100 SAPROPHYTISM, PARASITISM, AND PATHOGENISM 



Salivary Glands. The salivary glands of the mouth are sometimes 

 invaded by bacteria. 



Nasal Cavity. Large numbers of bacteria, indeed practically all 

 known bacteria may at one time or another gain access to the nose 

 through the inhalation of air containing dust, by droplet infection, 

 from the tear ducts, and in other ways. The air which is inhaled is 

 freed from bacteria before it enters the trachea, largely during its 

 tortuous passage over the turbinates; the moist surface of the nasal 

 mucosa effectively arrests the progress of bacteria, which adhere to 

 it. The constant secretion of mucus encloses many of these organ- 

 isms, which are removed mechanically with the mucus. There is no 

 evidence that the nasal secretions are germicidal. The permanent 

 nasal flora is very limited, however. The pseudodiphtheria bacillus 

 is very frequently found there and pneumococci, streptococci and 

 staphylococci are relatively common. The true diphtheria bacillus 

 is found in the nasal cavity of about 1 per cent, of healthy individuals. 



Lungs. The expired air in quiet, normal breathing is sterile: 

 also, the inhaled air is practically sterile before it reaches the bronchi, 

 for the moist tortuous passages of the nasal cavity mechanically 

 retain bacteria; the same mechanism prevents the expulsion of bac- 

 teria during exhalation, unless the breath is expelled forcibly either 

 through the nose or mouth. Bacteria leave the nose or mouth in 

 expired air only when the expiration is forcible enough to eject finely 

 divided droplets from the mouth or nose respectively. 



The lungs are protected from bacterial invasion not only by the 

 tortuous nasal air passages, but by the ciliated epithelium which 

 covers the surface of the mucosa of the bronchi and bronchioles. 

 The rhythmic contractions of these cilia carry upward and outward 

 those bacteria which may have penetrated so deeply into the respira- 

 tory passages. Inhibition of the activity of these cilia by cold or other 

 environmental conditions may be a potent factor in the establishment 

 of infection in the respiratory tract. Occasionally bacteria succeed 

 in reaching the terminal bronchioles and alveoli of the lungs : they 

 are normally removed by the phagocytic activity of leukocytes (micro- 

 phages) or of certain fixed tissue cells (macrophages) . In spite of 

 these barriers, however, the lungs occasionally become infected. The 

 pneumococcus and tubercle bacillus are the most common primary 

 invaders of the lungs. Streptococci are more frequently secondary 

 invaders, although many primary lobular pneumonias are caused by 

 this organism. 



