164 IMMUNITT. 



occur only when there is some break in the continuity of the cover- 

 ing of the body. Indeed some of the most powerful pathogenic 

 microorganisms are practically harmless when introduced into the 

 body subcutaneously, while they may speedily cause death when 

 taken into the alimentary canal. This is true of the vibrio of Asiatic 

 cholera. On the other hand tetanus and anthrax bacilli may do no 

 harm when taken into a healthy stomach, while they may cause 

 speedy death when injected subcutaneously. These examples make 

 it evident that it is not sufficient for a pathogenic microorganism, 

 even one to which man is highly susceptible, to come in contact with 

 the human body in order to cause disease, but it must first find its 

 suitable port of entry, and even after this has been reached, there 

 are other dangers to the life of the invader with which he may have 

 to meet before he reaches the place where he can entrench himself 

 and begin the struggle. Such guards are fiirnished, as has already 

 been seen, by the mechanical movements of epithelial cilise, by the 

 tortuosity of the passage, by coming in contact with such fluids as 

 the gastric juice, or by being htirried through the body and cast out 

 in the excretions, as undoubtedly sometimes happens to typhoid 

 bacilli when taken into the alimentary canal. Moreover every toxi- 

 cogenic germ is not capable of maintaining even for a short time a 

 parasitic existence. The obligate saprophytes, which are numerous, 

 and some of which undoubtedly produce powerful toxins, are capable 

 of harming man only under exceptional conditions which are spe- 

 cially favorable to liieir growth and multiplication. Indeed, some 

 members of this class do not appear to be capable of growth and 

 reproduction when introduced into any part of the animal body, and 

 consequently they are harmful to man only when their already formed 

 toxins are introduced into his body. These bacteria in and of them- 

 selves are not able to affect the health or endanger the life of man. 

 Other powerful toxicogenic bacteria have their intracellular toxins 

 locked up in cell walls which under ordinary conditions are imper- 

 meable ; and it is only when some unusual condition gives oppor- 

 tunity for the disintegration of the bacterial cell that its toxin is 

 liberated and may do harm. This seems to be true of the colon 

 bacillus, which is a normal habitant of the intestines of man, and 

 which contains a powerful intracellular poison. Under ordinary 

 conditions this bacterium is harmless to man, because its toxin does 

 not diffuse from the bacterial cell, and consequently cannot be ab- 

 sorbed by the intestinal wall ; but if the colon bacillus finds its way 

 into the peritoneal cavity, where its cell wall is disintegrated by 

 phagocytic or other action, its toxin is liberated -to the great danger 

 of the host. 



Animals differ widely not only in their susceptibility to bacterial in- 

 fection, but also to bacterial intoxication. For instance certain ani- 

 mals, especially birds and reptiles, are not susceptible to the toxin of 



