202 RESPIRATION. 



(fig. 59), and the irregular oblong space in its centre, 

 with which the air-cells communicate, an intercellular 



Fig. 59.* The air-cells may be 



placed singly, like recesses 

 from the intercellular pas- 

 sage,, but more often they 

 are arranged in groups or 

 even in rows, like minute 

 sacculated tubes; so that 

 a short series of cells, 

 all communicating with 

 one another, open by a 

 common orifice into the 

 tube. The cells are of 

 various forms, according 

 to the mutual pressure to 

 which they are subject ; 

 their walls are nearly in contact, and they vary from 

 ^^ to 1^ of an inch in diameter. Their walls are formed 

 of fine membrane, similar to that of the intercellular 

 passages, and continuous with it, which membrane is 

 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 bronchial 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, I and c). The 

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

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



* Fig. 59. Two small groups of air-cells, or infundibula, a a, with 

 air-cells, b b, and the ultimate bronchial tubes, c c, with which the air- 

 cells communicate. From a new-born child (after Kolliker), 



