CH. XXIV.] THE AIR VESICLES 349 



called air-sacs (fig. 323, b). Such a funnel-shaped terminal branch 

 of the bronchial tube, with its group of pouches or air-sacs, is called 

 an infundibulum (figs. 323, 324), and the irregular oblong space in 

 its centre, with which the air-sacs communicate, an intercellular 

 passage. 



The air-sacs, or air-vesicles, may be placed singly, like recesses 

 from the intercellular passage, but more often they are arranged in 

 groups, or 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 



FIG. 325. 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.) 



walls are nearly in contact, and they vary from ^th to ^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. 325) not provided with cilia. Outside the air-vesicles 

 a network of pulmonary capillaries is spread out so densely (fig. 326) 

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



