84 Infection 



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 orginisms 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 bacilli, 

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

 animals died more quickly than when they were injected into the 

 aorta, and that when the bacilli were injected into the peripheral 

 veins the animals Hved longest, the Uver seeming to be far less 

 destructive to streptococci than the lungs. 



The Susceptibility of the Host. — Susceptibility is liabiUty 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 diminishes 

 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 

 they escape detection, though at times they can be partly understood. 



The inhalation of noxious vapors. It has long been supposed 

 that sewer gas was responsible for the occurrence of certain in- 

 fectious diseases, and when the nature of these diseases was made 

 clear' by a knowledge of their bacterial causes, the old belief still 

 remained and many sanitarians continued to believe that defective 

 sewage is in some way connected with their occurrence. It is 

 difhcult to prove or disprove the matter experimentally. Men who 

 work in sewers and plumbers who breathe much sewer gas are 

 not apparently affected by it. Alessif found that rats, rabbits, and 



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

 t "Centralbl. f. Bakt.," etc., 1894, xv, p. 228. 



