Bronchus 
Figure 1.9.1-D. Schematic demonstrating the architecture of the walls of 
bronchi and bronchioles. Top . Cross-sectional view of a bronchus. It is 
comprised of three concentric tubes including an inner epithelial layer 
resting on a basement membrane, a middle sheath of smooth muscle cells and 
outer layer of connective tissue, islands of cartilage, and submucosal 
glands that are contiguous with the airway epithelium. Bottom . Cross-sec- 
tional view of the walls of a bronchiole. It is also composed of three 
concentric tubes but the outer layer contains no cartilage and no submuco- 
sal glands . 
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