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TEXT-BOOK OF PHYSIOLOGY. 



occupied by the heart and great blood-vessels (Fig. 189). This condition is 

 maintained by the pressure of the air within the lungs, the intra-pulmonic 

 pressure, which with the respiratory passages open, is that of the atmosphere, 

 760 mm. Hg. This relation persists so long as the thorax remains air- 

 tight. If the skin and muscles covering an intercostal space be removed the 

 lung can be seen in close contact with the parietal layer of the pleura gliding 

 by with each inspiration and expiration. If, however, an opening be now 

 made in the pleura sufficient to admit air, the lung immediately collapses and 

 a pleural cavity is established. The pressure of air within and without the 

 lung counterbalancing, at the moment the air is admitted, the elastic tissue 

 at once recoils and forces a large part of the air out of the lung. This is a proof 

 that in the normal condition, the lungs, distended by atmospheric pressure 

 from within, are in a state of elastic tension and ever endeavoring to pull the 



FIG. 189. SECTION OF THORAX WITH THE LUNGS, HEART, AND PRINCIPAL VESSELS. 5. 

 Catheter introduced into the pleural space and connected with a manometer. (After Moral and 

 Doyen.} 



visceral layer of the pleura away from the parietal layer. That they do not 

 succeed in doing so is due to the fact that the atmospheric pressure from 

 without is prevented from acting on the lung by the firm unyielding walls of 

 the thorax. 



Intra-thoracic Pressure. As a result of the elastic tension of the lungs a 

 fractional part of the intra-pulmonary pressure, 760 mm. Hg., is counter- 

 balanced or opposed, so that the heart and great vessels and other intra- thoracic 

 viscera are subjected to a pressure somewhat less than that of the atmosphere; 

 the amount of this pressure will be that of the atmosphere less that exerted by 

 the elastic tissue of the lung in the opposite direction, expressed in terms of 

 millimeters of mercury. In the thorax but outside the lungs, there then 

 prevails a pressure, negative to the pressure inside the lungs and which is 

 known as the intra-thoracic pressure. 



