344 TEXT-BOOK OF PHYSIOLOGY. 



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 counterbalanced 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, intra-thoracic pressure, negative to the pressure inside the 

 lungs. The amount of this intra-thoracic pressure can be approxi- 

 mately determined 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 Bonders as a measure of the force with which the lungs endeavor 

 to recoil. The intra-thoracic pressure would be, therefore, atmos- 

 pheric 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 extremities of the catheter and leads to a collapse 

 of the lungs may be subsequently aspirated, when the lung returns to 

 its normal position. The catheter is then placed in connection with 

 a water manometer. On establishing a communication between 

 them, by the turning of a stopcock, the water will rise in the proximal 

 and fall in the distal limb of the manometer, indicating a pressure 

 in the thorax negative to that in the lung. The difference in the 

 level of the water in the two limbs of the manometer, expressed 

 in millimeters of mercury, would also represent the force with which 

 the elastic tissue strives to recoil, the extent to which it opposes 

 the atmospheric pressure. This subtracted from the atmospheric 

 pressure would give the intra-thoracic pressure. In the living dog 

 this latter is less than the former, to the extent of from 3.5 to 5.5 mm. 

 For the same reason the superior surface of the diaphragm also ex- 

 periences a pressure less than that of the atmosphere. Owing to the 

 soft and yielding character of the abdominal walls the atmospheric 

 pressure is transmitted through the abdominal organs to the inferior 

 surface of the diaphragm. The pressure being greater from below 

 than above, the diaphragm is forced upward until it assumes the dome- 

 like appearance it usually presents. (These relations are shown in 

 Fig. 1 66.) 



The cause of the negativity of the intra-thoracic pressure is con- 



