THE AIR-VESICLES. 149 



connective tissue, and a thinned and interrupted muscularis 

 mucosse. 



The infimdibula : At their termination the bronchioles open 

 into clusters of large elliptical cavities, called infundibula, the 



/ 

 FIG. 62. 



Diagram of air-vesicles and termination of bronchi (Miller). B, terminal bronchi- 

 ole (the line leads from a terminal artery); A, P, alveolar passages; S, infundi- 

 bulum; C, air-vesicle ; T, beginning vein. 



walls of which are beset with air-vesicles. These infundi- 

 bula branch off from the sides and ends of the final portions 

 of the bronchioles, which are here somewhat expanded and 

 lined with flattened polygonal epithelium-cells, and which are 

 often called alveolar passages. 



Each infundibulum is a blind elliptical sac, opening at one 

 point into an alveolar passage, and with its walls formed 

 entirely of air-vesicles. 



The air-vesicles (air-sacs, air-cells, or alveoli) are small 

 hemispherical recesses forming the walls of the infundibula 

 and opening into the cavity of the latter. They make up the 

 great bulk of the lung-substance, and are the ultimate June- 



