THE LUNG 



THE PULMONARY ALVEOLI 



311 



The pulmonary alveoli are minute air cells, open toward the alveolar 

 ducts, whose extremely thin wall consists of a capillary network, a deli- 



FIG. 294. DIAGRAM OF THE THREE PULMONARY LOBULES CONNECTED WITH A 

 TERMINAL BRONCHIOLE (TB). 



The middle lobule is stippled. BR, bronchiole ;.RB, respiratory bronchiole of 

 the first order; TB, respiratory bronchiole of the second order (terminal bronchiole); 

 /, II, III, alveolar ducts; /, 2, and 3 a, b, c and d, atria; As, alveolar sacs with 

 pulmonary alveoli or air cells. (Adapted from Miller, Jour. Morph., 20, 4, 1913.) 



cate fibre-elastic reticulum, and a lining epithelium. The alveoli are 

 so densely clustered about the alveolar duct that the capillary plexus, in 

 the form of a reticulated membrane of wide capillary vessels, is exposed 

 to the air of two adjacent alveoli, being separated therefrom only by its 

 own endothelium and the epithelial lining of the alveolus. 



