90 Infection 



also defended against such micro-organisms as might reach the Veins 

 through lesions or accidents of the abdominal viscera, by the inter- 

 position of the portal capillary network of the liver, where the bac- 

 teria 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 phago- 

 cytes for the destruction 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 SQ 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 introduced through other 

 channels. 



The conditions differ, however, in different infections, for when 

 Roger experimented with streptococci instead of anthrax baciUi, 

 he found that if they were inoculated into the portal vein the 

 animals died more quickly than when they were injected into the 

 aorta, and that when they were injected into the peripheral veins 

 the animals lived longest, the liver seeming to be far less destruc- 

 tive to streptococci than the lungs. 



The Susceptibility of the Host. — SusceptibiHty is liabihty to in- 

 fection. It is a condition in which the host is unable to defend itself 

 against invading micro-organisms. Unusual or unnatural suscep- 

 tibility is also spoken of as predisposition or dyscrasia. 



Many animals and plants are naturally without any means of 

 overcoming the invasiveness of certain parasitic micro-organisms, 

 and are, therefore, naturally susceptible; others naturally resist 

 their inroads, but through various temporary or permanent physio- 

 logic changes may lose the defensive power. 



In general, it is true that any condition that depresses or dimin- 

 ishes the general physiological activity of an animal diminishes its 

 ability to defend itself against the pathogenic action of bacteria, and 

 so predisposes to infection. These changes are often so subtile that 

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



