24:2 



THE EESPIRATORY SYSTEM 



surrounded by the pulmonary alveoli. Neither the atria nor the 

 alveolar duct possess a true wall ; they are surrounded by the open 

 mouths of the pulmonary alveoli and the thin margins of the 

 alveolar walls. 



The pulmonary alveoli are minute air cells, open toward the 

 alveolar duct, whose extremely thin wall consists of a capillary 

 network, a delicate fibro-elastic reticulum, and a lining epithelium. 

 The alveoli are so densly clustered about the alveolar duct that 





^x 



FIG. 204 FROM A SECTION OF A CHILD'S LUNG. 



A, atriuin; B, brbnctyoles ending in terminal bronchioles, TB\ PA, pulmonary artery ; 

 PV, pulmonary vein. Hematein and eosin. Photo, x 50. 



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 endotheliuin and the 

 epithelial lining of the alveolus. 



