This image shows a cross section of a terminal bronchiole.  Within the lung, tertiary bronchi become progressively smaller and eventually form small airways called terminal bronchioles.  Note that this  bronchiole is lined with a thick layer of ciliated columnar epithelium whose cilia help to remove  foreign debris from the lungs.  A thin (but complete) layer of smooth muscle that can regulate  the diameter of the bronchiole can also be seen.