SERUM DIAGNOSIS. 325 



ultimate dilution will be approximately 1:15, and this is 

 the proportion generally to be recommended. 



If a preparation made by either of these methods is 

 examined at once under the microscope, the bacilli will 

 usually be found to be actively motile, darting about in all 

 directions. In a short time, however, if the serum is that 

 of a patient suffering from typhoid, their movements grad- 

 ually become slower, the bacilli begin to adhere to one 

 another, and ultimately become immobile and form clumps 

 by their aggregation. When the latter stage is reached the 

 reaction is said to be complete. The time required varies 

 in different cases ; in some the movements are effected 

 almost instantaneously, in others not for some minutes, and 

 the clumping is usually well marked within half an hour, 

 though occasionally it takes longer. 



A corresponding reaction, visible to the naked eye, may 

 be obtained by placing the mixture, composed of typhoid 

 serum and of bouillon containing the typhoid bacilli, in a 

 slender glass tube, and allowing it to stand in the upright 

 position. At the end of twenty-four hours the bacilli form 

 a mass at the foot like a precipitate, the upper part of the 

 fluid being clear. A similar preparation, made with normal 

 serum instead of typhoid serum, gives a diffuse turbidity at 

 the end of twenty-four hours. This test is usually called 

 the "sedimentation test." It has the disadvantage of 

 taking longer than the microscopic method, but is useful 

 as a control ; in nature it is similar. 



The reaction given by the serum in typhoid fever 

 usually begins to be observed about the seventh day 

 of the disease, though occasionally it has been found 

 as early as the fifth day. It gradually becomes more 

 marked as the disease advances, and it is still given by the 

 blood of convalescents from typhoid. How long it lasts 

 after the end of the disease has not yet been fully deter- 

 mined, but in many cases it has been found after several 

 months at least. As a rule the reaction is more marked 

 where the fever is of a pronounced character. In the 

 milder cases it is less pronounced. 



