238 THE HUMAN BODY. 



has been calculated that if the walls of the air-cells were 

 all spread out flat and placed side by side they would cover 

 an area of 2600 square feet. This great surface, therefore, 

 represents the area of the body by which oxygen is received 

 and carbon dioxide given off. 



The Pleura, The exterior of each lung, except where 

 its bronchus and blood-vessels enter it, is covered by a 

 thin elastic serous membrane, the pleura (Fig. 2). This 

 membrane also lines the inside of the chest. Its surface in 

 health is kept moistened by a small quantity of lymph. 

 In consequence of its smoothness and moisture, during the 

 breathing movements the chest wall and lung glide over 

 each other with hardly any friction. 



Pleurisy is inflammation of the pleura. In its early 

 stages it is usually associated with sharp pain on drawing the 

 breath. Later on a large quantity of lymph is often poured 

 out by the inflamed pleura, filling up the cavity which 

 should be occupied by the lung, and pressing the latter up 

 into a small mass, very inefficient for breathing purposes. 



The Elasticity of the Lungs. The lungs are so elastic as 

 to be like a thin india-rubber bag. If we tie a tube tightly 

 into a bronchus and blow in air the lung will dilate, but as 

 soon as we cease blowing and leave the tube open, it will 

 shrink up again. Yet in the chest the lungs always remain 

 so expanded as to completely fill up all the space left for 

 them by the heart and other things contained in the 

 thorax. How is this ? 



How large is the surface of the body set apart for oxygen recep- 

 tion and carbon dioxide elimiiiation? 



What is the pleura? What does it cover besides the outer surface 

 of each lung? What is its condition in health? What is its use? 



What is pleurisy? What symptom usually accompanies its early 

 stages? What happens later? 



What do the lungs resemble in their elasticity? How may this 

 elasticity be demonstrated? What is the natural condition of the 

 lungs in the chest? 



