OP THE HUMAN" SUBJECT. 79 



in tap water, the cover-glasses being then placed on edge to dry 

 or gently pressed between filter paper; when quite dry they are 

 mounted in xylol balsam. 



The diphtheria bacillns is depicted and described on Plate 

 XIX. 



If no diphtheria bacilli are found, a second series of prepa- 

 rations should be made from another 6se of the same culture, 

 and if a negative result is again obtained, a third set. Where 

 any doubt enters into the morphological diagnosis it is impera- 

 tive to test the virulence of the bacillus upon animals. With 

 this object a tube of broth is inoculated from a colony of the ba- 

 cillus, and after forty hours' incubation 2 cubic centimeters of 

 the shaken culture are injected into the subcutaneous tissue of 

 the anterior or lateral abdominal wall of a guinea pig weighing 

 500 grams. The broth used for the culture is prepared from 

 minced veal which is allowed to ferment in order to remove the 

 glucose present in it, and by so doing to reduce the amount of 

 acid formed by the bacillus, the production of which acts dele- 

 teriously upon the micro-organism and diminishes the amount of 

 toxin elaborated by it. 



It must be borne in mind that diphtheria bacilli present all 

 grades of virulence, the virulence at one end of the scale dimin- 

 ishing until it entirely vanishes. 



There are, that is to say, diphtheria bacilli of typical mor- 

 phological form which have, presumably, lost their virulence, 

 and the injection of a broth culture of which produces neither 

 local nor general results in the animal upon which they are 

 tested. In the lower grades of virulence a local swelling only 

 results; in the higher, death ensues in from twenty- four to forty- 

 eight hours; in the case of very high degrees of virulence, much 



