RESPIRATION 



399 



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 pulmonic layer of the pleura away from the thoracic 

 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. 



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

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

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





FIG. 185. SECTION OF THE THORAX OF A DOG WITH THE LUNGS, HEART, AND PRINCIPAL 

 VESSELS. S. Catheter introduced into the pi eural space and connected with a manometer. 

 (After Moral and Doyen.) 



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. 



The amount of this intra-thoracic pressure can be approximately deter- 

 mined in several ways. Thus, if shortly after death a mercurial manometer 

 be inserted air-tight into the trachea of a human being and the thorax opened, 

 the lungs will recoil and compress their contained air. The mercurial 

 manometer will at once show an excess of pressure in the trachea of about 

 6 mm. This was taken by Donders as a measure of the force with which 

 the lungs endeavor to recoil. The intra-thoracic pressure would be, there- 

 fore, atmospheric pressure, 760 mm., less 6 mm., or 754 mm. Hg. Another 

 method is to insert a rubber catheter through a small opening in an intercostal 

 space into the thoracic cavity. The air which enters through the open ex- 



