THE RESPIRATORY ORGANS. 



179 



epithelium lining- the bronchi gives place to a pavement-epithelium, 

 composed of small flattened cells disposed in a single laver. The 

 elastic tissue of the mucous membrane is continued through all the 

 divisions of the air-passages, and becomes a constituent part of the 

 alveolar walls of the lung itself. 



The alveolar passages open into spaces, called the " infundibula," 

 in the sides of which are the openings into the alveoli of the 

 lung, the ultimate destination of the inspired air. Here and there 



Fig. 155. 



Section of lung of the dog, showing a transverse section of a bronchiole: a, bronchiole (a 

 little mucus covers the epithelial lining) ; b, muscular layer of the mucous membrane ; c, 

 c, radicles of the pulmonary vein ; d, alveolar passage, just at its division to form infun- 

 dibula. An infundibulum extends from this passage toward the bronchiole. The wall 

 of the alveolar passage at this point is similar in structure to that of the pulmonary 

 alveoli, e, alveolar passage in oblique section. This passage is cut at a point further 

 from its opening into the infundibula, and has a somewhat thicker wall than d. The rest 

 of the section is made up of infundibula (the larger spaces) and pulmonary alveoli. 



stray alveoli open directly into the alveolar passages (Figs. 155, 156, 

 and 157). 



4. The pulmonary alveoli and the smaller air-passages are so 

 arranged that there are no vacant spaces ; and neighboring alveoli, 

 whether they belong to a group of infundibula springing from 

 the same alveolar passages or to separate groups, are so closely 

 situated that they have but one common wall dividing their cavities 



