STRUCTURE OF AIR-PASSAGES. 



323 



In the trachea a special muscle exists which can narrow the 

 FIG. 147. 



Transverse section of part of the wall of a medium-sized bronchial tube. 

 X 30. (F. E. Schultze.) a. Fibrous layer containing plates of cartilage, 

 glands, etc. b. Coat composed of unstriated muscle, c. Elastic sub-epithe- 

 lial layer, d. Columnar ciliated epithelium. 



FIG. 148. 



Section of a portion of Lung Tissue, showing part of a very small bronchus 

 cut across. (F. E. Schultze.) a. Fibrous layer containing bloodvessels. 6. 

 Layer of unstriated muscle, c. Layer of elastic fibres, d. Ciliated epithe- 

 lium. 



