178 RESPIRATION 



thrown into water ; these properties being due to the presence 

 of air in the minute air-cells of the lungs. To facilitate the 

 movements necessary to these organs, each of them is provided 

 with a double covering of an exceedingly smooth and delicate 

 membrane, called the jdeura. One layer of the pleura is 



FIG. 47. ORGANS OF THE CHEST 



A, Lungs D, Pulmonary Artery 



B, Heart E, Trachea 



attached to the walls of the chest, and the other to the lungs ; 

 and they glide, one upon the other, with utmost freedom. 

 Like the membrane which envelops the heart, the pleura 

 secretes its own lubricating fluid, in quantities sufficient to 

 keep it always moist. 



3. The Air-Passages. The lungs communicate with the 

 external air by means of certain air-tubes, the longest of 

 which the trachea, or windpipe runs along the front of 



8, Communication of the Jun^s with the external air ? Bronchial tubes ? 



