RESPIRATORY APPARATUS 



301 



LUNGS. 



The arrangement of the ultimate branches of a bronchiole is shown in 

 the diagram, Fig. 296. The respiratory bronchioles, 0.5 mm. or less in 

 diameter, at their beginning contain a simple columnar ciliated epithelium. 

 Further in their course the goblet cells disappear, cilia are lost, the cells 

 become cuboidal, and among them are found thin, non-nucleated plates of 

 different sizes. These plates constitute the respiratory epithelium. The 

 transition from the cuboidal to the respiratory epithelium occurs irregu- 

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



Bronchial artery. - 



Pulmonary vein. 



*~ Pulmonary artery. 



Respiratory bronchiole. 



Pleural capillaries 



(Lobule.) 



PIG. 296. DIAGRAM OF A LOBULB OF THE LUNG, SHOWING THE BLOOD VESSELS AND THE TERMINAL 



BRANCHES OF A BRONCHIOLE. 



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. 297, 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 contain 

 only the respiratory epithelium and are thickly beset with alveoli. 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 



