RELATIONS TO THE DISEASE. 449 



may occur after a similar interval. The spirilla begin to appear 

 in the blood shortly before the onset of the pyrexia, and during 

 the rise of temperature rapidly increase in number. They are 

 very numerous during the fever, a large number being often 

 present in every field of the microscope when the blood is 

 examined at this stage. They begin to disappear shortly before 

 the crisis; after the crisis they are entirely absent from the 

 circulating blood. A similar relation between the presence of 

 the spirilla in the blood and the fever is found in the case of 

 the relapses, whilst between these they are entirely absent. 

 Miinch in 1876 produced the disease in the human subject by 

 injecting blood containing the spirilla, 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 appearance 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 spi- 

 rilla increased somewhat in length, and formed a sort of felted 

 mass, but that no multiplication 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 the 

 first 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 pro- 

 duced 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 fol- 

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

 occasionally one of short duration occurs. The presence of spi- 

 rilla 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 mon- 



