100 SAPROPHYTISM, PARASITISM AND PATHOGENISM 
Salivary Glands.— The salivary glands of the mouth are sometimes 
invaded by bacteria. Indirect infection by way of the blood stream 
is not uncommon, particularly in typhoid fever. Epidemic parotitis 
(mumps) of unknown etiology is apparently transmitted by droplet 
infection. 
Nasal Cavity.— hsiTge 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 pseudo-diphtheria 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. 
Lvngs.— 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 bacteria dur- 
ing 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 respi- 
ratory passages. Inhibition of the activity of these cilia by cold or 
other environmental conditions may be a potent factor in the estab- 
lishment 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 
(microphages) 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. Streptoct)cci are more frequently secondary 
invaders, although many primary lobular pneumonias are caused by 
this organism. 
2. Mucous Membranes.— The moist surface of mucous membranes 
makes them excellent culture media for many bacteria which can 
grow at the temperature of the body. The physiological secretions 
which bathe these membranes, with the exception of the stomach, 
