RESPIRATION. 



181 



even in rows, like minute sacculated tubes; so that a short series of 

 vesicles, all communicating with one another, open by a common orifice 

 into the tube. The vesicles are of various forms, according to the mutual 

 pressure to which they are subject; their walls are nearly in contact, and 

 thcv vary from -^ to ^ of an inch in diameter. Their walls are formed 

 of fine membrane, similar to that of the intercellular passages, and con- 

 tinuous with it, which membrane is folded on itself so as to form a sharp- 

 edged border at each circular orifice of communication between con- 

 tiguous air-vesicles, or between the vesicles and the bronchial passages. 

 Numerous fibres of elastic tissue are spread out between contiguous air- 



Fio. 153. From a section of lung of a cat, stained with silver nitrate. A. D. Alveolar duct or in- 

 tercellular passage. S. Alveolar septa. N. Alveoli or air-cells, lined with large flat, nucleated cells, 

 with some smaller polyhedral nucleated cells. Circular muscular fibres are seen surrounding the in- 

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

 the bronchus. (Klein and Noble Smith.) 



cells, and many of these are attached to the outer surface of the fine 

 membrane of which each cell is composed, imparting to it additional 

 strength, and the power of recoil after distension. The cells are lined by 

 a layer of epithelium (Fig. 153), not provided with cilia. Outside the 

 cells, a network of pulmonary capillaries is spread out so densely (Fig. 

 154), that the interspaces or meshes are even narrower than the vessels, 

 which are, on an average, -g-gVff ^ an inch in diameter. Between the 

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

 venes but the thin walls of the 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 



