RESPIRATION. 



171 



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

 brane, within the layers of which are inclosed 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 cir- 

 cumference), and are deficient behind; the interval between their pos- 

 terior extremities being bridged over by a continuation of the fibrous 

 membrane in which they are inclosed (Fig. 144). The cartilages of the 

 trachea and bronchial tubes are of the hyaline variety. 



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



FIG. 146. Section of the 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, /; ft, 

 external investment of fine fibrous tissue. (S. K. Alcock.) 



muscular fibres, which extends, transversely, between the ends of the 

 cartilaginous 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 cartila- 

 ginous framework. 



