OF THE HUMAN SUBJECT. 55 



line reaction to blue and red litmus paper). There arises even 

 at this point the first possible source of fallacy. If such a broth 

 culture be examined by the hanging-drop method (fully de- 

 scribed in the previous volume), clumps of bacilli are not rarely 

 encountered, and these, at times, of considerable size. In all 

 cases, then, the broth culture is to be first passed through a 

 small double cone of filter-paper into a watch-glass, in order to 

 remove any such clumps present; and after this, a hanging drop 

 is to be prepared for comparison with the result obtained when 

 the reaction is tried for. One important difference to be ob- 

 served in the microscopic preparations is that although in the 

 broth culture clumps may be met with, the field between con- 

 tains varying numbers of bacilli in active movement; whereas 

 when clumping occurs from the action of the serum of a typhoid 

 patient, the bacilli become motionless between the bacterial isl- 

 ands, if any remain incoherent. 



How to test the Blood Serum. The reaction is obtainable 

 quite early in the course of the disease, as early as the fifth day, 

 and it persists during convalescence, but for an extremely vari- 

 able period afterward. Not rarely the reaction will continue for 

 a year, but it may last many years, and might without enquiry 

 into a patient's history be erroneously taken to prove the exist- 

 ence of typhoid on the occasion of some subsequent illness sug- 

 gestive of it. 



The blood to be used in the test is withdrawn either from, 

 the lobe of the ear or from the back of the finger near the root 

 of the nail; and the puncture is best made by means of a surgi- 

 cal or other neede with a cutting edge as well as a sharp 

 point. 



If the test cannot be carried out at the time, the blood must 



