The Cardinal Conditions of Infection 79 



have next to pass through the pulmonary capillaries, where 

 they are apt to be caught and destroyed without reaching 

 the arterial system, by which they might be distributed 

 to new fields of activity. The systemic circulation is 

 similarly defended against such micro-organisms as 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 through 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 protection of the host. 



These defenses, however, are of no avail against actively 

 invasive organisms which when injected directly into the 

 streaming blood produce their effects more rapidly and in 

 smaller numbers than when injected beneath the skin or 

 elsewhere, because the field of operation is immediately 

 reached instead of through a roundabout course. Taking 

 anthrax bacilli, whose invasiveness has already been dwelt 

 upon, as a typical illustration of this, Roger* found that 

 when these organisms were injected into the aorta, the 

 animals died much more quickly than when they were 

 injected into the veins and were so obliged to find their way 

 through the pulmonary capillaries to the general circulation. 

 If the injections were made into the portal vein, they 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 introduced through other channels. 



The defenses differ, however, in action upon different infec- 

 tious agents, for when Roger modified the experiment by 

 using 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 veins lived longest, the liver apparently 

 being far less destructive to streptococci than the lungs. 



4. The Susceptibility of the Host. Susceptibility is 

 liability to infection. It is a condition in which the host is 

 unable to defend itself against invading micro-organisms. 

 Unusual or unnatural susceptibility is also spoken of as 

 predisposition. 



Many animals and plants are naturally without any means 

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



