240 



HISTOLOGY. 



cells, and rests on a tunica propria which has many elastic fibers and lym- 

 phocytes. The latter may accumulate in nodules. 



Bronchioles are the small subdivisions of the bronchi, measuring 

 from 0.5 to i.o mm. in diameter. They are free from cartilage and glands 

 but have a columnar ciliated epithelium throughout. Obviously the dis- 

 tinction between the smaller bronchi and the bronchioles is arbitrary. 

 The terminal branches of the latter are called respiratory bronchioles. 



RESPIRATORY BRONCHIOLES, ALVEOLAR DUCTS, ALVEOLAR SACS, 



ALVEOLI. 



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

 the diagram, Fig. 271. The respiratory bronchioles, 0.5 mm. or less in 

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



Bronchial artery. 



Pulmonary vein. 



Pulmonary artery. 



-*^- Respiratory bronchiole. 



Pleural capillaries. 



(Lobule.) 



FIG. 271. DIAGRAM OF A LOBULE OP THE LUNG, SHOWING THE BLOOD VESSELS AND THE TERMINAL 



BRANCHES OP A BRONCHIOLE. 



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 together with the isolated cuboidal cells 

 remaining among them constitute the respiratory epithelium. The tran- 



