THE BESPIEATOSY TRACT. 



71.-, 



vessels, destined also for the nutrition of the adventitial coat 

 of the larger blood-vessels j their capillaries, however, inosculate 

 with those of the alveoli of the lung. 



The branches of the pulmonary artery, upon reaching the 

 lobules, rapidly decrease in diameter, and supply the alveoli 

 with a rich reticulum of capillary blood-vessels, which is located 

 directly underneath the flat, alveolar epithelium, and is often 

 seen, toward the central space, rising above the level of the 



FIG. 318. LUNG OF MAN. BLOOD-VESSELS INJECTED. 



A, pulmonary artery, C, capillaries of the alveoli ; IF, walls of the alveoli; B, bronchus, 

 in transverse section. Magnified 200 diameters. 



alveolar wall. In such bulging portions the endothelia of the 

 capillaries are easily discerned. (See Fig. 318.) 



In sections made through injected and hardened lungs, the 

 walls of the alveoli are marked by the presence of the pre- 

 capillary, arterial, and venous vessels, and by an apparently 

 more abundant reticulum of capillaries. Some of the alveoli 

 exhibit the capillary reticulum in parts or throughout their 



