320 THE ORGANS 



the large pulmonary vessels, one on either side of the bronchus, and 

 two or more small bronchial vessels in the walls of the bronchus 

 (Fig. 208). 



The pulmonary lobule forms a distinct "blood-vascular unit." A 

 branch of the pulmonary artery enters the apex of each lobule close 

 to the lobular bronchus, and almost immediately breaks up into 

 branches, one of which passes to each alveolar passage (Fig. 213). 

 From these are given off minute terminal arterioles which pass to the 



^ 



Fig. 218. — Parts of Four Alveoli from Section of Injected Human Lung. X200. 

 (Technic 5, p. 323.) a, Wall of ah^eolus seen on flat; c, same, but only small part of 

 alveolar wall in plane of section; b, alveoli in which plane of section includes only side 

 walls; alveolar wall seen on edge. 



central sides of the alveolar passages and alveoli, where they give 

 rise to a rich capillary network. This capillary network is extremely 

 close-meshed, and invests the alveoli on all sides (Fig. 218). Similar 

 networks invest the walls of the respiratory bronchi, the alveolar 

 ducts, and their alveoli. All of these capillary networks freely 

 anastomose. 



There are thus interposed between the blood in the capilaries 

 and the air in the alveoli only three extremely thin layers: (i) The 

 thin endothelium of the capillary wall; (2) the single layer of flat 



