RESPIRATION, 505 



'Donders found that the pressure at the end of quiet expiration was 6 mil- 

 limeters of Hg, and at the end of quiet inspiration 9 millimeters. Accord- 

 ing to these figures, the pressure on the heart, great blood-vessels, and other 

 thoracic structures lying between the lungs and the thoracic walls would be 

 754 millimeters of Hg (one atmosphere, 760 millimeters, 6 millimeters) at 

 the end of quiet expiration, and 751 millimeters of Hg at the end of quiet 

 inspiration. Corresponding values by Hutchinson are 3 millimeters and 

 4.5 millimeters. Arron l found in a case of a woman with emphysema that 

 the pressure at the end of expiration ranged from 1.9 to 3.9 millimeters, 

 and at the end of inspiration from 4 to 6.85 millimeters, according to the 

 position of the body, the pressure being lowest in the lying posture, higher 

 when sitting in bed, still higher when sitting on a chair, and highest when sit- 

 ting and when inspiration on the well side was hindered, thus throwing a larger 

 portion of the work on the diseased side, on which the measurements were 

 made. During inspiration negative pressure increases in proportion to the 

 depth of inspiration or, in other words, in relation to the amount of expan- 

 sion of the lungs while during expiration it gradually falls to the standard at 

 the beginning of inspiration. During forced inspiration it may reach 30 to 

 40 millimeters or more. The pressure thus observed within the thorax (out- 

 side of the lungs) is known as intratJioracic pressure, and must not be con- 

 founded with intrapulmonary or respiratory pressure, which exists within the 

 lungs and the respiratory passages (see p. 516). 



The thorax is capable of enlargement in all directions. It is cone-shaped, 

 the top of the cone being closed in by the structures of the neck ; the sides, 

 by the vertebral column, ribs, costal cartilages, sternum, and intercostal sheets 

 of muscular and other tissues ; and the bottom, by the arched diaphragm. It 

 is obvious that, since the thorax is an air-tight cavity and completely filled 

 by various structures, enlargement in any direction must cause a diminution of 

 pressure within the lungs, while a shrinkage would operate to bring about an 

 opposite condition of increased pressure. Since the trachea is the only means of 

 communication between the lungs and the atmosphere, it is evident that such 

 alterations in pressure must encourage either the inflow or the outflow of air, as 

 the case may be ; consequently, when the thoracic cavity is expanded the pres- 

 sure within the lungs is less than that of the atmosphere, and air is forced into 

 the lungs ; and when the thorax is decreased in size the reverse of the above 

 pressure relation exists, and the air is expelled. In fact, the thorax and the 

 lungs behave as a pair of bellows just as air is drawn into the expanding 

 bellows, so is air drawn into the lungs by the enlargement of the thorax ; 

 similarly, as the air is forced from the bellows by compression, so is air 

 forced from the lungs by the shrinkage of the lungs and the thorax. 



During the expansion of the thorax the lungs are- entirely passive, and by 

 virtue of their perfect elasticity merely follow the thoracic walls, from which 

 they are separated only by the two layers of the pleurae, which, being moist- 

 ened with lymph, slide over each other without appreciable friction. That 

 1 Virchoufs Archiv, 1891, vol. 126, p. 523. 



