SERUM DIAGNOSIS OF TYPHOID FEVER. 341 



causal bacterium. The principles on which the possession of 

 the faculty depends, and also its significance, are obscure, and 

 even in the case of the typhoid bacillus, where an enormous 

 amount of work has been done, we do not know the true inter- 

 pretation of some of the facts which have been observed. 



The methods by which the test can be applied have already 

 been described (p. 109). 



(1) It will be there seen that the loss of motility and clumping 

 may be observed microscopically. If a preparation be made by 

 the method detailed (typhoid serum in a dilution of, say, 1-50 

 having been employed), and examined at once under the micro- 

 scope, the bacilli will usually be found actively motile, darting 

 about in all directions. In a short time, however, these move- 

 ments gradually become slower, the bacilli begin to adhere to one 

 another, and ultimately become completely immobile and form 

 clumps by their aggregation, so that no longer are any free 

 bacilli noticeable in the preparation. When this occurs the 

 reaction is said to be complete. If the clumps be watched still 

 longer a swelling up of the bacilli will be observed, with a 

 granulation of the protoplasm, so that their forms can with 

 difficulty be recognised. In a preparation similarly made with 

 non-typhoid serum the individual bacilli can be observed separate 

 and actively motile for many hours. 



(2) A corresponding reaction visible to the naked eye is 

 obtained by the " sedimentation test," the method of applying 

 which has also been described (p. 112). Here at the end of 

 twenty-four hours the bacilli form a mass like a precipitate at the 

 bottom of the mixture of bacterial emulsion and diluted typhoid 

 serum, while the upper part remains clear. A similar prepara- 

 tion made with normal serum shows a diffuse turbidity at the end 

 of twenty-four hours. The test in this form has the disadvantage 

 of taking longer time than the microscopic method, but it is 

 useful as a control ; in nature it is similar. 



Such is what occurs in the case of a typical reaction. There 

 are several details, however, which require attention, and on which 

 the value of the method as a means of diagnosis largely depends. 

 The race of typhoid bacillus employed is important. All races do 

 not give uniformly the same results, though it is not known on 

 what this difference of susceptibility depends. The bacteriologist 

 must, therefore, apply a process of selection to the races at his 



