132 PRACTICAL PHYSIOLOGY 



CHAPTER XIX 

 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 as soon as the intra -pulmonary and pleural pressures become 

 equal. The intra-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 I- 

 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 

 inspiration to -f- 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. 



The trachea of a dead rat or rabbit is exposed, and a ligature 

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

 and the ribs exposed. The intercostal muscles are carefully separ- 

 ated between two ribs. 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 lungs becomes atmospheric. The elasticity of the distended 

 lung then comes into play and causes its collapse. Place a glass 

 tube in the trachea and perform artificial ventilation of the lungs. 



CHAPTER XX 



VENTILATION OF THE LUNGS 

 THE SPIROMETER AND THE STETHOGRAPH 



The ventilation of the lungs is determined by a gas-meter through 

 which the subject breathes by means of an anaesthetic mask, pro- 

 vided with inspiratory and expiratory valves. Meters with a very 



