THE FINER STRUCTURE OF THE LUNG 



273 



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 \vith the air of the lungs, but on the other 



FIG. 225. From a Section of the Lung of a Cat, Stained with Silver Nitrate. A. D, 

 Alveolar duct or intercellular passage; S, alveolar septa, N, alveoli or air-cells, lined with 

 large, flat, nucleated cells, with some smaller polyhedral nucleated cells; M, unstriped 

 muscular fibers. Circular muscular fibers are seen surrounding the interior of the alveolar 

 duct, and at one part is seen a group of small polyhedral cells continued from the bronchus. 

 (Klein and Noble Smith.) 



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 3 yo n 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. 



The air-vesicles situated nearest to the center of the lung are smaller 

 and their networks of capillaries are closer than those nearer to the circum- 

 18 



