The trachea is very slightly flattened dorsally. It contains forty to
forty-five =U=-shaped rings; the ends of the rings do not meet dorsally,
so that here the wall of the trachea is membranous and is composed of a
layer of transverse smooth muscle-fibers outside of the rings, the
fibrous membrane, and the mucous membrane.
The stem bronchi diverge at an obtuse angle, and each divides into two
branches before entering the lung. The anterior branch goes to the
apical and cardiac lobes, the posterior one to the diaphragmatic lobe,
supplying also the mediastinal lobe of the right lung. The anterior
branch of the left bronchus crosses under the pulmonary artery.