THE RESPIRATORY SYSTEM 



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pressure is greater, by about 730 mm., than that which tends to 

 collapse them. This explains why the lungs are kept expanded 

 and in contact with the chest wall under natural conditions. If 

 the thoracic wall is perforated and the external air is allowed to 

 rush into the pleural cavity, the lungs immediately collapse, driv- 

 ing a certain amount of air out through the trachea for now, in 

 addition to the contractile force of the lungs, there is added 

 the atmospheric pressure, against which is only the normal 

 pressure from the air in the lungs. A collapsed lung is practi- 

 cally useless. 



Fio. 35. — Apparatus to illustrate the mechanics of respiration: L, Lungs of 

 experimental animal; D, rubber diaphragm. At the left the lungs are collapsed 

 and the diaphragm relaxed, as occurs after expiration ; at the right the diaphragm 

 is contracted, which increases the negative pressure around the lungs and allows 

 the air to rush into and expand them. 



The mechanism of breathing may be best understood by study- 

 ing the movements of lungs recently removed from a living rabbit, 

 dog, or pig, and placed in an apparatus similar to that illustrated 

 in Fig. 35. The glass bell-jar represents the thoracic walls. 

 The jar is closed beneath by a sheet of rubber (D), to take the 

 place of the diaphragm. Conditions that closely approximate 

 the normal pleural cavity may be thus obtained. By partially 

 exhausting the air in the bell-jar through the tube with the pinch- 

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