RESPIRATION 189 



It is easily overcome by too great an inflating strain, as 

 happens in some forms of emphysema, and for this 

 reason artificial inflation of the lungs by bellows is 

 dangerous ; and it is also diminished when the lung is 

 floated up by fluid in the pleura or when it is the seat of 

 acute congestion. The normal * lung note ' obtained on 

 percussing the healthy chest depends on a proper ' tone ' 

 in the lungs, and when that is diminished, as happens, 

 for example, in the upper part of the lung when there is 

 an effusion into the pleural cavity or where the lung is 

 the seat of acute congestion or miliary tuberculosis, a 

 more resonant and even tympanitic percussion note is 

 obtained (Skodaic resonance). 



The surface of the lung is bound to the chest wall by 

 an ' atmospheric ligament,' which acts in the same way 

 as a rubber sucker (Keith). This permits of the lung 

 gliding freely over the interior of the thoracic wall with- 

 out the bond being broken. The weight required to 

 separate the visceral from the parietal pleura over both 

 lungs amounts to about a ton, which is far greater than 

 any force the inspiratory muscles can bring to bear. 

 Hence by no muscular effort can the pleural bond be 

 broken. 



The contractility of the lungs causes them to exert a 

 certain ' pull ' on the chest wall and on the mediastinum 

 (Fig. 10). A sort of * tug-of-war,' indeed, goes on between 

 the two lungs, with the mediastinum and its contents as 

 the rope, so that if one lung becomes collapsed e.g., in 

 pneumothorax the other, being no longer opposed, pulls 

 over the heart and other mediastinal organs to its own 

 side. It is mainly in this way that displacement of the 



