240 ACQUIRED IMMUNITY 



now in use. Rabies develops slowly in an infected person. The 

 microbes inhabit especially the great nervous centres, the brain and 

 spinal cord. Pasteur dried a series of cords from infected rabbits, 

 made emulsions from them, and injected the emulsions into in- 

 dividuals who had acquired the disease, but had not as yet 

 developed the symptoms. He began with a cord which had been 

 artificially dried for fourteen days. Next he used in succession 

 fresher cords, till he injected material from one which was abso- 

 lutely fresh, and which would have infallibly communicated the 

 disease had the previous treatment not been undergone. As a 

 result the individual, so far from acquiring the disease, developed 

 immunity. 



402. (h) Pasteur subjected the bacilli of anthrax, for a longer 

 or shorter time, to an abnormal degree of heat. He found they 

 became * attenuated' (i.e. lost their virulence) in proportion. If, 

 now, sheep were inoculated, first with bacilli of little virulence, 

 and then in succession with those of greater virulence, they could 

 eventually be rendered immune to those of the greatest virulence. 

 Attempts to pass directly from weak to strong bacilli led to the 

 death of the infected sheep, (z) A few microbes are not, as a rule, 

 sufficient to infect even a very susceptible individual. They are 

 destroyed by the phagocytes, and tend merely to induce an in- 

 crease of resisting power. Rabbits are susceptible to anthrax, and 

 young animals more susceptible than older individuals. It has 

 been found that not less than 16,000 bacilli must be injected into 

 a young rabbit to induce the disease. 



403. (/) If diphtheria bacilli be cultivated in broth, outside the 

 body, they render the broth intensely poisonous with their toxins. 

 If the broth be injected, at first in small but afterwards in larger 

 doses, into a horse, the animal acquires immunity, and remains in 

 good health during the rest of the treatment. But immediately 

 after a large dose his blood-serum is poisonous to other animals 

 which have not been similarly treated. If, however, the serum be 

 withdrawn from him after a fit period, it is not only harmless, but 

 helps another animal (e.g. man), who is suffering from the actual 

 disease, to recover ; or, if not suffering, it renders him, for a time 

 at least, insusceptible. This is the celebrated antitoxin treatment 

 which has greatly reduced the mortality from diphtheria, and is 

 yearly being extended to other diseases, the medical treatment of 

 which was formerly of little value. (/) Roux found it possible to 

 obtain at different times two lots of serum of equal value from a 

 prepared horse, though no intermediate injection of toxin had 



