536 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



seldom enter the body generally. From the locus of the infection they 

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

 toxin is probably carried over the body along the sheaths of the motor 

 nerves; in diphtheria the toxin may be carried by the lymph, occasionally by 

 the blood; in the case of cholera the blood and lymph both may serve to 

 carry the toxic agents. In diphtheria and cholera the microorganisms very 

 frequently extend along the mucous membranes from the original point 

 of infection. There are other infections in which the causal microorgan- 

 isms extend only from the point of original invasion into the surrounding 

 areas. Such is the case of the Strept. pyo genes, of erysipelas, in the skin 

 and of the Bad. influenza through the respiratory tract. Many of the 

 infectious agents are carried by the blood, occasionally by the lymph, 

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

 monia and the septicaemias due to the pyogenic cocci. It is possible, 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. 



THE METHODS BY WHICH INFECTIOUS AGENTS ARE ELIMINATED 



FROM THE BODY. 



The causal microorganisms of the various infectious diseases may be 

 eliminated from the body in two general ways, namely, by direct method 

 and by an indirect method. For a microorganism to be directly elimi- 

 nated from the body it is necessary for the focus of the infection to com- 

 municate with the outside of the body in some way or other. In the case 

 of infections of the mucous membranes and the skin there is, of course, 

 direct communication with the outside. In diseases of the respiratory 

 organs and the intestines the infectious agents are discharged into the 

 lumen of the air passages and the intestines and then thrown out from 

 these passages. Examples of the direct elimination from the skin may 

 be found in such diseases as smallpox, measles, syphilis, scarlet fever, 

 lupus and in suppurative conditions such as carbuncles and furuncles. The 

 significance of transmission of smallpox, measles and scarlet fever from 

 the skin cannot be stated positively because contradictory evidence exists; 

 of the other diseases mentioned, little can be said of a satisfactory nature. 

 As examples of diseases where direct elimination from the various mucous 

 membranes occurs may be mentioned infections such as typhoid fever, 

 tuberculosis, cholera and dysentery from the membranes of the intestines; 



