148 THE RESPIRATORY SYSTEM. 



the ultimate alveoli of the lungs, and are analogous to the 

 ducts of glands. They are surrounded by a considerable 

 amount of firm fibrous connective tissue, the peribronchial 

 tissue ; each bronchus is accompanied in the same peribron- 

 chial sheath by correspondingly large pulmonary vessels, the 

 artery on one side, the vein on the other, and by the smaller 

 bronchial vessels and nerves, usually a bronchial artery, vein, 

 and nerve being on each side of the bronchus. 



The structure of the bronchi gradually changes as they vary 

 in size. At the beginning they are about the same in struct- 

 ure as the trachea. 



The medium-sized bronchi are made up, from within out- 

 ward, of an epithelial layer, a tunica propria, muscularis 

 mucosse, and a submucosa, which contains plates of cartilage, 

 mucous glands, and lymphoid tissue, and extends to the peri- 

 bronchial tissue. 



The bronchi are lined with stratified ciliated, epithelium, un- 

 derlying which is the tunica propria, consisting largely of 

 elastic tissue. These two layers are thrown into longitudinal 

 folds. 



The muscularis mucosce is a thin but well-marked layer or 

 ring of involuntary muscle, arranged circularly ; it separates 

 the tunica propria from the submucosa, and is not included 

 in the longitudinal folds of the mucosa. 



The submucosa consists of areolar tissue, merging into the 

 firmer peribronchial connective tissue. It contains small 

 racemose mucous glands, whose ducts discharge on the epithelial 

 surface; occasional nodules of lymphoid tissue; and unequal 

 curved plates of hyaline cartilage, the representatives of the 

 tracheal rings, which in cross-sections appear as arcs incom- 

 pletely surrounding the bronchi. 



Terminal bronchioles : As the bronchi decrease in size the 

 cartilaginous plates diminish and finally disappear entirely ; 

 the stratified ciliated epithelium becomes reduced to a single 

 layer of ciliated cells, and still later to a layer of low columnar 

 non-ciliated cells; the connective-tissue layers become dimin- 

 ished in thickness, and the peribronchial tissue and mucous 

 glands disappear. Thus, the terminal bronchioles are lined 

 with a single layer of low columnar non-ciliated or flattened 

 polygonal epithelium-cells, a slight amount of fibro-elastic 



