350 RESPIRATION [CH. XXIV. 



which are, on an average, ^V^th of an inch (8/x) in 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. 233 (p. 222). 



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. 326. Capillary network of the pulmonary blood-vessels in the human lung. 

 X 60. (Kolliker.) 



Blood-supply. The lungs receive blood from two sources, (a) the 

 pulmonary artery, (b) the bronchial arteries. The former conveys 

 venoics blood to the lungs to be arterialised, 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, etc. ; 

 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: 1. 

 Irregular lacun?e in the walls of the alveoli or air-sacs. The lym- 

 phatic vessels which lead from these accompany the pulmonary 

 vessels towards the root of the lung. 2. Irregular anastomosing 

 spaces in the walls of the bronchi. 3. Lymph-spaces in the pul- 

 monary pleura. The lymphatic vessels from all these irregular 



