390 THE HUMAN BODY 



tached to each lung is its visceral, and that attached to the 

 chest-wall its parietal layer. Each pleura thus forms a closed 

 sac surrounding a pleural cavity, in which, during health, there 

 are found a few drops of lymph, keeping its surfaces moist. This 

 lessens friction between the two layers during the movements 

 of the chest-walls and the lungs; for although, to insure dis- 

 tinctness, the visceral and parietal layers of the pleura are rep- 

 resented in the diagram as not in contact, that is not the nat- 

 ural condition of things; the lungs are in life distended so that 

 the visceral pleura rubs against the parietal, and the pleural 

 cavity is practically obliterated. This is due to the pressure of 

 the atmosphere exerted through the air-passages on the interior 

 of the lungs. The lungs are extremely elastic and distensible, and 

 when the chest cavity is perforated each shrivels up just as an 

 india-rubber bladder does when its neck is opened; the reason 

 being that then the air presses on the outside of each with as 

 much force as it does on the inside. These two pressures neutral- 

 izing one another, there is nothing to overcome the tendency of 

 the lungs to collapse. So long as the chest-walls are whole, how- 

 ever, the lungs remain distended. The pleural sac is air-tight 

 and contains no air, and the pressure of the air around the Body 

 is borne by the rigid walls of the chest and prevented from reach- 

 ing the lungs; consequently no atmospheric pressure is exerted 

 on their outside. On their interior, however, the atmosphere 

 presses with its full weight, equal to about 90 centigrams on a 

 square centimeter (14.5 Ibs. on the square inch), and this is far 

 more than sufficient to distend the lungs so as to make them 

 completely fill all the parts of the thoracic cavity not occupied by 

 other organs. Suppose A (Fig. 115) to be a bottle closed air- 

 tight by a cork through which two tubes pass, one of which, b, 

 leads into an elastic bag, d, and the other, c, provided with a stop- 

 cock, opens freely below into the bottle. When the stop-cock, c, 

 is open the air will enter the bottle and press there on the outside 

 of the bag, as well as on its inside through b. The bag will there- 

 fore collapse, as the lungs do when the chest cavity is opened. 

 But if some air be sucked out through c the pressure of that re- 

 maining in the bottle will diminish, and of that inside the bag 

 will be unchanged, and the bag will thus be blown up, because 

 the atmospheric pressure on its interior will not be balanced by 



