260 HANDBOOK OF PHYSIOLOGY. / 



intercellular passages, and continuous with it, which membrane is folded 

 on itself so as to form a sharp-edged border at each circular orifice of 

 communication between contiguous air-vesicles, or between the vesicles 

 and the bronchial passages. Numerous fibres of elastic tissue are spread 

 out between contiguous air-cells, and many of these are attached to the 

 outer surface of the fine membrane of which each cell is composed, im- 

 parting to it additional strength, and the power of recoil after disten- 

 tion. The cells are lined by a layer of epithelium (fig. 208), not pro- 

 vided with cilia. Outside the cells, a network of pulmonary capillaries 

 is spread out so densely (fig. 209) that the interspaces or meshes are 

 even narrower than the vessels, which are, on an average, 30 * 00 of an 

 inch (8ft) in diameter. Between the atmospheric air in the cells and 

 the blood in these vessels, nothing intervenes but the thin walls of the 



Fig. 209. Capillary network of the pulmonary blood-vessels in the human lung. X 60. (Kolliker.) 



cells and capillaries; and the exposure 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, (b) the bronchial arteries. The former conveys venous 

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



