312 THE BESPIBATOEY SYSTEM 



The lining epithelium of the alveoli, continuous through the alveolar 



ducts with that of the respiratory bronchioles, consists of flattened cells 



and broad protoplasmic non-nucleated plates. These cells are narrower 



A B and thicker (cubi- 



^ cal) in the prenatal 



*J? &, J lung and when the 



f* ""' ^ Sfts & 



$ m i$$ lung is collapsed, 



. % A * broader and thin- 



%* % 4 * ner when it is fully 



y~- - % * 



I * expanded. T h e 



C ^$r*. $_ 4t ^ completely expand- 



|fe A * ed alveolus in full 



%*^j respiration is two 



to three times the 

 size of the collapsed 

 FIG. 295. Two ALVEOLI OF A CHILD'S LUNG. or retracted alveo- 



In A, the wall is cut- across and viewed in profile; B, a his of full expira- 



tangential section showing the cup-shaped bottom of tion (Kolliker). 



the alveolus and the pulmonary epithelium in surface rp, l f fil f 



view; c, a pulmonary venule. Hematein and eosin. X 



425. the alveolar Avail 



form a delicate net 



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

 found. The normal respiratory epithelium does not become phagocytic 

 (Miller, 1911). 



An alveolar duct with its atria, alveolar saccules, blood-vessels, lymph- 

 vessels and veins forms a natural unit of structure, the pninnry pul- 

 monary lobule. 



Pores leading from one alveolus to another have been described, but 

 Miller denies their presence in the lung of the cat (Jour. Morph., 24, 4, 

 1913). 



THE PLEURA 



The pleura is a serous membrane whose visceral layer (pleura pul- 

 monalis) envelops the lung, and whose parietal layer (pleura costalis, 

 diaphragmatis et mediaatinalis) lines the thoracic cavity. 



The surface of the pleura is clothed with a layer of mesothelium 

 which rests upon a 'subserous' layer of connective tissue. The mesothe- 

 lium contains frequent 'stonmtn' which in the costal pleura are only 

 present over the intercostal spaces. 



