RESPIRATION. 



Cilia. This current of mucus is caused by the cilia 

 projecting from the lining cells of the trachea. They are 

 little hairlike projections, in countless numbers, like a field 

 of grass, each stalk having the power of bending back and 

 forth, making a quick stroke toward the throat, then a 

 slower recover stroke. Thus the united wavelike action 

 of the myriads of lashing cilia paddles the mucus head- 

 ward. It is a very common error to suppose that the cilia 

 produce air currents. This is not their function, and it 

 can readily be seen that they cannot create currents of air, 

 as they are wholly submerged, like grass growing on the 

 bottom of a shallow pond of slimy water. 



Location of Mucous Membrane. All the cavities and 

 passages in the body to which the air has access, such as 

 the digestive and respiratory passages, etc., are lined by 

 mucous membrane (not all 

 ciliated). 



The Pleura. The out- 

 side of each 

 lung is cov- 

 ered by a thin 

 adherent mem- 

 brane, the pleu- 

 ra, which com- 

 pletely invests 

 it, except at the root of 

 the lung, where the bron- 

 chus and blood tubes 

 enter. Here the pleura 

 turns toward and adheres to the inner wall of the chest, 

 forming its lining (still called the pleura), and below passes 

 over the anterior surface of the diaphragm. The lung is 



Trachea 



Pleural Space 

 (Exaggerated) 



Chest Wall 



= = - Plc-ura 



Fig. 40. Diagram of the Lungs and Pleurae. 



