-tissue, Pasteur has determined by 

 several experiments that when a large quantity of 

 virus (that is to say, of the medulla oblongata of a 

 rabid rabbit pounded up in a perfectly neutral or 

 sterilised broth) is injected into the veins of a dog, 

 the incubation period is seven or eight days ; by using 

 a smaller cjuantity he obtained an incubation period 

 of twenty days, and by using a yet smaller quantity 

 one of thirty-eight days. It is very important to note 

 that by using a still smaller dose Pasteur found that 



