STRUCTURE OF THE LUNGS. 



203 



work of pulmonary capillaries is spread out so densely 

 (fig. 6 1), that the interspaces or meshes are even narrower 

 than the vessels, which are, on an average, ^V(r of an 

 inch in diameter. Between the atmospheric air in the 

 cells and the blood in these vessels, nothing intervenes 

 Fig. 60.* 



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. 



* Fig. 60. Air-cells of lung, magnified 350 diameters, a. Epithelial 

 lining of the cells ; b. Fibres of elastic tissue ; c. Delicate membrane of 

 which the cell- wall is constructed with elastic fibres attached to it (after 

 Kolliker). 



