THE RESPIRATORY ORGANS. 



251 



noted ; when the diameter of the twig no longer exceeds one milli- 

 metre the tube is termed 



a terminal bronchus ; FIG. 286. 



these divisions open into 

 the somewhat larger al- 

 veolar passages, the 

 walls of which are beset 

 with air-sacs, and from 

 which extend blind ir- 

 regular or pyramidal 

 spaces, the i n f u n d i- 

 bula ; each infundibu- 

 lum is surrounded on all 

 sides by the air-sacs, 

 which communicate 

 freely with the former 

 cavity, but not directly 

 with each other. Greater 

 exactness suggests ad- 

 ditional subdivisions of 

 the alveolar duct into 

 vestibule, atrium, and infundibular passage (Miller). 



The walls of the terminal bronchial tubes consist at first of a 

 single layer of ciliated columnar epithelium, 

 outside of which the mucosa contains longitudinal 

 elastic fibres and thin, irregular, annular bun- 

 dles of non-striped muscle. The mucous 

 glands and the cartilaginous plates are want- 

 ing in the terminal tubes. Within the latter 

 the ciliated cells disappear, a simple columnar 

 epithelium existing for some distance, which, in 

 turn, is replaced by cuboidal cells on approach- 

 ing the alveolar ducts. 



The walls of the alveolar ducts suffer still 

 further reduction, the fibrous coat becoming 

 greatly thinned, while the mucosa is reduced to 

 a delicate tunica propria of fibro-elastic tissue, in 

 which bundles of non-striped muscle remain. 



The epithelium of the alveolar passage, at 

 first low cuboidal in character, rapidly assumes a 

 flat polygonal type ; towards the infundibula 

 large flat plates appear among the smaller 

 polygonal cells, and become more numerous as 

 divisions of the air-passages are neared. 



Section of portion of bronchus of child : a, epithelium ; 6, 

 basement-membrane ; c, stroma of mucosa ; d, layer of in- 

 voluntary muscle ; e , submucosa ; f, acini of mucous glands ; 

 A, blood-vessels ; i, obliquely-cut duct of mucous glands. 



FIG. 287. 



Diagram of terminal 

 compartments of air- 

 passage : T.B., terminal 

 bronchus; A.D., alveo- 

 lar ducts ; Inf., infundi- 

 bula, into which open 

 air-sacs. The general 

 character of the epithe- 

 lial lining is indicated. 



these terminal 



