250 



RESPIRATION 



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

 has just been described, #, figure 210, 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. 225. From a Section of the Lung of a Cat, Stained with Silver Nitrate. A. D, Alveolar 

 duct or intercellular passage; S, alveolar septa, N, alveoli or 'air-cells, lined with large flat, 

 nuleated cells, with some smaller polyhedral nucleated cells; M, unstriped muscular fibers Cir- 

 cular muscular fibers are seen surrounding the interior of the alveolar duct, and at one part is seen 

 a group of small polyhedral cells continued from the bronchus. (Klein and Noble Smith.) 



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

 preparation can be cut across to illustrate the intercellular passage, the in- 

 fundibulum, 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 



