CHAP. XII.] RESPIRATION. 135 



inch, in consequence of the diaphragm rising 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 base of each lung is at- 

 tached, by means of the pleura, to the diaphragm, and the apex 

 is attached above the margin of the first rib. 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 lungs to pull away 

 the layer of the pleural sac which adheres to them from the 

 layer which is attached to the chest wall. If, however, 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 

 65), 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 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, 



