RESPIRATION 



385 



outside by a double (pleura!) membrane. Their shape is the same 

 as the chest cavity, the upper part of which they completely fill. 

 Between them is the heart and below is the diaphragm which 

 is a muscular partition curving upward so that the lower lung 

 surface is sharply concave. The pleura] membrane that covers 

 the lungs and lines the chest cavity is constantly moist and per- 

 mits free motion of the lungs, within the chest, for breathing. 



?ui.noMATiy A fir. 



TUBE 



T~o PULMOMAHY Vei 



-BLOOD 



Fid. 123. Exchanges between blood and air in lungs. After Colton. 



Pleurisy is an inflamed condition of these membranes which makes 

 breathing very painful and difficult. 



Blood Supply. The pulmonary artery brings the dark (de-oxy- 

 genated) blood to the lungs, where it divides into an extensive 

 network of capillaries, completely surrounding each air cell. The 

 thin walls of both cell and capillary make easy the osmotic ex- 

 change of oxygen from air to blood, and of carbon dioxide and 

 water from blood to air, so that the pulmonary vein returns its 

 blood to the heart, purified and laden with oxygen for the tissues. 



