458 RESPIRATION 



they illustrate in a most convincing manner the principle upon which 

 the mechanics of respiration are based. They prove first of all that the 

 lungs are held in firm contact with the internal surface of the chest wall 

 by a definite force, the removal of which immediately allows the pul- 

 monary tissue to separate itself from the wall of the thorax. In the 

 second place, they prove in an unmistakable manner that the tissue 

 of the lung is elastic, and that it is constantly held in a state of hyper- 

 distention. On this account the wall of the chest is always exposed to 

 the elastic recoil of the lungs, the tendency of which is to allow the 

 stretched interalveolar fibers to regain their normal length and shape. 

 Obviously, therefore, these organs are always kept in an expanded state 

 by a force resting upon their external surface and not by a force acting 

 upon the walls of the respiratory passage from within. In other, 

 words, the lungs are not inflated by a current of air forced in through 

 the trachea, but are expanded from without, this movement causing 

 air to flow into their recesses. Consequently, excepting the elastic 

 recoil during expiration, the lungs do not actually participate in an 

 active way in the shifting of the respiratory air. 



The force which keeps the lungs in contact with the internal sur- 

 face of the chest wall is the pressure prevailing in the intrapleural 

 space. In the nature of things, it is the pressure which the elastic 

 power of the lungs would have to overcome in order to pull the pleural 

 layers apart, but as the capillary space between the latter is closed, 

 the recoil of the pulmonary tissue is much too slight to overcome this 

 resistance. It follows, therefore, that the lungs and the chest wall 

 must remain in the closest possible opposition. Supposing, moreover, 

 that the pressure in the passages of the lung amounts to one atmos- 

 phere, or 760 mm. Hg, the pressure to which the heart, great vessels, 

 and other structures of the thoracic cavity are exposed, must be less 

 than this figure, because the elastic tension of the pulmonary tissue 

 constantly opposes and counterbalances the atmospheric pressure. 

 Thus, it must be clear that the pressure prevailing immediately out- 

 side the surfaces of the lungs, ^.e., in the intrapleural space, must be 

 that of the atmosphere minus the elastic recoil of the lung tissue. 



In attempting to measure this elastic pull of the pulmonary tissue 

 Bonders^ connected the trachea of a dead animal with a mercurial 

 manometer and permitted the lungs to collapse by perforating the 

 chest wall. Quite naturally, the recoiUng lungs placed the air within 

 them under a certain pressure, a fact which was most clearly betrayed 

 by the outward movement of the column of mercury. It was found 

 in this way that the lungs are capable of counterbalancing a mercurial 

 column about 6 mm. in height and hence, if this figure is subtracted 

 from 760 mm., the resulting value of 754 mm. indicates the pressure 

 prevailing in the intrapleural space and other regions of the thoracic 

 cavity outside the respiratory channel. 



1 Zeitschr. fur rat. Med., iii, 287. 



