312 TH RESPIRATORY 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 



&' **i < ^ iuug anc ^ w ^ en t ne 



^ :<& w$ iuug * s c ii a p seo ^ 



? S*,i %* broader and thin- 



' *e A-: 



* *<k ner wnen it i g ^ u iiy 



ji; $$| expanded. The 



completely expand- 

 A gfc* ed alveolus in full 



^Nfe^ *.i% ^ 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 lus of full expira- 



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



the alveolus and the pulmonary epithelium in surface m, -, I 



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



425. the alveolar wall 



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 nerves forms a natural unit of structure, the primary 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, 

 1313). 



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 mediastinalis) 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 'stomata' which in the costal pleura are only 

 present over the intercostal spaces. 



