152 IMMUNIZATION 



on the distance of the infected wound from the central 

 nervous system, and may sometimes last for one 

 month. Owing to this it was possible to Pasteur 

 to check the illness by inoculation of weakened 

 rabies virus. 



After active immunization of an animal which has 

 been immunized before against the same bacilli the 

 time of incubation is sometimes characterized by a 

 diminution of the content of antibody — this is the 

 so-called " negative phase." The said decrease is 

 regularly observed with the immunization of horses 

 against diphtheric toxin, whereby antidiphtheric 

 serum is prepared. Thus Salomonsen and Madsen 

 found that the antitoxin content of a horse went 

 down one time from ioo to 65 units, another time 

 from 120 to 105 units, to rise subsequently above its 

 initial value. A similar decrease was also observed 

 after every bleeding (at which seven litres of blood 

 were taken for the preparation of antidiphtheric 

 serum); in one case the fall was from 120 to 105 

 units, another time from 85 to 70 units. 



After the end of the time of incubation an enor- 

 mous increase of the quantity of antitoxin takes 

 place. As an example may be cited the following 

 table from Madsen's and Jorgensen's investigation, 

 illustrated by the diagram, Fig. 36. The animal 

 treated was a goat which had been used for similar 

 experiments before, so that it contained a little 

 initial quantity — designated as four units — at the 

 time of injection. This quantity decreases a little 

 — one unit — in the first day; this is the "negative 



