BRONCHI AND LUNGS. 207 



and end partly under and partly in the epithelium. On the 

 lower surface of the epiglottis there are small taste buds 



present. 



B. BRONCHI AND LUNGS. 



The trachea divides to form the bronchi, of which the 

 largest are quite similar in structure to the trachea. 



The mucous membrane is thrown into longitudinal folds, and 

 covered on the surface with a many-layered ciliated epithelium 

 containing a considerable number of goblet cells (Fig. 159). 

 The mucous membrane of the smaller bronchial branches con- 

 sists of a single layer of ciliated epithelium. The tunica pro- 

 pria consists of connective tissue with elastic fibres and leuco- 

 cytes. The smooth muscle cells here form a circular layer. 

 The mucous glands break through this muscle layer, and are 

 first absent in bronchial twigs as small as 1 mm. in diameter. 

 This is also about the place where cartilage ceases to exist in the 

 bronchi. In larger bronchi the cartilage has the form of half 

 rings, while in smaller branches it usually appears as irregular 

 plates which are arranged on all sides of the wall. Toward the 

 outside the cartilage masses are surrounded by a fibrous mem- 

 brane which contains elastic tissue, blood-vessels, and nerves. 



In bronchioli 0.5 mm. in diameter the cartilage and 

 mucous glands are absent, and the mucous membrane consists 

 of a single layer of ciliated epithelial cells, among which are 

 mingled many goblet cells. The muscle layer surrounds these 

 bronchioles as a circular sheath, and during contraction throws 



o 



the surface into longitudinal folds. 



By a division of the bronchioles there are formed the respi- 

 ratory bronchioles, from which thin-walled diverticula, the 

 alveoli, are developed (Fig. 160). These are covered by the so- 

 called respiratory epithelium. The epithelium at the beginning 

 of the respiratory bronchioles is ciliated, and becomes gradually 

 cubical and then flat. The respiratory epithelium, which con- 

 sists of flat non-nucleated cells, begins in the form of small 

 islands among the low cubical epithelium of the respiratory 

 bronchioles. 



W. S. Miller has given us a new conception of the lobule 



