ELEMENTARY DEMONSTRATIONS 141 



CHAPTER XXXVIII. 

 INTRA-THORACIC PRESSURE. 



Intra-thoracic Pressure. The thoracic cavity, when opened, is far 

 larger than its contents, for the lungs, owing to their elasticity, collapse 

 so soon as the intra-pulmonary and pleural pressures became equal. 

 The inbra-pleural pressure is less than the atmospheric pressure by that 

 amount of the atmospheric pressure which is required to overcome the 

 elasticity of the lungs and distend these organs to the size of the 

 thoracic cavity. The intra-thoracic pressure or elastic traction exerted 

 by the lungs on the thoracic wall varies as follows : 



Normal inspiration about - 10 mm. Hg. 

 ,, expiration - 7 



Deep inspiration - - 40 



expiration - 



inspiration with air-way closed - 100 



expiration +100 



The intra-tracheal pressure varies from - 1 mm. Hg. in quiet inspira- 

 tion to + 1 mm. Hg. in expiration. During forced breathing with the 

 air-way closed the intra-tracheal pressure is greater than the intra- 

 thoracic pressure by the amount of the elastic traction exerted by the 

 lungs. All the structures, e.g. heart and blood-vessels, are affected by 

 the respiratory variations of pressure. 



DEMONSTRATION. The trachea of a dead rabbit is exposed, and a 

 ligature tied round it. The skin is divided over the thorax on one 

 side, and the ribs exposed. One inch of two adjoining ribs is removed. 

 Note that the lung is in contact with the thoracic wall. The 

 ligature round the trachea is now divided; the air escapes, and the 

 lung, owing to its elasticity, will collapse. On opening the pleural 

 cavity the pressure within and without the lung becomes atmospheric. 

 The elasticity of the distended lung then comes into play and causes its 

 collapse. 



DEMONSTRATION. In the rabbit anaesthetised with urethane or 

 chloral the skin is divided over an intercostal space. The intercostal 

 muscles are then separated with care, and a piece of rib removed, 

 while the parietal pleura is left quite uninjured. The lung will not 

 collapse so long as the pleural cavity is not opened. On the contrary 

 it will be seen gliding to and fro with each movement of respiration. 

 Note how easily the pleural surface of the lung glides over the 

 parietal pleura. A glass cannula attached to a water manometer is 



