The Cardinal Conditions of Infection 81 



into the body. Should any survive, they may be transported 

 to the lymph -nodes and there destroyed, passing through 

 these barriers without destruction, and reaching the venous 

 channels, they have next to pass through the pulmonary 

 capillaries, where they are apt to be caught and destroyed. 

 Finally, should any escape all these defences and reach 

 the arterial system, it is to find the endothelium of the 

 capillaries prone to collect and detain them until destruction 

 is finally effected. The systemic circulation is also defended 

 against such micro-organisms as might reach the veins 

 through lesions or accidents of the abdominal viscera, by 

 the interposition of the portal capillary network of the liver, 

 where the bacteria are caught and many of them destroyed, 

 or passing which, the pulmonary capillary system acts as a 

 second barrier against them. The deeper the penetration, the 

 more active the defense becomes, the blood itself furnishing 

 agglutinins, bacterio-lysins, and phagocytes for the destruc- 

 tion of the micro-organisms and the protection of the host. 



These defenses, however, are of no avail against actively 

 invasive organisms provided with the means of overcoming 

 them all through aggressins that destroy the germicidal 

 humors or toxins that kill or paralyze the cells. When 

 these are injected directly into the streaming blood they 

 produce their effects more rapidly than when injected 

 beneath the skin or elsewhere, because the field of operation 

 is immediately reached instead of through a roundabout 

 course in which so many defenses have to be overcome. 

 Taking anthrax bacilli, whose invasiveness has already been 

 dwelt upon, as an example, Roger * found that when the 

 organisms were injected into the aorta, animals died more 

 quickly than when they were injected into the veins and 

 obliged to find their way through the pulmonary capillaries 

 to the general circulation. If the injections were made into 

 the portal vein, the animals stood a good chance of recovery, 

 the liver possessing the power of destroying sixty-four 

 times as many anthrax bacilli as would prove fatal if intro- 

 duced through other channels. 



The conditions differ, however, in different infections, 

 for when Roger experimented with streptococci instead of 

 anthrax bacilli, he found that animals inoculated into the 

 portal vein died more quickly than others injected into 

 the aorta, and that animals injected into the peripheral 



* "Introduction to the Study of Medicine," p. 151. 

 6 



