ORGANS OF RESPIRATION. 



(See below.) The walls of the respiratory bronchioles are rela- 

 tively thin, consisting of fibre-elastic connective tissue and nonstri- 

 ated muscle. Our knowledge of the further divisions of the 

 bronchioles and of their relation to the terminal air-spaces has been 

 increased greatly by Miller, who has made use of Born's method 

 of wax-plate reconstruction in the study of these structures. His 

 account is here followed. According to Miller, the respiratory 

 bronchioles divide into or become the terminal bronchioles or alveo- 



Section of al 



veolus of 

 lung. 



Respiratory 

 bronchiole. 



Fig. 250. Internal surface of a human respiratory bronchiole, treated with silver 

 nitrate ; X 2 34 ( after Kolliker). 



lar ducts. These are somewhat dilated at their distal ends and 

 communicate, by means of three to six round openings, with a cor- 

 responding number of spherical cavities, known as atria. Each 

 atrium communicates with a variable number of somewhat irregu- 

 lar spaces or cavities, the air-sacs, the walls of which are beset with 

 numerous somewhat irregular hemispheric bulgings, the air-cells 

 or lung alveoli. The air-cells or alveoli are also numerous in the 

 walls of the atria and the terminal bronchioles or alveolar ducts, 



