A histological cross section of a parent bronchus at a point of branching. The entire cross section of the bronchus is shown (Left panel, Whole Airway) indicating the point at which the parent bronchus opens out into the daughter. The parent bronchial wall (PW) and the wall at the shoulder of a branching point (BP) are indicated by arrows. Middle and right panels are magnified images of the branching point and of the parent bronchial wall respectively. Substantial thickening of the cartilage plate (CP) at the branching point is apparent, which is accompanied by an increase in smooth muscle (SM) mass and a more oblique orientation of muscle cells.