CH. XXIV.] 



THE TRACHEA AND BRONCHI 



345 





tend 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 is bridged over by a continuation of the 

 fibrous membrane in which they are enclosed (fig. 318). The carti- 

 lages of the trachea and bronchial tubes are of the hyaline variety. 



Immediately within this tube, 

 at the back, is a layer of un- 

 stripad muscular fibres, which 

 extends, transversely, between tlie 

 ends of the cartilaginous rings 

 to which they are attached, and 

 opposite the intervals between 

 them also; their function is to 

 diminish, when required, the 

 calibre of the trachea by ap- 

 proximating the ends of the 

 cartilages. Outside these are a 

 few longitudinal bundles of mus- 

 cular tissue, which, like the pre- 

 ceding, are attached both to the 

 fibrous and cartilaginous frame- 

 work. 



The mucous membrane con- 

 sists to a great extent of loose 

 lymphoid tissue, separated from 

 the ciliated epithelium (fig. 322) 

 which lines it by a homogeneous 

 basement membrane. In the 

 deeper part of the corium of the 

 mucous membrane are many 

 elastic fibres, between which lie 

 connective-tissue corpuscles and 

 capillary blood-vessels. 



Numerous mucous glands are 

 situated in the substance of the 

 mucous membrane of the trachea ; 

 their ducts perforate the various 

 structures which form the wall 

 of the trachea, and open through the mucous membrane into the 



FIG. 319. Section of the trachea. , columnar cili- 

 ated epithelium ; 6 and c, corium of the mucous 

 membrane, containing elastic fibres cut across 

 transversely ; <J, submucous tissue containing 

 mucous glands, e, separated, from the hyaline 

 cartilage, g, by fine fibrous tissue, /; h, external 

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



interior (fig. 319). 



The two bronchi into which ' the trachea divides,* of which the 

 * J right is shorter, broader, and more horizontal than the 'left (fig. 317), 

 \jesemble the trachea in structurer^vith the difference that in them 

 there is a distinct layer of unstriped muscle arranged circularly 

 beneath the mucous membrane, forming the muscularis mucosce. On 



