344 TYPHOID FEVER. 



be described in the chapter on Immunity. We shall find 

 that in many diseases the serum has this property of causing 

 agglutination of cultures of the 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 interpreta- 

 tion of some of the facts which have been observed. 



The methods by which the test can be applied have 

 already been described (p. 118). 



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

 clumping may be observed microscopically. If a prepara- 

 tion be made by the method detailed (typhoid serum in a 

 dilution of, say, 1 130 having been employed), and examined 

 at once under the microscope, the bacilli will usually be 

 found actively motile, darting about in all directions. In a 

 short time, however, these movements 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 apply- 

 ing which has also been described (p. 120). Here at the end 

 of twenty-four hours the bacilli form a mass like a precipi- 

 tate at the bottom of the mixture of bacterial emulsion and 

 diluted typhoid serum, while the upper part remains clear. 

 A similar preparation made with normal serum gives 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. 



