THE LUNGS. 



193 



fibres and intervening lymphoid cells. As in the trachea and the bronchi, 

 the current produced by the cilia is directed centrally, and thus tends to 



Bronchiole 



Bronchiole 



Cartilage 



Alveoli- 

 FIG. 240. Section of lung, showing small air-tubes and branch of pulmonary artery. X 35. 



carry mucus and particles of dust from the smaller tubes towards the bronchi 

 and trachea. Mucous glands, similar to those of the trachea, are present in 

 decreasing number and size 

 until the bronchiole attains 

 a diameter of i mm. , after 

 which they usually disap- 

 pear. Their chief location ^j/ n 



Epithelium 

 Goblet-cell 



their journey to gain the 

 free surface where they 

 open in minute depressions 

 within the epithelium. In 

 addition to the lymphoid 

 cells diffused through the 

 mucosa, more definite ag- 

 gregations occur as minute 

 lymph-nodules along the 

 bronchi, the points of bifur- 

 cation being their favorite 

 seats. The epithelium lin- 

 ing the air-tubes retains the 

 ciliated columnar type as 

 far as the smaller bronchi- 

 oles. Within the latter 

 the ciliated cells are replaced by simple columnar elements, which, in turn, 

 give way to low cuboidal cells within the proximal part of the terminal 



13 



Fibrous tissue 

 Alveolar wall 



Cartilage . 



FIG. 241. Portion of wall of small bronchus. X 180. 



