284 RESPIRATION 



On entering a lobule, the small bronchial tube, the structure of which 

 has just been described, a, figure 223, divides and subdivides; its walls at 

 the same time becoming thinner and thinner, until at length they are formed 

 only of a thin membrane of areolar and elastic tissue, lined by a layer of 

 squamous epithelium, no longer provided with cilia. At the same time they 

 are altered in shape; each of the minute terminal branches widening out 

 funnel-wise, and its walls being pouched out irregularly into small saccular 

 dilatations, called air-cells, figure 223, b. Such a funnel-shaped terminal 

 branch of the bronchial tube, with its group of pouches or air-cells, has been 

 called an infundibulum, figures 223 and 224, and the irregular oblong space 

 in its center, with which the air-cells communicate, an intercellular passage. 



FIG. 223. FIG. 224. 



FIG. 223. Terminal Branch of a Bronchial Tube, with its Infundibula and Air-cells, 

 from the Margin of the Lung Injected with Quicksilver; Monkey, a, Terminal bronchial 

 twig; 6, 6, infundibula and air-cells. X 10. (F. E. Schulze.) 



FIG. 224. Two Small 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. (Kolliker.) 



An inflated and dried turtle's lung illustrates the homologue of a lobule. 

 Such a preparation can be cut across to illustrate the intercellular passage, 

 the infundibulum, and the air-cells. 



The air-cells, or air-vessels, are sometimes placed singly, like recesses 

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

 or even 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 walls are nearly in contact, and they vary from o . 3 

 to 0.5 mm. in diameter. Their walls are formed of fine membrane similar 

 to that of the intercellular passages and continuous with it. The membrane 

 is folded on itself so as to form a sharp-edged border at each circular orifice 



