Chap. XIII.] RESPIRATIOK 157 



inch, in consequence of the diaphragm ri.sing higher on the 

 right side to accommodate the liver. The right lung is divided 

 by fissures into three lobes. The left lung is smaller, narrower, 

 and longer than the right, and has only two lobes. Each lung 

 is enclosed in a serous sac, the pleura, one layer of which is 

 closely adherent to the walls of the chest and diaphragm ; the 

 other closely covers the lung. The two layers of the pleural 

 sacs, moistened by lymph, are normally in close contact ; they 

 move easily upon one another, and prevent the friction that 

 would otherwise occur between the lungs and the walls of the 

 chest with every respiration. 



The pressure of the atmospheric air upon the lungs through 

 the air-passages is greater than it can possibly be upon them 

 from the outside through the chest walls, on account of the 

 resistance which the solid chest walls offer to this pressure ; 

 and, ordinarily, it is impossible for the distended lungs to 

 pull away the layer of the plural sac which adheres to them 

 from the layer which is attached to the chest wall. If, how- 

 ever, the chest wall be punctured, the air from the outside 

 will rush in, distend the pleura, and, squeezing the air out of 

 the air-sacs into the air-passages, cause the lungs to shrivel up 

 and collapse. 



Respiration. — The lungs, then, are placed in an air-tight tho- 

 rax, which they, together with the heart and great blood-vessels, 

 completely fill. By the contraction of certain muscles (see page 

 66), the cavity of the thorax is enlarged ; the lungs are cor- 

 respondingly distended to fill the enlarged cavity, and, by this 

 distension, the air within the air-sacs becomes expanded and 

 more rarefied than the air outside. Being thus expanded and 

 rarefied, the pressure of the air within the lungs becomes less 

 than that of the air outside, and this difference of pressure 

 causes the air to rush through the trachea into the lungs, until 

 an equilibrium of pressure is established between the air inside 

 the lungs and that outside. This constitutes an inspiration. 

 Upon the relaxation of the inspiratory muscles, the elasticity 

 of the lungs and of the chest walls causes the chest to return to 

 its original size, in consequence of which the air within the 

 lungs becomes more contracted and denser than the air outside, 

 the pressure within becomes greater than the pressure without, 

 and the air rushes out of the trachea until equilibrium is once 



