I9 2 KESPIRATIOX. 



folded on itself so as to form a sharp-edged border at 

 each circular orifice of communication between contiguous 

 air-cells, or between the cells and the Jbronchial passages. 

 Numerous fibres of elastic tissue are spread out between 

 contiguous air-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 (fig. 60, b and c). The 



Fig. 60.* 



cells are lined by a layer of squamous or tessellated epithe- 

 lium, not provided with cilia. Outside the cells, a net- 

 work of pulmonary capillaries is spread out so densely 

 (fig. 6l), that the interspaces or meshes are even narrower 



* Fig. 60. Air-cells of lung, magnified 350 diameters, a, Epithelial 

 lining of the cells ; b, Fibres of elastic tissue ; c, Delicate membrane of 

 which the cell-wall is constructed with elastic fibres attached to it (after 

 Kcilliker). 



