WHY THE LUNGS DO NOT COLLAPSE. 357 



<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 Indian-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 neutralizing one an- 

 other, there is nothing to overcome the tendency of the 

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

 however, the lungs remain distended. The pleura! 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 reaching the lungs; consequently no 

 -atmospheric pressure is exerted on their outside. On their 

 interior, however, the atmosphere presses with its full 

 weight, equal (see Physics) 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. 107) to be a bottle closed air-tight 

 by a cork through which two tubes pass, 

 one of which, b, leads into an elastic bag, 

 4 9 and the other, c, provided with a stop- 

 cock, opens freely below into the bottle. 

 If the stop-cock, c. is open the air will FTO. 107. Diagram 



, , , illustrating the pres- 



enter the bottle and press there on the sure relationships of 



, -I , i i ., . the lungs in the tho- 



outside oi the bag, as well as on its in- rax. 

 side through b. The bag will therefore 

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

 J3ut if some air be sucked out of c the pressure of that remain^ 

 ing in the bottle will diminish, while that inside the bag 

 will be the same, and r.he bag will thus be blown up, because 

 the atmospheric pressure on its interior will not be balanced 

 by that on its exterior. At last, when all the air is sucked 

 out of the bottle and the stop-cock on c closed, the bag, if 

 sufficiently distensible, will be expanded so as to completely 

 fill the bottle and press against its inside, and the state 



