222 A MANUAL OF PHYSIOLOGY 



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 inspiration 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 pleura! 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 (atelectatic), and 

 there is no negative intrathoracic 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 completely collapse again. 



Amount and Variations of the Respiratory or Intrapulmonary 

 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 conditions a negative pressure of about 

 75 mm. of mercury (i.e., a pressure less than that of the atmo- 

 sphere by this amount) has been found, and in deep expiration a 

 somewhat greater positive pressure* (Practical Exercises, p. 292). 



But with ordinary breathing, the variations of pressure as 

 measured by this method do not exceed 5 to 10 mm. of mercury 

 above or below the pressure of the atmosphere. 



When the external openings are not obstructed, as, for example, 

 when the lateral pressure is taken in the trachea of an animal 

 by means of a cannula with a side-tube connected with a mano- 

 meter, still smaller, and doubtless truer, values have been found 

 (2-3 mm. of mercury, as the positive expiratory pressure and 



* The maximum negative pressure in deepest inspiration averaged for 

 49 students, - 73 mm. (highest observation - 137 mm.) of mercury ; the 

 maximum positive pressure in deepest expiration, + 80 mm. (highest 

 observation + 140 mm.). 



