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HAND-BOOK OF PHYSIOLOGY. 



Structure. The trachea is essentially a tube of fibro-elastic membrane, 

 within the layers of which are enclosed a series of cartilaginous rings, from 

 sixteen to twenty in number. These rings extend only around the front 

 and sides of the trachea (about two-thirds of its circumference), and are 

 deficient behind; the interval between their posterior extremities being 

 bridged over by a continuation of the fibrous membrane in which they 

 are enclosed (Fig. 145). The cartilages of the trachea and bronchial 

 tubes are of the hyaline variety. 



FIG. 147. Section of trachea, a, columnar ciliated epithelium; 6 and c, proper structure of the 

 mucous membrane, containing elastic fibres cut across transversely; d, submucous tissue containing 

 mucous glands, e, separated from the hyaline cartilage, g, by a fine fibrous tissue, /; /i, external in- 

 vestment of fine fibrous tissue. (S. K. Alcock.) 



Immediately within this tube, at the back, is a layer of unstriped 

 muscular fibres, which extends, transversely., between the ends of the car- 

 tilaginous rings to which they are attached, and opposite the intervals 

 between them, also; their evident function being to diminish, when re- 

 quired, the calibre of the trachea by approximating the ends of the car- 

 tilages. Outside these are a few longitudinal bundles of muscular tissue 

 which, like the preceding, are attached both to the fibrous and cartilagi- 

 nous framework. 



