210 A MANUAL OF PHYSIOLOGY 



mercury in the expiratory position to 9 mm. in the in- 

 spiratory. So far as can be judged from observations made 

 on persons suffering from various diseases of the respiratory 

 organs, the alterations during ordinary breathing do not 

 amount to more than 3 or 4 mm. of mercury. But when an 

 attempt is made in the dead body to imitate a deep in- 

 spiration by making traction on the chest-walls so as to 

 expand the lungs, the intra-thoracic pressure may fall to 

 30 mm. of mercury; and in a living rabbit during a 

 deep natural inspiration, a pressure of 20 mm. has been 

 seen. 



The reason why the lungs collapse when the chest is 

 opened is that the pressure is now equal on the pleural and 

 alveolar surfaces, being in both cases that of the atmosphere. 

 There is therefore nothing to oppose the elasticity of the 

 lungs, which tends to contract them. So long as the chest 

 is unopened, the pressure on the pleural surface of the lungs 

 is less than that on the alveolar surface, and the elastic 

 tension can only cause them to shrink until it just balances 

 this difference. 



In intra-uterine life, and in stillborn children who have 

 never breathed, the lungs are completely collapsed (atelec- 

 tatic), and there is no negative intra-thoracic pressure. 

 They are kept in this condition by adhesion of the walls of 

 the bronchioles and alveoli. If the lungs have been once 

 inflated, this adhesion ceases to act, and they never com- 

 pletely collapse again. 



Amount and Variations of the Respiratory or Intra-pulmonary 

 Pressure. As we have already remarked, the pressure in the 

 alveoli and air-passages is less than that of the atmosphere 

 while the inspiratory movement is going on, greater than 

 that of the atmosphere during the expiratory movement, 

 and equal to that of the atmosphere when the chest-walls 

 are at rest. When the external air-passages are closed, e.g., 

 by connecting a manometer with the mouth and pinching 

 the nostrils, the greatest possible variations of pressure are 

 produced. In the deepest inspiration under these condi- 

 tions a negative pressure of about 75 mm. of mercury \i.e., a 

 pressure less than that of the atmosphere by this amount) 



