RESPIRATION". 



259 



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

 which has been just described (a, fig. 206), 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, not 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 (fig. 206, b). 

 Such a funnel-shaped terminal branch of the bronchial tube, with its 



Fig. 208. 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, nucleated cells, 

 with some smaller polyhedral nucleated cells. M. Unstriped muscular fibres. Circular muscular 

 fibres 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.) 



group of pouches or air-cells, has been called an infundibulum (figs. 206, 

 207), and the irregular oblong space in its centre, with which the air- 

 cells communicate, an intercellular passage. 



The air-cells, or air-vesicles, may be placed singly, like recesses from 

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

 even in rows, like minute sacculated tubes; so that a short series of ves- 

 icles, 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 -g^ to -fa of an inch (.5 to .3 mm.) in diam- 

 eter. Their walls are formed of fine membrane, similar to that of the 



