CHAP, ii.] RESPIRATION. 315 



into the chest and of removing carbonic acid from it. In this way, 

 by the ebb and flow of the tidal air, and by diffusion between it 

 and the stationary air, the air in the lungs is being constantly 

 renewed through the alternate expansion and contraction of the 

 chest. 



In ordinary respiration, the expansion of the chest never reaches 

 its maximum ; by more forcible muscular contraction, by what is 

 called laboured inspiration, an additional thoracic expansion can be 

 brought about, leading to the inrush of a certain additional quantity 

 of air before equilibrium is established. This additional quantity 

 is often spoken of as complemental air. In the same way, in 

 ordinary respiration, the contraction of the chest never reaches its 

 maximum. By calling into use additional muscles, by a laboured 

 expiration, an additional quantity of air, the so-called reserve or 

 supplemental air, may be driven out. But even after the most 

 forcible expiration, a considerable quantity of air, the residual air, 

 still remains in the lungs. The natural condition of the lungs in 

 the chest is in fact one of partial distension. The elastic pulmonary 

 tissue is always to a certain extent on the stretch ; it is always, so 

 to speak, striving to pull asunder the pulmonary from the parietal 

 pleura ; but this it cannot do, because the air can have no access to 

 the pleural cavity. When however the chest ceases to be air-tight, 

 when by a puncture of the chest-wall or diaphragm, air is introduced 

 into the pleural chamber, the elasticity of the lungs pulls the pul- 

 monary away from the parietal pleura, and the lungs collapse, 

 driving out by the windpipe a considerable quantity of the residual 

 air. Even then, however, the lungs are not completely emptied, 

 some air still remaining in the air-cells and passages. It need 

 hardly be added that when the pleura is punctured, and air can 

 gain free admittance from the exterior into the pleural chamber, 

 the effect of the respiratory movements is simply to drive air in 

 and out of that chamber, instead of in and out of the lung. There 

 is in consequence no renewal of the air within the lungs under 

 those circumstances. 



In man the pressure exerted by the elasticity of the lungs alone 

 amounts to about 5 mm. of mercury. This is estimated by tying a 

 manometer into the windpipe of a dead subject and observing the 

 rise of mercury which takes place when the chest-walls are punc- 

 tured. If the chest be forcibly distended beforehand, a much larger 

 rise of the mercury is observed, amounting, in the case of a dis- 

 tension corresponding to a very forcible inspiration, to 30 mm. In 

 the living body this mechanical elastic force of the lungs is assisted 

 by the contraction of the plain muscular fibres of the bronchi ; the 

 pressure however which can be exerted by these probably does 

 not exceed 1 or 2 mm. 



When a manometer is introduced into a lateral opening of the 

 windpipe of an animal, the mercury will fall, indicating a negative 

 pressure as it is called, during inspiration, and rise, indicating a 



