244 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



even in the smallest tubes whose diameters measure 

 0.2 mm. 



Structure of the Lung. The structure of the 

 smaller bronchi, which form a part of the lung tis- 

 sue, has just been described. Our knowledge of the 

 terminal air passages has recently been greatly in- 

 creased by the work of Dr. Miller, of the University 

 of Wisconsin, whose account will be followed. The 

 smallest bronchioles, whose diameters are 0.2 mm., 

 are called terminal, or respiratory bronchi. In these 



Fat cells. 



Fig. 187. Section from lung showing portion of small bronchus and 

 adjacent lung tissue. 



tubes the character of the epithelial lining changes. 

 Patches of small pavement epithelial cells appear 

 among the ciliated cells, while at the end of the res- 

 piratory bronchus all are of the pavement variety. 

 At this point the tube, slightly dilated, opens into 

 three to six distinct chambers called atria. Each 

 atrium opens into a variable number (2 to 5) of 

 larger irregular spaces called air sacs, the walls of 

 which have concave spherical depressions called air 



