252 RESPIRATION 



The air-vesicles situated nearest to the center of the lung are smaller 

 and their networks of capillaries are closer than those nearer to the circum- 

 ference. The vesicles of adjacent lobules dp not communicate. Those of 

 the same lobule or proceeding from the same intercellular passage com- 

 municate as a general rule only near angles of bifurcation, so that when any 

 bronchial tube is closed or obstructed the supply of air is lost for all the blood- 

 vessels of that lobule and its branches. 



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

 pulmonary artery; b, the bronchial arteries. The former conveys venous 

 blood to the lungs for its oxidation, and this Wood takes no share in the 

 nutrition of the deeper pulmonary tissues through which it passes. The 

 branches of the bronchial arteries are nutrient arteries which ramify in the 



FIG. 227. Capillary Network of the Pulmonary Blood-vessels in the Human Lung. X 60. 



(Kolliker.) 



walls of the bronchi, in the walls of the larger pulmonary vessels, and in the 

 interlobular connective tissue, etc. The blood of the bronchial vessels is re- 

 turned chiefly through the bronchial, but partly through the pulmonary, veins. 



Lymphatics. The lymphatics are arranged in three sets: i. Ir- 

 regular lacunae in the walls of the alveoli or air-cells. The lymphatic 

 vessels which lead from these accompany the pulmonary vessels toward 

 the root of the lung. 2, Irregular anastomosing spaces in the walls of the 

 bronchi. 3, Lymph-spaces in the pulmonary pleura. The lymphatic vessels 

 from all these irregular sinuses pass in toward the root of the lung to reach 

 the bronchial glands. 



Nerves. The nerves of the lung are to be traced from the anterior 

 and posterior pulmonary plexuses, which are formed by branches of the 

 vagus and sympathetic. The nerves follow the course of the blood-vessels 

 and bronchi, and many small ganglia are situated in the walls of the latter. 



