METHODS AND CHANNELS OF INFECTION 677 



are injected into the circulation of susceptible animals, tubercle 

 formation occurs showing that these proteins are poisonous. 



OTHER POSSIBLE EXACT CAUSES. In certain infectious diseases it 

 is also claimed by certain writers that enzymes are responsible. This 

 lacks substantiation. It is also stated that in such infections as anthrax 

 the mechanical effect of the bacteria plugging up the capillaries and 

 producing mycotic emboli is a factor. This may be true but in addition 

 other factors are concerned as previously mentioned. 



In mixed infections of two or more organisms, which frequently 

 occurs, the infected individual may have within the body soluble 

 toxins, endotoxins, and toxic bacterial proteins and in such a case it is 

 difficult to differentiate their action. 



THE METHODS BY WHICH INFECTIOUS MICROORGANISMS ARE 



DISSEMINATED 



The microorganisms of some of the infectious diseases such as diph- 

 theria and Asiatic cholera and usually' tetanus remain local and seldom 

 enter the body generally. From the locus of the infection they dis- 

 seminate their toxic or poisonous products. In the case of tetanus the 

 toxin is carried over the body along the sheaths of the motor nerves; 

 in diphtheria the toxin is usually carried by the lymph, occasionally 

 by the blood; and in the case of cholera the blood and lymph both serve 

 to carry the toxic agents. In diphtheria and cholera the microorgan- 

 isms very frequently extend along the mucous membranes from the 

 original point of infection. There are other infections in which the 

 causal microorganisms extend only from the point of original invasion 

 into the surrounding areas. Such is the case with Strept. pyogenes in 

 the infection of the lymphatics of the skin in erysipelas and of Eact. 

 influenza in all infection through the respiratory tract. Many infec- 

 tious agents are carried by the blood and occasionally by the lymph, 

 as for example, in tuberculosis, syphilis, glanders, plague, leprosy, 

 pneumonia, and the septicemias due to the pyogenic cocci. It is pos- 

 sible in certain cases that the leucocytes acting as phagocytes may 

 carry virulent infectious agents through the blood and lymph from one 

 part of the body to the other. 



