INFECTIONS OF THE DIGESTIVE TRACT 177 



taken from both horses and mixed in equal volumes. 

 "By this means the best universal serum can be ob- 

 tained" (Shiga). 



I have mentioned that the dysentery bacilli resemble 

 those of typhoid fever in being hemiparasites capable 

 of infecting a susceptible individual only if present 

 in such considerable numbers that his anti-aggressive 

 powers are neutralized, thus clearing the way for a free 

 multiplication of the bacilli. There are, however, some 

 apparently fundamental differences in the biological 

 characters of these two important pathogenic forms. 

 While the typhoid bacillus, an actively motile organism, 

 is highly invasive and sets up a bacillsemia with con- 

 sequences already noted, the dysentery bacillus, a non- 

 motile organism, is only slightly invasive, remains almost 

 limited to the intestinal mucous membrane, and leads 

 to neither bacillsemia nor bacilluria. The typhoid bacilli 

 produce soluble poisons with difficulty, at least in fluid 

 cultures. Some dysentery bacilli, namely those of the 

 original Shiga or promptly alkali-making variety, show 

 a considerable capacity to yield soluble poisons in fluid 

 media. They differ in this respect from the acid-making 

 varieties, which yield their active toxic product only 

 within the animal organism. 1 The chemical nature of 

 the poison is at present wholly unknown, but significant 

 facts in respect to its mode of action in animals, including 

 probably the human species, have been lately brought 

 to light, chiefly through the experimental studies of 



1 Dr. Flexner tells me he has lately succeeded in obtaining toxins 

 from his dysentery bacilli by means of a special procedure. 



