352 



RESPIRATION 



[CH. XXYI. 



walls are nearly in contact, and they vary from -^th to T Vth of an 

 inch ('5 to *3 mm.) in diameter. Their walls are formed of fine 

 membrane, like those of the intercellular passage; this 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 fibres of elastic 

 tissue are spread out between contiguous air-sacs, and many of these 

 are attached to the outer surface of the fine membrane of which each 

 sac is composed, imparting to it additional strength and the power of 

 recoil after distension. The vesicles are lined by a layer of pavement 

 epithelium (fig. 288). Outside the air-vesicles a network of pulmonary 



FIG. 288. Section of lung stained with silver nitrate. A. D., alveolar duct or intercellular passage ; 

 S, alveolar septa; N, alveoli or air-sacs, lined with large flat cells, with some smaller polyhedral 

 cells ; M, plain muscular fibres surrounding the alveolar duct. (Klein and Noble Smith.) 



capillaries is spread out so densely (fig. 289) that the interspaces or 

 meshes are even narrower than the vessels, which are, on an average, 

 ainnrth of an inch (8 /uC) in diameter. Between the air in the sacs 

 and the blood in these vessels nothing intervenes but the thin walls 

 of the air-sacs and of the capillaries ; and the exposure of the blood 

 to the air is the more complete, because the folds of membrane 

 between contiguous air-sacs, and often the spaces between the walls 

 of the same, contain only a single layer of capillaries, both sides of 

 which are thus at once exposed to the air. The arrangement of the 

 capillaries is shown on a larger scale in fig. 205 (p. 223). 



Area of the Surface of the Lung. The object of the compli- 

 cated structure of the lung is to provide a very large surface, for the 



