CH. XXIV.] 



THE LUNGS. 



345 



diameter. Between the air in the sacs and the blood in these 

 vessels nothing intervenes but the thin walls of the air-sacs and 

 of the capillaries*; and the exposure of the blood to the air is the 

 more complete, because the folds of membrane between contiguous 

 air-sacs, and often the spaces between the walls of the same, 

 contain only a single layer of capillaries, both sides of which are 

 thus at once exposed to the air. The arrangement of the 

 capillaries is shown on a larger scale in fig. 224 (p. 220). 



The vesicles of adjacent lobules do not communicate ; so that, 

 when any bronchial tube is closed or obstructed, the supply of 

 air is lost for all the sacs opening into it or its branches. 



Fig. 316. Section of lung stained with silver nitrate. A. D., alveolar duct or intercellular 

 passage ; 8, alveolar septa ; N, alveoli or air-Macs, lined with large flat cells, with some 

 smaller polyhedral cells; M, plain muscular fibres surrounding the alveolar duct. 

 (Klein and Noble Smith.) 



Blood-supply. The lungs receive blood from two sources, 

 (a) the pulmonary artery, (f>) the bronchial arteries. The former 

 conveys venous blood to the lungs to be artericdized, and this 

 blood takes no share in the nutrition of the pulmonary tissues 

 through which it passes. The branches of the bronchial arteries 

 convey arterial blood from the aorta for the nutrition of the walls 

 of the bronchi, of the larger pulmonary vessels, of the interlobular 

 connective-tissue, fec.; the blood of the bronchial vessels is returned 

 chiefly through the bronchial and partly through the pulmonary 

 veins. 



Lymphatics. The lymphatics are arranged in three sets : 

 i. Irregular lacunae in the walls of the alveoli or air-sacs. The 



