FUNCTION OF THE PLEURA. 339 



ing inspiration, and the apices would receive but little addition 

 to their contained air. This condition is often found in disease of 

 the pleura, leading to adhesion of the visceral and parietal layers. 

 When such cases live for some time after the pleurisy and the 

 adhesions persist, the air cells of the lower margins of the lungs 

 are commonly found to be distended and bloodless (i. e., local 

 emphysema from habitual over-distention) ; while, on the other 

 hand, the apices become abnormally dense, and the alveoli are 

 contracted and airless. 



The surface of the soft, elastic lung tissue is normally quite 

 free, being encased in a serous membrane, the smooth surface of 

 which can slide uninterruptedly and freely over the similar lining 

 of the costal wall. That this motion of the lung actually occurs 

 may be seen from watching the lung through the exposed parietal 

 pleura, or recognized by studying the sounds produced by a 

 roughness of the pleura, such as occurs in inflammation, when a 

 " friction " can be detected by the ear. 



The lungs move in a definite direction. From the most fixed 

 points of the thorax, namely, the apex and vertebral margin, 

 they pass toward the more movable inferior costal and sternal 

 regions. In short, the anterior part of the lungs passes down- 

 ward and forward to fill up the gap made by the descent of the 

 diaphragm and by the passing of the costal wall upward and 

 forward. 



The position of the inferior margin of the lung may be easily 

 recognized by percussion over the liver, and may thus be shown 

 to be moving up and down with expiration and inspiration 

 respectively. By percussion we also find that the space between 

 the two lungs in front is increased during expiration and dimin- 

 ished during inspiration, so that the heart is more or less covered 

 by lung, and the precordial dullness is altered every time we 

 draw a breath. 



By means of this free movement of the lungs in the cavities 

 lined by serous membrane, the air exerts equal force on the walls 

 of all the air cells, whether they are situated in the apex or base 

 of the lung, and the alveoli are all equally filled with air. 



If the pleural cavity be brought into contact with the air, 



