RESPIRATION. 



posure of the blood to the air is the more complete, because the folds 

 of membrane between contiguous cells, 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 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 pas- 

 sage, 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 

 pulmonary artery, (t>) the bronchial arteries. The former conveys ven-< 

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



FIG. 153. Capillary network of the pulmonary blood-vessels in the human lung, x 60. (Kol- 

 liker.) 



share in the nutrition of the pulmonary tissues through which it passes. 

 (b) The branches of the bronchial arteries ramify for nutrition's sake in 

 the walls of the bronchi, of the larger pulmonary vessels, in the inter- 

 lobular connective tissue, etc. ; the blood of the bronchial vessels .being 

 returned chiefly through the bronchial and partly through the pulmo- 

 nary veins. 



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

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

 sels 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 pulmonary pleura. The lymphatic 

 vessels from all these irregular sinuses pass in towards the root of the 

 lung to reach the bronchial glands. 

 12 



