138 RESPIRATION 



and the air and blood being separated only by the very thin 

 walls of the capillary and vesicle. The elastic tissue is very 

 important in expelling the air from the cells when the in- 

 spiratory effort has ceased. 



For the nutrition of the bronchi and lung substance ar- 





FIG. 50.- Terminal branch of a bronchial tube, with its infundib- 

 ula and air-sacs, from the margin of the lung of a 'monkey, injected 

 with quicksilver. 



a, terminal bronchial twig; b, b, air-sacs; c, c, infundibula. X 10. (Kirkes 

 after E. E. Schulse.) 



terial blood is furnished by the bronchial artery, which en- 

 ters and ramifies with the bronchi. The entire mass of 

 venous blood passes directly from the heart through the 

 pulmonary artery to the lungs to be arterialized, and it is 

 the capillaries of this artery which furnish the abundant net- 

 work between the air cells. 



The lungs have the shape of irregular cones, their bases 

 resting on the diaphragm and their apices extending to 

 points a little above the clavicles. They are completely sep- 

 arated from each other by the mediastinum and their exter- 

 nal surfaces are covered by the pleura, a serous membrane 

 similar to the peritoneum and reflected from the thoracic 

 wall. The right lung is divided by fissures into three lobes 

 and the left into two. Superficially the lung substance is 

 seen to be subdivided into areas about Y$ in. in diameter 

 called the lobules. Each lobule is composed of a number of 

 lobulettes as above mentioned. 



