BRONCHIAL TUBES. 95 



bronchial twigs, terminal or capillary bronchi, from one-one hundredth 

 to one-two hundredth of an inch in diameter, as follows: 



The dichotomous subdivision is continued until the resulting 

 branches become reduced to about one-sixth of an inch in diameter, 

 when this mode of division ceases, and the resulting tubes are projected 

 radially toward the periphery of the lung. As the straight tubes 

 pursue their course, side branches are given off in spiral succession. 

 The side tubes themselves give off branches which divide dichoto- 

 mously into the terminal bronchi. The straight tubes constantly 

 dimmish in size, and ultimately divide and result also in terminal 

 bronchi. The diagram (Fig. 69) is intended to illustrate this plan 

 of subdivision, but it is purely schematic. 



A typical bronchial tube (Fig. 71) presents four coats as follows: 



1. Epithelial. 



2. Internal fibrous or mucosa. 



3. Muscular or muscularis mucosce. 



4. External fibrous or submucosa. 



The lining epithelium is composed of cylindrical cells, provided on 

 their free extremities with delicate hair-like appendages the cilia. 

 Between the pointed, attached end of the ciliated cells, small ovoid 

 cells are wedged, and the whole rests upon a layer of round cells. 

 The epithelium pursues a wavy course, so that the lumen of a tube 

 appears stellate rather than circular in transverse section. This 

 greatly increases the extent of surface. 



The internal fibrous coat or mucosa is composed of a small amount 

 of connective tissue, which, just beneath or outside the epithelium, 

 sustains collections of adenoid or lymphoid tissue. In the pig, a con- 

 siderable quantity of yelloiv elastic tissue is found in the mucosa out- 

 side the adenoid tissue, but the amount is smaller in man. The fibres 

 are for the most part disposed longitudinally. Many nutrient vessels 

 from the bronchial artery, capillaries, venules, and lymph-spaces are 

 also found in this coat. 



The muscular coat muscularis mucosse does not differ from the 

 same layer in other mucous membranes. Its thickness varies in pro- 

 portion to the size of the bronchus, the smaller tube possessing rel- 

 atively the thicker walls. The fibres pass circularly, and are of 

 the non-striated or involuntary variety. 



The external coat or submucosa is largely composed of loose con- 

 nective tissue, the fibres being mostly arranged circularly. A few 

 delicate elastic fibres run longitudinally. The external fibres, like 

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

 ing connection with the organ or part traversed; so that it is often 



