250 



NORMAL HISTOLOGY. 



a layer of transversely-disposed bundles of non-striped muscle. 

 These latter extend for some little distance along the inner side of the 

 cartilages, to whose perichondrium they are attached. The muscle 

 not only exists between the ends of the cartilaginous plates, but also 

 passes across in the intervals between these, thus constituting a con- 

 tinuous layer, which serves to narrow the tube. In addition to 

 the transverse bundles, a few longitudinal muscular bands are 

 present. The outer surface of the fibrous tunic is connected with 

 the surrounding structures by loose areolar tissue. 



The larger blood-vessels pass to the submucosa, from which 

 smaller twigs are given off to supply the mucous membrane and, 

 partially, the fibrous and cartilaginous structures. The vessels termi- 

 nate within the mucosa in a net-work beneath the epithelium ; the 

 acini of the mucous glands within the submucous layer are surrounded 

 by capillaries. 



The lymphatics of the trachea are numerous within the mucous 

 and submucous coats, where they constitute plexiform arrangements 

 of large, irregular, thin-walled channels. Lymphatic tissue in the 

 form of solitary follicles also occurs. 



The nerves contain both medullated and non-medullated fibres. 

 The larger trunks pass within the submucosa and send smaller fibres 

 into the mucosa, where they course as minute naked fibrillae ; the 

 exact mode of their ending is unknown. 



THE BRONCHI. 



The larger bronchial tubes repeat almost exactly the structure 

 of the trachea, with such modifications as result from the slighter 

 general development of the several coats incidental to the gradual 

 reduction in the size of the tubes. 



On reaching the small bronchi the epithelium is reduced to 

 a single layer of ciliated columnar cells. The thickness of the 

 mucosa at first is not greatly diminished, since the loss sustained in 

 the thinning of the elastic tissue of the tunica propria is compensated 

 by the appearance of an additional layer of non-striped muscle 

 situated at the outer border of the mucosa, next the submucosa ; this 

 layer, which corresponds to a muscularis mucosae, forms a com- 

 plete investment, especially conspicuous when the cartilaginous plates 

 diminish. The ring-cartilages of the bronchi become reduced in 

 size, then broken up, and finally replaced by irregular short plates ; 

 these, becoming smaller and infrequent, embrace gradually less of 

 the circumference of the tube, until in the bronchial twigs of the 

 diameter of about one millimetre they altogether disappear. 



By repeated division the bronchial tubes become greatly reduced 

 in size, the reduction being accompanied by the changes already 



