182 HAND-BOOK OF PHYSIOLOGY. 



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

 thus at once exposed to the air. 



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

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

 cumference. The vesicles of adjacent lobules do not communicate; and 

 those of the same lobule or proceeding from the same intercellular passage, 

 do so 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 cells opening into it or its branches. 



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

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

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

 the nutrition of the pulmonary tissues through which it passes. (#) The 



FIG. 154. Capillary network of the pulmonary blood- vessels in the human lung. x60. (Kolliker.) 



branches of the bronchial arteries ramify for nutrition's sake in the walls 

 of the bronchi, of the larger pulmonary vessels, in the interlobular con- 

 nective tissue, etc.; the blood of the bronchial vessels being returned 

 chiefly through the bronchial and partly through the pulmonary veins. 



Lymphatics. The lymphatics are arranged in three sets: 1. Irreg- 

 ular 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 vessels and 

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



