SEC. 2. THE MECHANICS OF PULMONARY RESPIRATION. 



324. The lungs are placed, in the air-tight thorax, the 

 cavity of which they, together with the heart, great blood vessels 

 and other organs, completely fill. By the contraction of certain 

 muscles the cavity of the thorax is enlarged. As the result of this, 

 in order to fill up the increased thoracic space, the contents of the 

 thorax are enlarged, and nearly the whole of this enlargement falls 

 on the lungs. The heart and great blood vessels it is true, as 

 we shall see, are to a certain extent distended by the enlargement 

 of the thorax because that enlargement draws blood into them from 

 the vessels outside the thorax. But that which we speak of as the 

 pleural cavity around each lung cannot be enlarged. The pulmo- 

 nary pleura over the lung is separated from the parietal pleura 

 lining the chest wall by nothing more than an exceedingly thin 

 layer of fluid, of lymph; the two membranes are virtually in 

 contact and the pleural cavity between them is a potential rather 

 than an actual space. The enlargement of the thorax cannot 

 enlarge the pleural cavity either by drawing more lymph into it 

 (or only to an inappreciable extent), or, so long as the membranes 

 are intact, by drawing air into it, or in any other way; in the 

 enlargement of the thorax the pulmonary pleura still keeps close to 

 the parietal pleura. That is to say, the lungs must follow the 

 enlargement of the thorax and be themselves enlarged. The 

 enlargement of the lung consists chiefly in an enlargement or 

 expansion of the pulmonary alveoli, the air in which becomes by 

 the expansion rarefied. That is to say the pressure of the air 

 within the lungs becomes less than that of the air outside the 

 body, becomes as it is said 'negative'; and this difference of 

 pressure causes a rush of air through the trachea into the lungs 

 until an equilibrium of pressure is established between the air 

 inside the lungs and that outside. This constitutes inspiration. 

 Upon the relaxation of the inspiratory muscles (the muscles whose 

 contractions have brought about the thoracic expansion), the 

 elasticity of the lungs and chest-walls, aided perhaps to some extent 



