BRONCHUS OF PIG. 97 



sweat glands of the skin. The ciliated epithelium of the bronchus 

 is continued down the beginning of the tube for a short distance, 

 after which the cells are shortened, and lose their cilia. The coiled, 

 gland-part of the tube is lined with conical cells, which are so large 

 as to leave the lumen very small. Sometimes, and especially in the 

 aged, an ampulliform dilatation of the tube may be seen during its 

 passage through the mucosa. 



The description just given will apply to large and medium-sized 

 bronchi. Very important changes take place as we pass to the ter- 

 minal tubes. 



As the tubes decrease in size^the first coat to diminish in thickness 

 is the outer, or submucosa. We have already alluded to the disap- 

 pearance of the cartilage, and the mucous glands are lost at about 

 the same time. The outer coat becomes, in the small bronchi, so 

 thin as to be no longer distinctly demonstrable. The muscular coat 

 is the last to disappear. It remains a prominent feature of the tube 

 as long as separate coats can be distinguished. The epithelial cells 

 lining the tubes toward the termini become shortened, and, getting 

 lower and lower, at last result in flat, pavement epithelium. 



The walls of terminal bronchi (diameter one-one hundredth to one- 

 two hundredths of an inch) are composed of a slight amount of con- 

 nective tissue in which an occasional non-striated muscle-cell and 

 yellow elastic fibre can be distinguished. They are lined with a 

 single layer of flat cells. No definite layers are distinguishable in 

 these bronchi. In a transverse section the lumen would appear cir- 

 cular. 



PRACTICAL DEMONSTRATION. 



The histology of the bronchi can be studied to best advantage, 

 using tissue from a freshly killed pig or sheep. Short pieces of tubes, 

 about one-quarter of an inch in diameter, from which most of the 

 lung substance has been cut away, should be hardened quickly in 

 strong alcohol. Transverse sections can be made free-handed, or 

 the tissue may be infiltrated with bayberry tallow or celloidin, and 

 cut with the microtome. Stain with hsema. and eosin, and mount 

 in dammar. 



TRANSVERSE SECTION OF PORTION OF BRONCHUS 



OF PIG. (Fig. 71.) 

 OBSERVE: 



(L.) 



1. The epithelial lining: (a) The wavy course, (b) Regions 

 occupied by beaker or goblet cells. (The letter E ID t 1 ^ drawing 



