ALVEOLI OF THE LUNG. 



241 



sition from the cuboidal to the respiratory epithelium occurs irregu- 

 larly, so that a bronchiole may have cuboidal epithelium on one side 

 and respiratory epithelium on the other; or one sort of epithelium 

 may form an island in the midst of the other. Hence the respiratory 

 bronchioles contain a mixed epithelium (Fig. 272, A). The respiratory 

 epithelium steadily gains in extent until the cuboidal epithelium has 

 disappeared. 



At irregular intervals along the bronchioles the respiratory epithelium 

 forms hemispherical outpocketings or alveoli. The alveolar ducts, from 

 i to 2 mm. long, differ from the respiratory bronchioles in that they con- 

 tain only the respiratory epithelium and are thickly beset with alveoli. 



Pores. Cuboidal epithelial cells. Non-nucleated 



Non-nucleated 

 plates. 



Border of an alveolus. B 

 FIG. 272. FROM SECTIONS OF THE HUMAN LUNG. X 240. 



n alveolus. 



A, Mixed epithelium of a respiratory bronchiole; B, an alveolus sketched with change of focus; the 

 border of the alveolus is shaded ; it is covered by the same epithelium as that of the (clear) fundus of 

 the alveolus; the nuclei of the cells are invisible. (Silver nitrate preparation.) 



The layer of smooth muscle fibers may be traced to the end of the alveolar 

 ducts, where it terminates. Since the muscles do not extend over the 

 alveoli, but merely surround the main shaft of the duct, the layer is greatly 

 interrupted, and some consider that it ends in the course of the duct. 

 The respiratory bronchiole may be continued as a single alveolar duct or 

 may divide into two or more. 



The alveolar ducts branch to produce alveolar sacs [infundibula] 

 which are cavities in the center of clusters of alveoli. The sacs resemble 

 the ducts as shown in Fig. 271. According to Professor Miller, who has 

 made reconstructions of these structures in the human lung, an atrium or 

 round cavity should be recognized between the alveolar duct and the alveo- 

 lar sacs. The alveolar duct opens sometimes into five atria from each of 



16 



