124 STUDENTS HISTOLOGY * 



in proportion to the size of the bronchus, the smaller tubes possess- 

 ing relatively the thicker walls. The fibers pass circularly, and 

 are of the non- striated or involuntary variety. 



The external coat, or submucosa, is largely composed of loose 

 connective tissue, the fibers being mostly arranged circularly. A 

 few delicate elastic fibers run longitudinally. The external fibers, 

 like those of all tubes, ducts, and vessels, are for the purpose of 

 establishing connection with the organ or part traversed ; so that 

 it is often difficult to demonstrate the exact external limit of a 

 bronchus. This coat is liberally supplied with nutrient branches 

 from the bronchial artery. 



The elasticity and strength of the larger and medium -sized 



FIG. 84. TRANSVERSE SECTION OF A PORTION OF HUMAN LUNG, SHOWING A 

 SMA.L.L BRONCHIAL TUBE (X 60). STAINED WITH H^EMATOXYLIN. 



A. Lumen of bronchus. 



B. Ciliated columnar epithelium. 



C. Internal fibrous layer mucosa. 



D. Muscular coat. 



E. External fibrous layer submucosa. 



F. Pulmonary artery. 



G. Nerve. 



H, H, H. Pulmonary alveoli surrounding bronchus. 



bronchial tubes are greatly increased by the presence of cartilage 

 in the form of plates, which are imbedded in the external coat. 

 They are not uniform in size, neither are they placed regularly. 

 They frequently overlap one another, and two or three may be 

 superposed. As the tubes become reduced in size the plates be- 

 come diminished in frequency disappearing altogether when a 



