Lung, light micrograph. Upper left is a small bronchus in cross-section enclosed by a thin ring of smooth muscle stained red. A fragment of hyaline cartilage (blue-purple) lies externally. The longitudinal airway is a terminal bronchiole lacking accompanying cartilage. It bifurcates into respiratory bronchioles that lead to alveolar ducts and clusters of alveoli. Paraffin section, haematoxylin and eosin stain. Magnification: x40 when width printed at 10cm.