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CLINICAL BACTERIOLOGY AND HEMATOLOGY 



taking care to put the loop flat on the surface of the glass ; this 

 is done more easily if the wire is slightly bent, or if a flat side is 

 used instead of the watch-glass. 



Next heat the platinum loop in a flame ; this is to burn off any 

 blood which might remain on it and contaminate the emulsion. 

 Take up a loopful of the emulsion, and place it on the watch-glass 

 by the side of the drop of serum, but not touching it. Repeat this 

 until you have placed twenty-nine drops of emulsion round the 

 serum. Mix the whole together by stirring them thoroughly 

 with the platinum loop, place a droplet of the mixture on a clean 

 cover-glass, making a hanging-drop specimen, and examine as 

 before. 



If the blood comes from a case of typhoid fever (with certain 

 restrictions which will be discussed below), the microscopic 



Fig. 23. 

 a, Negative Widal's reaction ; b, positive Widal's reaction. 



appearances will be quite different from those seen in the drop of 

 emulsion which was previously examined. The bacilli will no 

 longer swim about rapidly in all directions ; they will become 

 paralyzed, and remain quite motionless. Further, they will collect 

 into clumps, each clump consisting of a larger or smaller number 

 of bacilli, arranged in a felted network, resembling that seen in 

 a heap of "spellicans'' (Fig. 23, b). This is the complete positive 

 reaction ; it consists of two parts, clumping and paralysis, and is 

 given only (in the dilution used) by the blood of a patient who is 

 suffering or who has suffered from typhoid fever. If this is not 

 the case, the bacilli will continue to move about just as before, 

 and will not collect into clurnps. 



In the process which has been described above, the blood has 

 been diluted to thirty times its volume, and this is the best dilution 

 to use for diagnostic purposes. But the reaction is given earlier 



