THE RESPIRA TOR Y APPARA TUS. 1 73 



passages, which are grouped around the terminal 

 bronchi. 



These bronchi enter the lobules in an irregular 

 manner; some enter the lobule at the end nearest 

 the root of the lung ; others at the side ; others run 

 along its side and send branches into it at right 

 angles. Upon entering a lobule the broches breaks 

 up into irregular tubular cavities, called air-passages, 

 which branch and anastomose, and from which ir- 

 regular-shaped vesicles, called air-vesicles or alveoli, 

 open out. All of these cavities are closely crowded 

 together, and their walls intimately joined. It is in 

 the walls of the air-vesicles, air-passages, and the 

 portion of the bronchi lined with respiratory epithe- 

 lium that the interchange of material between the 

 air and blood occurs, which is the essential factor in 

 respiration. 



We have now to consider the structure of the 

 walls of the air-passages and air-vesicles. The grad- 

 ual thinning which we have observed in the walls of 

 the bronchi as they approach their termination, is 

 still more marked as we pass over into the air-pas- 

 sages. Here, the walls consist of little else than a 

 thin, delicately striated, membranous basement sub- 

 stance, in which numerous elastic fibres ramify, and 

 a few connective-tissue and smooth muscle-cells are 

 imbedded, the whole being lined with flattened 

 epithelium. At the opening of the air-passages and 

 air-vesicles the elastic fibres are grouped to form 



