THE TRACHEA AND BRONCHI 



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The diameter of the lumen of the trachea when distended to a cyhndrical form has been 

 measured by See : — 



New-born 4.12 to 5.6 mm. 



Infant 2 years 7.5 to 8 mm. 



Infant 4 to 7 8 to 10.5 mm. 



Over 20 years, male 16 to 22.5 mm. 



Over 20 years, female 13 to 16 mm. 



Structure of the trachea and bronchi (figs. 978, 988, 989, 991).— The walls of 

 the trachea and bronchi are composed of a series of cartilages having the form of 

 incomplete rings, held together and enclosed by a strong and elastic fibrous mem- 

 brane. Posteriorly, where the rings are deficient, this membrane remains as the 

 membranous wall [paries membranacea] ; between the cartilages it constitutes 

 the annular ligaments [hgg. annularia (trachealia)]. 



Fig. 991. — Schematic Longitudinal Section of the Wall of the Trachea. (Gegenbaur.) 



Fibrous membrane [ 



Annular ligament 

 Tracheal glands 



Tracheal cartilage 



Epithelium 



A tracheal cartilage [cartilage trachealis] comprises a little more than two- 

 thirds of a circle. Its ends are rounded, its outer surface flat, while the inner sur- 

 face is convex from above downward; the upper and lower margins are nearly 

 parallel. The cartilages are from sixteen to twenty in number. The first is 

 usually broader than the type, and is connected by the crico-tracheal ligament 

 with the cricoid cartilage. Sometimes these two cartilages are in part continuous. 

 The last cartilage is adapted to the bifurcation of the trachea and presents at 

 the middle of its lower margin a hook-like process. This turns backward between 

 the origins of the bronchi, and in the majority of cases gives a cartilaginous basis 

 to the tracheal carina. Some of the tracheal cartilages vary from the type by 

 bifurcating at one end. The cartilages keep the lumen of the trachea patent for 

 the free passage of the air. Calcification occurs as with the laryngeal cartilages, 

 but much later in life. 



A mucous coat [tunica mucosa], soft and pinkish-white in colour, covers the 

 inner surface of the trachea; posteriorly it is thrown into longitudinal folds. 

 Mucous secreting tracheal glands [gl. tracheales] are present in the elastic sub- 

 mucous coat [tela submucosa] between the cartilages and at the back of the 

 trachea. A thin layer of transversely disposed smooth muscle-fibres, stretching 

 between the ends of the cartilages in the posterior wall, constitutes the muscular 



