RESPIRATION 117 



between the outer surface of the lung and the thoracic wall on each 

 side is the so-called "pleural cavity." Physiologically speaking, this 

 is a misleading misnomer. Each of the pleurse is a sac, one of whose 

 sides lines the ribs and intercostal muscles, while the other encloses a 

 lung. It is only because this sac is completely closed, air-tight, that the 

 thoracic wall on expanding outward does not draw apart its two 

 sides and leave the lung unenlarged. Boys sometimes amuse them- 

 selves with a scientific toy made of a circular piece of stout leather 

 five or six inches in diameter, through the center of which is knotted 

 a strong string. The leather being wet and pressed carefully over 

 the smooth surface of a boulder, a considerable rock can be lifted by 

 drawing upward on the string. Theoretically about as many pounds 

 of rock can be raised as there are square inches of leather multiplied 

 by 14.7, which is the weight in pounds of the atmosphere over a square 

 inch of this sucker. The boy's muscles lift the rock, but the weight 

 of the air keeps the leather meanwhile in contact with the stone. 

 Exactly so in inspiration: the muscles (external intercostals chiefly) 

 horizontally enlarge the thorax, but the atmospheric pressure keeps 

 the outer surface of the lungs in contact with the expanding thoracic 

 wall, thus distending the lungs against their elasticity. Thus, two 

 forces combine to draw the air through the nostrils and into the 

 larger bronchi, muscular contraction providing space into which the 

 atmosphere is forced by its own weight (gravitation}. 



The essential importance of the air-tightness of the pleural cavities, 

 so-called, is demonstrated only too often by accidents to people in 

 which the pleural sac is punctured in a way that the tissues cannot 

 immediately close. By a bullet, a dagger, or a corroding ulcer, air 

 is let into a pleural "cavity" which then for the first time becomes 

 a real cavity. The elasticity of the lung-tissue now draws together 

 the lung in a collapsed condition, and it no longer enlarges in inspira- 

 tion, no longer ventilates. The two surfaces, visceral and parietal, 

 of the pleura are drawn apart and separated by air instead of by a 

 mere lubricating layer of lymph or plasma such as alone is present 

 in the uninjured animal. If inflammation does not occur the opening 

 in the thoracic wall soon heals together, the air in the cavity is 

 absorbed by the tissues, and the lung gradually resumes its bellows-like 

 expansion and contraction and its normal functions. If both pleural 

 "cavities" are punctured, and skilled and unusual means be not close at 

 hand, the patient promptly dies of asphyxia, both lungs being then 

 undilatable. This is the condition known in surgery as pneumothorax, 

 single and double respectively. 



The force which causes the oxygen of the bronchial tidal air to 

 continue its course to the alveoli on its way to the capillary blood 

 there, seems to be an addition to this the physical force of diffusion. In 

 respiration this force may be said to be the preeminent physical prin- 

 ciple, if the term be used to include osmosis, the passage through a 

 membrane. The diffusion of gases is that process by which two 



