INOCULATION INTO LOWER ANIMALS. 351 



often a matter that requires the careful application of 

 all the different tests. 



.'Probably the most trustworthy, certainly the most 

 recently described, reaction of the typhoid bacillus is 

 that seen when it is brouglit in contact with the blood- 

 serum from liuman beings sick of typhoid fever, or from 

 animals that have survived inoculation with cultures of 

 this organism. This reaction consists of a peculiar 

 alteration in the relation of the organisms to one 

 another in the fluid. As ordinarily seen in a hanging 

 drop of bouillon the typhoid bacillus appears as single, 

 actively motile cells; when to such a drop a drop of 

 dilute serum from a case of typhoid fever is added the 

 motility of the organism gradually becomes lessened, 

 and finally ceases, and the bacteria congregate together 

 in larger and smaller clumps. The reaction may also 

 be made in another way, viz. , by adding to about 4 or 

 5 c.c. of a twenty-four-hour-old bouillon culture of 

 typhoid bacilli in a narrow test-tube about eight drops 

 of serum from a case of typhoid fever, after which the 

 tube is placed in the incubator. After a few hours the 

 normally clouded culture is seen to have undergone a 

 change; instead of the diffuse cloud caused by the 

 growth, the fluid is found to be clear and to contain 

 within it flocculent masses of the bacteria that have 

 agglutinated together as a result of the specific action of 

 the serum used. When employed conversely — i.e., for 

 deciding if the serum used is from a case of typhoid 

 fever or not — the reaction constitutes what is known as 

 " Widal's serum diagnosis of typhoid fever." For this 

 latter purpose it is often necessary to test several cul- 

 tures of genuine typhoid bacilli, from different sources 

 and of varying degrees of vitality, before a culture is 



