1082 THE KESPIEATOKY SYSTEM. 



The mucous membrane is laid smoothly over the interior of the tube upon a layer of 

 submucous areolar tissue. Lymph tissue enters largely into the composition of the 

 tracheal mucous membrane, and its inner surface is lined with columnar ciliated epithelial 

 'cells. The action of the cilia exercises an important influence in producing an upward 

 movement of the mucus which is present on the surface of the mucous membrane. 



Numerous longitudinal bundles of elastic tissue are present in the dorsal wall of 

 the trachea, more particularly in its inferior part, between the mucous membrane and the 

 trachealis muscle. 



In connexion with the mucous membrane there is a plentiful supply of acinous mucous 

 glands. These are placed in the submucous tissue, and also, on the dorsal aspect of the 

 tube, on the exterior of the trachealis as well as amidst its muscular bundles. They 

 send their ducts to the surface of the mucous membrane, where they open by trumpet- 

 shaped mouths. 



BRONCHI. 



The two chief bronchi proceed obliquely downwards and laterally from the ter- 

 mination of the trachea, each towards the hilum of the corresponding lung. Like the 



rachea 



FIG. 861. DRAWING OF A STEREOSCOPIC SKIAGRAPH OF THE TRACHEA AND BRONCHI INJECTED 



WITH STARCH AND RED LEAD. 



trachea, they are kept permanently patent by the presence of cartilaginous bars in 

 their walls. These bars are deficient dorsally, so that each bronchus exhibits 

 flattened dorsal surface, in every respect similar to the trachea. The two 

 bronchi differ from each other, not only in the relations which they present 

 surrounding structures, but also in length, in width, and in the direction which 

 they pursue (Fig. 858, p. 1079). 



The first collateral branch of the right bronchus (ramus bronchialis eparterialis 

 arises much nearer the trachea than the first branch of the left bronchus, 

 difference determines the length of the primary divisions of the trachea, anc 

 although there is much variation in the matter, it may be said that, as a rule, tl 



