STRUCTURE OF THE LUNGS. 



159 



FIG. 59. 



centre, with which the air-cells communicate, an intercellular 

 passage. 



The air-cells may be placed singly, like recesses from the in- 

 tercellular passage, but more often they are arranged in groups 

 or even in rows, like minute sac- 

 culated tubes ; so that a short se- 

 ries 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 pres- 

 sure to which they are subject ; 

 their walls are nearly in con- 

 tact, and they vary from jf G to 

 Jfi of an inch in diameter. 

 Their walls are formed of fine 

 membrane, similar to that of 

 the intercellular passages, and 

 continuous with it, which mem- 

 brane is folded on itself so as to 

 form a sharp-edged border at 

 each circular orifice of commu- 

 nication between contiguous air- 

 cells, or between the cells and 

 the bronchial passages. Nu- 

 merous fibres of elastic tissue 

 are spread out between con- 

 tiguous 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 cells are lined by a 

 layer of squamous or tessellated epithelium, not provided with 

 cilia. Outside the cells, a network of pulmonary capillaries 

 is spread out so densely (Fig. 61), that the interspaces or 

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

 average, -g^ 1 ^ of an inch in diameter. Between the atmo- 

 spheric air in the cells and the blood in these vessels, nothing 

 intervenes but the thin membranes of the cells and capilla- 

 ries and the delicate epithelial lining of the former ; 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 same, contain only a single 

 layer of capillaries, both sides of which are thus at once ex- 

 posed to the air. 



The cells situated nearest to the centre of the lung are 



Two small groups of air-cells, or 

 infundihvla, a a, with air cells, b 6, and 

 the ultimate bronchial tubes, c c, with 

 which the air-cells communicate. 

 From a new-born child (after Kolliker). 



