190 



APPLIED PHYSIOLOGY 



heart is produced in cases of pneumothorax and pleural 

 effusion. 



The 'pull' of the lungs on the chest wall entails a 

 slightly negative pressure (amounting to about 6 milli- 



FIG. 10. 



The arrows represent the direction of the ' pull ' exerted by the lungs 

 by means of their contractility. 



metres Hg) in the pleural cavity and on the heart.* 

 This favours the effusion of fluid (hydrothorax) when 



* The following table exhibits approximately the variations in 

 the intrathoracic pressure under different conditions : 



Milligrammes 

 Hg. 



Normal inspiration 10 



Normal expiration 7 



Deep inspiration 40 



Deep expiration ... ... ... ... - 



Deep inspiration with air passages closed - 100 



Deep expiration with air passages closed + 100 



