IMMUNITY 441 



to infection. In the former animals the disease is characterised 

 by several relapses, in the latter there is, however, no relapse. 



Numerous attempts to cultivate this organism outside the 

 body have all been attended with failure, and it has been 

 abundantly shown that it does not grow on any of the media 

 ordinarily in use. Koch found that on blood serum the 

 filaments of the spirilla increased somewhat in length, and 

 formed a sort of felted mass, but that no multiplication took 



FIG. 150. Spirillum Obermeieri in blood of infected mouse. x 1000. 



place. Recently Norris, Pappenheimer, and Flournoy have 

 found that a considerable amount of multiplication may take 

 place in the citrated blood of man and the rat. 



Immunity. Metchnikoff found that during the fever the 

 spirilla were practically never taken up by the leucocytes in the 

 circulating blood, but that at the time of the crisis, on dis- 

 appearing from the blood, they accumulated in the spleen and 

 were ingested in large numbers by the microphages or poly- 

 morphonuclear leucocytes. Within these they rapidly under- 

 went degeneration and disappeared. It is to be noted in this 

 connection that swelling of the spleen is a very marked feature 

 in relapsing fever. These observations were entirely confirmed 



