SEC. 1. THE MECHANICS OF PULMONARY KESPIRA- 



TIOK 



255. The lungs are placed, in a state which is always one 

 of distension, sometimes greater, sometimes less, 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. The lungs must follow this enlargement and be 

 themselves enlarged , otherwise the pleural cavities would be 

 enlarged, but this is impossible so long as the thoracic walls 

 are intact 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 , and this difference of pressure causes 

 a rush of air through the. trachea into the lungs until an equi- 

 librium 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 by the contraction of certain muscles, causes the chest 

 to return to its original size ; in consequence of this the pres- 

 sure 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 form a respiration. The fresh air 

 introduced into the upper part of the pulmonary passages by 

 the inspiratory movement contains more oxygen and less car- 

 bonic 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 diffu- 

 sion between it and the stationary air, the whole air in the 



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