564 PUNCTURE OF PLEURA. [BOOK n. 



by the contraction of certain muscles, causes the chest to return to 

 its original size; in consequence of this the pressure within the 

 lungs now becomes greater than that outside, and thus air rushes 

 out of the trachea until equilibrium is once more established. 

 This constitutes expiration; the inspiratory and expiratory act 

 together forming a respiration. The fresh air introduced into the 

 upper part of the pulmonary passages by the inspiratory movement 

 contains more oxygen and less carbonic acid than the old air 

 previously present in the lungs. By diffusion the new or tidal 

 air, as it is frequently called, gives up its oxygen to, and takes 

 carbonic acid from, the old or stationary air, as it has been called, 

 and thus when it leaves the chest in expiration has been the means 

 of both introducing oxygen 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 whole air in the 

 lungs is being constantly renewed through the alternate expansion 

 and contraction of the chest. 



325. In ordinary respiration, the expansion of the chest 

 never reaches its maximum; by more forcible muscular contrac- 

 tions, 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. We said a little back that when the thorax is 

 enlarged the pressure within the pulmonary alveoli becomes 

 ' negative ' and in consequence air rushes into the lungs. The 

 pressure within the alveoli is negative because the whole of the 

 pressure within the thorax, the intrathoracic pressure, is negative. 

 It is the difference between the (negative) intrathoracic pressure 

 and the pressure of the atmosphere which causes the inrush of air 

 in inspiration. But a portion of this difference of pressure is spent 

 in the act of distending the alveoli, is used up in stretching the 

 elastic walls of the alveoli. Hence though after the entrance of 

 air in inspiration equilibrium is established within the cavities of 

 the alveoli, it is not established in the parts of the thoracic cavity 

 lying outside the alveoli, in the pleural spaces, the mediastinum 

 and the like; the pressure outside the alveoli remains negative, it 

 falls short of the pressure of the atmosphere by the amount of 

 pressure used up in putting the walls of the alveoli on the stretch. 

 Moreover the chest is so constructed that even in the condition of 

 rest, before the enlargement of the thorax in inspiration takes place, 



