External Respiration. 9 1 



action of the myriads of lashing ciliums paddles the mucus 

 headward. 



The Pleura. The outside of each lung is covered by a 

 thin membrane, the pleura, which completely surrounds it, 

 except at the root of the lung, where the bronchus and 

 blood tubes enter. Here the pleura turns toward and 

 becomes attached to the inner wall of the chest, forming 

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



Trachea 



Pleural Space 

 (Exaggerated) 



Chest Wall 



Pleura 



Fig. 55. Diagram of the Lungs and Pleuras. 



the upper surface of the diaphragm. The lung is thus 

 free, except at its root, where the air and blood tubes 

 enter. A very small quantity of liquid moistens the sur- 

 faces of the pleuras on the outside of the lung and the 

 inside of the chest wall, so they move easily one upon the 

 other during respiration. As the lungs are always dis- 

 tended enough to fill the chest cavity, these two surfaces 

 are always in contact. 



Pleurisy. Pleurisy is an inflammation of the pleura. 

 In breathing there is pain from friction or adhesion of the 

 pleuras of the lungs and chest wall. 



