180 



NORMAL HISTOLOGY. 



from each other. Notwithstanding this general compactness of 

 arrangement, the lungs are divided by delicate septa of fibrous 

 tissue into more or less well-defined lobules, corresponding to the 

 smallest bronchi or the bronchioles. 



The alveolar walls are made up of a delicate, loose areolar tissue, 

 containing numerous elastic fibres and supporting the abundant 

 capillary plexus in which the blood suffers the gaseous exchanges 

 with the air that constitute the function of respiration (Fig. 158). 



Fig. 156. 





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Section of lung of the dog : a, alveolar passage opening into an infnndibulum and also into 

 a solitary alveolus ; b, cross-section of an infnndibulum. The dotted line indicates the 

 limits of the infundibular space. Opening into it are a number of alveoli. Were the 

 dotted line removed, the infundibular cross-section and the alveoli around it would form 

 a stellate space in the section, ('.junction of two radicles of the pulmonary \ein. At 

 the top of the section, to the right, is an oblique section of a bronchiole. 



Covering the two surfaces of the alveolar wall is a layer of very 

 thin cellular plates (pavement-epithelium, see Fig. 30), among 

 which are scattered a few cells resembling those lining the alveolar 

 passages. This cellular investment is continuous with the lining 

 of the infnndibulum, which is of similar character, and thence with 

 the epithelium covering the inner surface of the alveolar passage. 

 It is to be regarded as a special modification of epithelium, fitting 

 it for usefulness in this situation. 



