THE RESPIRATORY SYSTEM. 369 



themselves are covered mth. a tough hut thin membrane lined 

 with flattened cells, which secrete a small quantity of fluid 

 that serves to maintaia the surrounding parts in a moist con- 



FiG. 291. — Mold of a terminal bronchus and a ^oup of air-cells moderately distended 

 by injection, from the human subject (Robm). 



dition, thus lessening friction. The importance of this ar- 

 rangement is well seen when, in consequence of inflammation 

 of this pleura, it becomes dry, giving rise during each respira- 

 tory movement to a friction-sound and a painful sensation. 

 It will not be forgotten that this membrane extends over the 

 diaphragm, and that, in consequence of the lungs completely 

 filling all the space (not occupied by other organs) during every 

 position of the chest-walls, the costal and pulmonary pleural 

 surfaces are in constant contact. By far the greater part of 

 the lung-substance consists of elastic tissue, thus adapting the 

 principal respiratory organs to that amount of distention and 

 recoil to which they are ceaselessly subjected during the entire 

 lifetime of the animal. 

 24 



