224 ENTRANCE OF PATHOGENIC ORGANISMS 



{g) The external genitalia are the usual points of entrance 

 for the venereal organisms in man (gonococcus, Treponema 

 pallidum, and Ducrey's bacillus). The bacillus of contagious 

 abortion and probably the trypanosome of dourine are 

 commonly introduced through these channels in animals. 



C. Lungs. — The varied types of pneumonia due to many 

 different organisms (tubercle, glanders, influenza, plague 

 bacilli, pneumococcus, streptococcus, micrococcus and many 

 others) show how frequently these organs are the seat of a 

 localized infection, which may or may not be general. 

 Whether the lungs are the actual point of entrance in these 

 cases is a question which is much discussed at the present 

 time, particularly with reference to tuberculosis. The 

 mucous secretion of the respiratory tract tends to catch 

 incoming bacteria and other small particles and the ciliary 

 movement along bronchial tubes and trachea tends to carry 

 such material out. "Foreign body pneumonia" shows clini- 

 cally, and many observers have shown experimentally that 

 microorganisms may reach the alveoli even though the 

 exchange of air between them and the bronchioles and larger 

 bronchi takes place ordinarily only by diffusion. The pres- 

 ence of carbon particles in the walls of the alveoli in older 

 animals and human beings and in those that breathe dusty 

 air for long periods indicates strongly, though it does not 

 prove absolutely, that these came in with inspired air. On 

 the other hand, experiment has shown that tubercle bacilli 

 introduced into the intestine may appear in the lungs and 

 cause disease there and not in the intestine. It is probably 

 safe to assume that in those diseases which are transmitted 

 most readily through close association though not neces- 

 sarily actual contact, the commonest path is through the 

 lungs, which may or may not show lesions (smallpox, scar- 

 let fever, measles, chicken-pox, whooping-cough, pneumonic 

 plague in man, lobar and bronchopneumonias and influenza 

 in man and animals, some cases of glanders and tubercu- 

 losis). On the other hand, the fact that the Bacillus typhosus 

 and Bacillus coli may cause pneumonia when they evi- 

 dently have reached the lung from the intestinal tract, and 

 the experimental evidence of lung tuberculosis above men- 



