RESPIRATION. 357 



PRESSURE IN THE AIR=PASSAGES DURING RESPIRATION. 



It has been previously stated that even after the deepest expira- 

 tion the lungs are never completely collapsed. They are still "on 

 the stretch" by reason of the elastic fibers contained in them. 



The reason for the collapsing of the lungs when the chest is 

 opened is that the pressure upon the pleural and alveolar surfaces 

 is now equal, being that of the pressure of the atmosphere. The 

 pressure of the residual air was sufficient to overcome the elasticity 

 of the muscular fibers of the lungs. As long as the chest-wall was 

 unopened the lungs contracted only until their elasticity was just 



Fig. 125. Number of Respirations by Man at Different Ages. . 

 (QUETELET.) (From Tigerstedt's "Human Physiology," copyright, 1906, 

 by D. Appleton and Company. ) 



Read from left to right. 



balanced by the outward pressure of the contained air. In intra- 

 uterine life, and in stillborn children who have never breathed, the 

 lungs are completely collapsed (atelectasis). If the lungs be once 

 inflated they never completely collapse so long as the thoracic walls 

 be not pierced. 



When a manometer was attached to the trachea of an animal 

 so that its respirations proceeded unchecked, every inspiration 

 showed a negative pressure, every expiration a positive pressure. 

 An observer placed a U-shaped manometer tube in one of his nos- 

 trils, closed his mouth, let the other nostril open, and then respired 

 quietly. During every inspiration there was a negative pressure of 



