PARASITIC BACTERIA. 149 



cies. Some few species, however, are not thus 

 destroyed by the hostile agencies of the tissues 

 of the animal, but are capable of growing and 

 multiplying in the living body. These alone are 

 what constitute the pathogenic bacteria, since, of 

 course, these are the only bacteria which can pro- 

 duce disease by growing in the tissues of an ani- 

 mal. The fact that the vast majority of bacteria 

 can not grow in the living organism shows clearly 

 enough that there are some conditions existing in 

 the living tissue hostile to bacterial life. There 

 can be little doubt, moreover, that it is these same 

 hostile conditions, which enable the body to resist 

 the attack of the pathogenic species in cases 

 where resistance is successfully made. 



What are the forces arrayed against these in- 

 vaders ? The essential nature of the battle ap- 

 pears to be a production of poisons and counter 

 poisons. It appears to be an undoubted fact that 

 the first step in repelling these bacteria is to flood 

 them with certain poisons which check their 

 growth. In the blood and lymph of man and 

 other animals there are present certain products 

 which have a direct deleterious influence upon the 

 growth of micro-organisms. The existence of 

 these poisons is undoubted, many an experiment 

 having directly attested to their presence in the 

 blood of animals. Of their nature we know very 

 little, but of their repressing influence upon bac- 

 terial growth we are sure. They have been named 

 alexines, and they are produced in the living tis- 

 sue, although as to the method of their pro- 

 duction we are in ignorance. By the aid of 

 these poisons the body is able to prevent the 

 growth of the vast majority of bacteria which 

 get into its tissues. Ordinary micro-organisms 



