446 RELAPSING FEVER. 



and this experiment has been several times repeated with 

 the same result ; after a period of incubation the spirilla 

 begin to appear in the circulating blood, and their appear- 

 ance is soon followed by the occurrence of pyrexia. 



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 multi- 

 plication took place. Additional proof, however, that the 

 organism is the cause of the disease has been afforded by 

 experiments on monkeys, and facts of considerable interest 

 have been thus established. Carter in 1879 was tne ^ rst 

 to show that the disease could be readily produced in 

 these animals, and his experiments were confirmed by 

 Koch. In such experiments the blood taken from patients 

 and containing the spirilla was injected subcutaneously. 

 In the disease thus produced there is an incubation period 

 which usually lasts about three days. At the end of that 

 time the spirilla rapidly appear in the blood, and shortly 

 afterwards the temperature quickly rises. The period of 

 pyrexia usually lasts for two or three days, and is followed 

 by a marked crisis. As a rule there is no relapse, but 

 occasionally one of short duration occurs. The presence 

 of spirilla in the blood has the same relation to the pyrexial 

 period as in the human subject. 



For a long time the place and mode of destruction of 

 the spirilla were quite unknown, but valuable light was 

 thrown on these points by Metchnikoff, who produced 

 the disease in monkeys and killed them at various stages 

 of the fever. He found that during the fever the spirilla 

 were practically never taken up by the leucocytes in the 

 circulating blood, but at the time of the crisis the spirilla, 

 on disappearing from the blood, accumulated in the spleen 

 and were ingested in large numbers by the microphages or 

 polymorpho-nuclear leucocytes. Within these they rapidly 

 underwent degeneration and disappeared. Metchnikoff also 



