108 



LUNGS AND PLEURA. 



Trachea 



Pleural Space - - - 

 (Exaggerated) 



Chest Wall- -- 



=*- Pleura 



has access, such as the digestive and respiratory passages, etc., 

 are lined by mucous membrane (not all ciliated). 



The outside of each lung is covered by a thin adherent 

 membrane, the pleura, which 

 completely invests it, except 

 at the root of tiie lung, 

 where the bronchus and 

 blood tubes en- 

 ter. Here the 

 pleura turns 

 toward and 

 adheres 'to the 

 inner wall of 

 the chest, form- 

 ing its lining (still called 

 the pleura), and below 

 passes over the anterior 

 surface of the diaphragm. 

 The lung is thus free, ex- 

 cept at its root, where the air and blood tubes enter. A very 

 small quantity of liquid moistens the contiguous surfaces of 

 the pleurae on the outside of the lung and the inside of the 

 chest wall, so they move easily one upon the ether during 

 respiration. As the lungs are always distended enough to 

 fill the chest cavity, these two surfaces are always in con- 

 tact. In pleurisy (inflammation of the pleurae), pain is felt 

 in breathing from friction or adhesion of these surfaces. 



In studying respiration, let us constantly keep in mind 

 these facts : 



1. The lungs are highly elastic, and 



2. Highly porous, each air vesicle being in direct commu- 

 nication with the outer air by means of : 



3. Air tubes that always stand open, 



Fig. 42. Diagram of the Lungs and Pleurae. 



