SEC. 2. THE MECHANICS OF PULMONARY RESPIRATION. 



324. 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 ; other- 

 wise the pleural cavities would be enlarged, but this is impossible 

 so long as the 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 rarified. 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 

 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 contrac- 

 tions have brought about the thoracic expansion), the elasticity of 

 the lungs and chest-walls, aided perhaps to some extent by the con- 

 traction 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 consti- 

 tutes expiration ; the inspiratory and expiratory act together form- 

 ing a respiration. The fresh air introduced into the upper part of 

 the pulmonary passages by the inspiratory movement contains more 

 nxygen 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 



