312 



THE EESPIRATORY SYSTEM 



y&T A f m 



Vv ?4 f*-l 



v ~ *;**"" 



FIG. 295. Two ALVEOLI OF \ CHILD'S LUNG. 



In A, the wall is cut across and viewed in profile; B, 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 

 425. 



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 



*^ --^***.' ^ ^ g$*j -. lung and when the 



;/ ! ^ ^|$ % lung is collapsed, 



.& broader and thin- 



* ^ ner wlicn it is fully 



*$ expanded. The 



^M fl completely expand- 



ed alveolus in full 

 respiration is two 

 to three times the 

 size of the collapsed 

 or retracted alveo- 

 lus of full expira- 

 tion (Kolliker). 

 The elastic fibers of 

 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, 

 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 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. 



