THE FINER STRUCTURE OF THE LUNG 251 



they are subject. Their walls are nearly in contact, and they vary from 0.5 

 to 0.3 mm. in diameter. Their walls are formed of fine membrane similar 

 to that of the intercellular passages and continuous with it. The 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 fibers of elastic tissue are spread 

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

 to the outer surface of the wall of which each cell is composed, imparting to 

 it additional strength and the power of recoil after distention. 



The air-cells are lined by a layer of epithelium, figure 225, the cells of 

 which are very thin and plate-like. The thin epithelial membrane is free on 

 one side, where it comes in contact with the air of the lungs, but on the other 



FIG. 226. Section of Injected Lung, Including Several Contiguous Alveoli. (F. E. Schulze.) 

 Highly magnified, a, a, Free edges of alveoli; c, c, partitions between neighboring alveoli, seen in 

 section; b, small arterial branch giving off capillaries to the alveoli. The looping of the vessels to 

 either side of the partitions is well exhibited. Between the capillaries is seen the homogeneous 

 alveolar wall with nuclei of connective-tissue corpuscles and elastic fibers. 



side a network of pulmonary capillaries is spread out so densely, figure 226, 

 that the interspaces or meshes are even narrower than the vessels. These 

 are on an average -g-oVo of an inch, or 8 micromillimeters, in diameter. Be- 

 tween the atmospheric air-cells and the blood in these vessels, nothing in- 

 tervenes but the thin walls of the cells and capillaries. The exposure of the 

 blood to the air is the more complete because the wall between contiguous 

 air-cells, and often the spaces between the walls of the same, contain only 

 a single layer of capillaries both sides of which are at once exposed to the air, 



