THE RESPIRATORY ORGANS. 



249 



THE TRACHEA. 



The trachea in its general structure resembles the lower part of the 

 larynx : it consists of a fibrous tube, lined by the mucous mem- 

 brane, and strengthened and "kept open by a series of incomplete 

 cartilaginous rings. 



The mucous membrane of the trachea is lined by stratified 



FIG. 285. 



ciliated columnar epithelium, 



among whose elements lie num- 

 bers of goblet-cells. The current 

 established by the ciliated epithe- 

 lium tends to expel mucus or other 

 substances. 



The tunica, propria is conspic- 

 uous on account of the large 

 amount of elastic tissue which 

 it contains ; owing to the disposi- 

 tion of the elastic fibres, two zones 

 are recognizable, an inner loosely- 

 thrown-together fibrous layer, 

 containing some elastic tissue, vas- 

 cular loops, and nerve-fibres, to- 

 gether with numerous lymphoid 

 cells, and an outer layer, next the 

 submucosa, made up largely of 

 close net-works of longitudinal 

 elastic fibres./ The elasticffibres 

 are particularly robust and abun- 

 dant along the posterior membra- 

 nous wall of the trachea, between 

 the ends of the cartilages. 



The submucosa is loosely ar- 

 ranged, and connects the mucosa with the fibrous sheath, as well 

 as supports the glands and larger blood-vessels, lymphatics, and 

 nerve-trunks. The tracheal glands are represented by numerous 

 small groups of racemose structures which occupy the submucous 

 layer and communicate with the mucous surface by means of the 

 long excretory ducts. The latter are lined with low columnar 

 epithelium, while the acini contain cuboidal cells. 



The fibrous coat lies external to the submucosa and forms a 

 complete investment in which the cartilaginous rings are embedded. 

 These latter are C-shaped masses of hyaline cartilage, embracing 

 almost three-fourths of the tracheal tube. The remaining cleft is 

 bridged by the continuation of the fibrous tunic supplemented by 



Section embracing trachea and oesophagus of 

 child : a, b, tracheal and oesophageal surfaces ; 

 c, tracheal epithelium ; d, stroma of mucosa ; e, 

 submucosa ; .//mucous glands; /, part of ring 

 cartilage ; g; its perichondrium ; /, fibrous 

 tissue ; k, fibro-muscular tissue of oesophagus ; 

 /, oesophageal epithelium. 



