7Y//<: RESPIRATORY APPARATUS. 175 



actual respiratory surface in the lungs is greater 

 than we have been wont to believe. 



In the foetus the air-vesicles are lined at first with 

 a distinct layer of cylindrical or cuboidal epithelial 

 cells, which become gradually flattened, and when 

 respiration is established assume the form of a layer 

 of very thin polygonal cells ; these change their 

 character as the animal matures, until in adult life 

 we have the forms above described. 



Just beneath the epithelial cells, and separated by 

 them alone from the air within the vesicles, lies the 

 rich blood-capillary net-work, which, in bulk as in 

 importance, is the most essential element in the 

 walls of the vesicles. The lungs are supplied with 

 blood through the pulmonary and the bronchial 

 arteries. The blood from the latter is chiefly dis- 

 tributed to the walls of the bronchi and larger 

 blood-vessels, and to the connective tissue of the 

 lungs. A large part of the blood from the bron- 

 chial arteries, after passing through various sets of 

 capillaries, returns through the bronchial veins ; 

 but a certain portion of it finds its way into the 

 pulmonary veins ; indeed, the two sets of vessels 

 seem to be in communication in various parts of 

 the lungs. 



The pulmonary artery, following the course of the 

 bronchi, divides and subdivides, until on reaching 

 the lobules the small trunks break up into smaller 

 branchlets, which pass along the alveolar passages 



