PRESSURE CONDITIONS IN LUNGS AND THORAX. 585 



of the atmosphere, the intrathoracic pressure is less than an 

 atmosphere by an amount equal to the recoil of the lungs. The 

 negative pressure in the thorax is, therefore, equal to the elastic 

 force of the lungs, and is larger the more the lungs are put upon 

 a stretch, that is, the deeper the inspiration. The amount of this 

 negative pressure has been measured upon both animals and men 

 by two methods : First by Donders method of attaching a manom- 

 eter to the trachea and then opening the thoracic walls so as to 

 allow the atmosphere to press upon the exterior face of the lungs. 

 In this way the elastic force of the lungs is determined, and, as 

 explained above, this is equivalent to the negative pressure. Second, 

 by thrusting a trocar through the thoracic wall so that its open end 

 may lie in the pleural or mediastinal cavity, the other end being 

 appropriately connected with a manometer. The older observers 

 (Hutchinson) also made experiments upon freshly excised human 

 lungs, determining their elastic force when distended by known 

 amounts of air. The figures obtained by these different methods 

 have shown some variations, but the following quotations give 

 an idea of the average extent of this negative pressure. Heynsius,* 

 making use of the figures obtained by Hutchinson, estimates that 

 in man the negative pressure in the thorax at the end of inspiration 

 is 4.5 mms. Hg, while at the end of an inspiration it is equal to 

 7.5 mms. Hg, a variation during respiration, therefore, of 3 mms. 

 Hg. That is, assuming that the atmospheric pressure is 760 mms. 

 Hg, the conditions of pressure in the thorax and lungs at the end of 

 inspiration and expiration are as follows: 



At the End of Inspiration. At the End of Expiration. 

 Intrapulmonic pressure . . . 760 mms. Hg. 760 mms. Hg. 



Intrathoracic pressure .... 752.5 " " 755.5 " " 



Aron gives results obtained from a healthy man in whom a can- 

 nula was connected directly with the pleural cavity. f From 36 

 determinations he obtained the average result that at the end of 

 quiet inspiration the negative pressure is 4.64 mms. Hg (755.36) 

 and at the end of expiration 3.02 mms. Hg (756.98), results 

 considerably lower than those estimated by Heynsius. It should 

 be borne in mind, however, that these values depend upon the 

 condition of expansion of the chest, that is, the position of the 

 body and the depth of inspiration. On dogs Heynsius reports as 

 follows: At end of inspiration, 9.4 mms. Hg; end of expiration, 

 3.9 mms. On rabbits, 4.5 mms. and 2.5 mms. Hg. 



Variations of Intrathoracic Pressure with Forced and Unusual 

 Respirations. After the most forcible expiration, when the air- 



* "Archiv f . die gesammte Physiologie, " 29, 265, 1882. 



t Aron, quoted from Emerson, "Johns Hopkins Hospital Reports," 11, 

 194, 1903. 



