SEO. 1. THE MECHANICS OF PULMONARY RESPIRATION. 



The lungs are placed, in a semi-distended state, 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 muscjes the cavity of the thorax is enlarged ; in conse- 

 quence 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 and 

 that outside the lungs. This constitutes inspiration, tlpon the 

 relaxation of the inspiratory muscles (the muscles whose contraction 

 has brought about the thoracic expansion), the elasticity of the 

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

 tion 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 

 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 



