The trachea (see figure) is a cylindrical tube, four or five inches long, reaching from the larynx to the point of division into the bronchial tubes. It is formed of from sixteen to twenty cartilaginous rings, united by elastic ligamentous tissue. It is lined with mucous membrane continuous with that of the larynx, which is extremely vascular, and covered with numerous follicles.

The bronchi or bronchial tubes are essentially of the same structure and arrangement as the trachea; the right bronchus is shorter and of larger diameter than the left. The bronchial tubes ramify into numerous sub-divisions, which finally terminate in the lobules of the lungs.

In front of the first two rings of the trachea and upon the sides of the larynx is the thyroid gland. It is sometimes much enlarged, constituting goitre.