MECHANICAL PHENOMENA OF EXTERNAL RESPIRATION 237 



nothing to oppose the elasticity of the lungs, which tends to con- 

 tract 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 oollapsed (atelectatic), and there 

 is no negative intrathoracic pressure. They are kept in this con- 

 dition 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 in- 

 spiratory movement is going on, greater than that of the atmosphere 

 during the expiratory movement, and equal to that of the atmo- 

 sphere 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 atmosphere by this amount) has been 

 found, and in deep expiration a somewhat greater positive pressure* 

 (Practical Exercises, p. 298). 



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 manometer, 

 still smaller, and doubtless truer, values have been found (2-3 mm. 

 of mercury as the positive expiratory pressure and I mm. as the 

 negative inspiratory pressure in dogs). But since the respiratory 

 passages are abruptly narrowed at the glottis, the variations of 

 pressure must be greater below than above it, and in general they 

 must increase with the distance from that orifice, being greater, for 

 instance, in the alveoli than in the bronchi. 



The mechanical phenomena of respiration having been described, 

 it might seem logical to consider next the nervous mechanism by 

 which the respiratory movements are controlled ; but the regulation 

 of these movements through the nervous system is in so important 

 a degree a chemical regulation that it cannot be properly understood 



* The maximum negative pressure in deepest inspiration averaged for 49 

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

 mum positive pressure in deepest expiration, -f 80 mm. (highest observation 

 + 140 mm.). 



