THE PLEUEA 243 



The lining epithelium of the alveoli, continuous through the 

 alveolar ducts with that of the terminal bronchioles, consists of 

 flattened or cuboidal cells and broad protoplasmic plates. These 

 cells are narrower and thicker when the lung is collapsed, broader 



FIG. 205. Two ALVEOLI OF A CHILD'S LUNG. 



In A, the wall is cut across and viewed in profile ; & a tangential section showing 

 the cup-shaped bottom of the alveolus and the pulmonary epithelium in surface view ; 

 c, a pulmonary venule. Hematein and eosin. x 640. 



and thinner when it is fully expanded. The completely expanded 

 alveolus in full inspiration is two to three times the size of the 

 collapsed or retracted alveolus of full expiration (Kolliker). The 

 elastic fibres of the alveolar wall form a delicate net among the 

 capillaries; in the meshes of this net a few white fibres are 

 found. 



THE PLEURA. The pleura is a serous membrane whose vis- 

 ceral layer (pleura pulmonalis) envelops the lung, and whose 

 parietal layer (pleura costalis, diaphragmatis, et mediastinalis) 

 lines the thoracic cavity. 



The surface of the pleura is clothed with a layer of endothe- 

 lium which rests upon a " subserous " layer of connective tissue. 

 The endothelium contains frequent stomata which in the costal 

 pleura are only present over the intercostal spaces. The con- 



