158 The Vertebrate Organ Systems 



ing occurs when the soft palate vibrates due to the individual's respiring 

 through the mouth. Yawning is a phenomenon which has neither ex- 

 planation nor known cause, except possibly a dull lecture or book. 

 Perhaps it is a useful survival mechanism ! 



Colds and Coughs. — The respiratory tract is in close contact with 

 the outside environment; hence is subject to inflammations and irrita- 

 tions. In our temperate climates where the temperature changes are 

 abrupt, colds are a frequent nusiance. These cause irritation and ex- 

 cessive secretion by the mucous membranes of the nasal passages and 

 sinuses. Bacteria that may enter with the air can cause inflammation 

 of various parts of the respiratory passageways, such as bronchitis 

 (inflammation of the bronchiole tubes), or even of the lungs themselves. 

 This latter may result in infections such as pneumonia in which fluids 

 often accumulate in the alveoli, or in tuberculosis, an infectious disease, 

 characterized by the formation of tubercles in the lung tissues. 



