124 



RESPIRATION. 



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 



FIG. 34. ORGANS or THE CHEST. 



A, Lungs. 



B, Heart. 



D, Pulmonary Artery. 



E, Trachea. 



pleura. One layer of the pleura is 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 mem- 

 brane which envelops the heart, the pleura secretes its own 

 lubricating fluid, in quantities sufficient to keep it alwaye 

 moist. 



3. The Air-Passages. The lungs communicate with 

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



3. Communication of the lungs with the external air ? Bronchial tubes ? 



