IMMUNITY. 169 



Pasteur has employed another method of attenuat- 

 ing the virus of rabies. The spinal cord of inoculated 

 rabbits is removed with all possible precautions, and 

 portions a few centimetres in length are suspended 

 in flasks in which the air is dried by fragments 

 of potash. By this process the virulence is found 

 to gradually diminish and finally disappear. 



Animals inoculated with portions of these cords, 

 after suspension for a certain time, are rendered 

 refractory to inoculation with virulent cords. 

 Having rendered dogs, which had been previously 

 bitten, free from the supervention of symptoms of 

 hydrophobia by means of protective inoculation, 

 Pasteur proceeded to apply the same treatment to 

 persons bitten by rabid animals, with results which 

 tend to the belief that a prophylactic for rabies has 

 been found, though this must still be considered to 

 be sub judice. 



The question as to what constitutes immunity 

 is a vexed one. 



Raulin has shown that Aspergillm niger develops 

 a substance which is prejudicial to its own growth 

 in the absence of iron salts in the nutrient soil. 

 Pasteur has suggested^ that in rabies side by side 

 with the living and organised substance there is 

 some other substance which has, as in Raulin's 

 experiment, the power of arresting the growth 

 of the first substance. If we accept the theory 

 of arrest by some chemical substance, we must 

 suppose that in the acquired immunity afforded by 



