STUUCTURE OF THE LUNGS. 



193 



than the vessels, which are, on an average, -njVzr f an 

 inch in diameter. Between the atmospheric air in the 

 cells and the blood in these vessels, nothing intervenes 

 but the thin membranes of the cells and capillaries and 

 the delicate epithelial lining of the former ; 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, con- 

 tain only a single layer of capillaries, both sides of which 

 are thus at once exposed to the air. 



The cells situated nearest to the centre of the lung are 

 smaller, and their networks of capillaries are closer than 

 those nearer to the circumference, in adaptation to the 



Fig. 6 1.* 



mure ready supply of fresh air to the central than the 

 peripheral portion of the lungs. The cells 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, 



* Fig. 61. Capillary net-work of the pulmonary blood-vessels in the 

 human lung (from Kolliker) 6 T . 



o 



