ELASTICITY OF THE LUNGS AND INTRATHORACIC PRESSURE 311 



The following methods have been used for determining intrathoracic pres- 

 sure. A manometer may be connected terminally with the trachea of a corpse, 

 and the thoracic cavity opened without injuring the lungs. Since the pressure 

 within and without the lungs is thereby equalized, the lungs contract in virtue 

 of their elasticity, the force of the contraction being measured by the pressure 

 which the air column exerts on the mercury of the manometer. This is evi- 



Fio. 120. A simple experiment with the lungs of a rabbit to illustrate the normal expansion 

 and collapse of the lungs in response to variations of the intrathoracic pressure. When the 

 rubber membrane representing the diaphragm is drawn down (A) a negative pressure is 

 produced inside the bell jar and the air enters the lungs through the glass tube tied into the 

 trachea. When the membrane is released (B) the pressure inside the bell jar becomes less 

 negative and the lungs collapse in virtue of their own elasticity, forcing the air out. The 

 elastic recoil of the membrane, which tends to increase the pressure inside the jar, may be 

 taken to represent the elastic recoil of the abdominal wall (cf. p. 317). A monometer can 

 be connected through a second opening in the rubber stopper and the actual variations of 

 intrathoracic pressure demonstrated at the same time. 



dently equal to the pressure which was previously necessary to expand the lungs 

 to their original volume. If the thorax were dilated more or less before being 

 opened, the value of the pressure obtained on contraction would vary accordingly 

 (Bonders) . 



The intrathoracic pressure can be determined on a living animal also, by 

 introducing a flattened cannula through a slitlike opening into the pleural cavity, 

 care being taken to prevent the entrance of air (Fredericq). 



