478 



ORGAN SYSTEMS OF MAN 



corfiloge 



larynx 

 vocal cords 



trocbca 



capillones 

 oround olv«olus 



Fig. 18-2. A schematic view of the breathing system of man with one alveolus and associated parts greatly enlarged. 



after working in dust-laden air. The mu- 

 cus secretion also has some hygienic value, 

 because bacteria are caught by this mech- 

 anism as well, thus preventing them from 

 reaching the lungs where they might cause 

 disease. However, some bacteria and 

 viruses may establish themselves in the 

 nasal areas, as in the case of head "colds." 

 Later they may make their way into the 

 deeper portions of the respiratory tract 

 where characteristic symptoms appear. 



The nasal secretions have some bacteria- 

 destroying action, and in head colds the cells 

 of these areas are stimulated to produce 

 large amounts of mucus to prevent the 

 organisms from getting a foothold. This 

 reaction accounts for the "running nose" at 

 the beginning of a cold. Unfortunately, this 

 is frequently carried to such extremes, that 

 swelling of the walls closes the air passages, 

 forcing the victim to breathe through his 

 mouth. This makes him liable to infection 



