VO PRACTICAL MICROSCOPY. 



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 bronchi 

 are greatly increased by the presence of cartilage in the form of 

 plates, which are imbedded in the external coat. They are not uni- 

 form 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 become diminished in fre- 

 quency disappearing altogether when a diameter of about one- 

 twentieth of an inch has been reached. The cartilage is of the 

 hyaline variety; and each plate is covered with a dense fibrous coat, 

 the perichondrium, which unites it with contiguous parts. 



FIG. 70. -TRANSVERSE SECTION OF A PORTION OP HUMAN LUNG, SHOWING A SMALL BRONCHUS. 



Stained with Haema. 



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. X 60. 



The principal bronchi are provided with a great number of mucous 

 glands, which are located in the external coat or submucosa. They 

 are simple, coiled tubular glands; commencing on the inner surface, 

 penetrating the mucosa and muscularis mucosae, and terminating in 

 the submucosa, generally within the cartilage where they are coiled 

 in short, close turns resembling, in sections, somewhat the larger 



