The trachea is a rather rigid and constantly open tube, 10 to 12 cm- long and 11 to 18 mm. wide, which leaves the larynx at the level of the intervertebral disc between the sixth and seventh cervical vertebrae and extends to that of the disc between the fourth and fifth thoracic vertebrae. In the latter situation or in front of the fifth thoracic vertebra it divides into two branches, the bronchi, which form a right angle with each other. This division is called the tracheal bifurcation.
Throughout its entire length the trachea is situated almost in the median line, the larger portion being in the neck and the smaller in the thorax.
In the neck it lies in the thyreoid and suprasternal regions and in the jugular fossa, and its anterolateral portion is covered by muscles (sternohyoideus and sternothyreoideus). Near the median line its anterior wall is separated from the skin only by the superficial layer of the deep cervical fascia, except where it is crossed by the isthmus of the thyreoid gland, but toward the sternum it gradually assumes a deeper position, and in the lower portion of the neck, it is placed behind the thyreoid venous plexus and the vena thyreoidea ima. In rare cases also behind the inconstant arteria thyreoidea ima.
The thoracic portion of the trachea lies behind the manubrium sterni and the left innominate vein, other structures in front of it in this situation being the innominate artery, which crosses it at an acute angle, and a part of the left common carotid Gateral wall of the trachea). The tracheal bifurcation is situated immediately behind the aortic arch, and the entire length of the posterior (membranous) wall of the trachea is in immediate contact with the oesophagus.
The tracheal skeleton consists of cartilaginous rings, the tracheal cartilages, which are open posteriorly, where from a third to a fifth of their circumference is wanting. Their number varies between sixteen and twenty, and they form the anterior and lateral walls of the trachea, which are cylindrical, while the posterior wall is flattened. The rings frequently bifurcate (the lower more often than the upper) or neighboring rings anastomose with each other; in rare instances they are perforated. Their posterior extremities have markedly rounded apices and are of about the same width as the remaining portion of the ring. The uppermost ring is the widest and is never bifurcated; it is connected with the inferior margin of the cricoid cartilage by the cricotracheal ligament, and the remaining cartilages are connected with each other by elastic membranes, the annular ligaments, which pass from the inferior margin of one cartilage to the superior margin of the underlying one, each margin usually being slightly beveled.
The posterior wall of the trachea has no cartilaginous skeleton and is consequently called the membranous wall. It is flattened and consists chiefly of nonstriated muscular tissue which is stretched between the extremities of the tracheal rings.
The mucous membrane of the trachea is a direct continuation of that of the larynx and exhibits the same characteristics. It rests upon a thick elastic foundation, the continuation of the elastic membrane of the larynx, which contains marked longitudinal fasciculi. The mucous tracheal glands are particularly plentiful in the region of the annular ligaments and of the membranous wall, and in the latter situation the large glands usually extend through the musculature and are visible upon the outer surface of the trachea as nodules the size of a millet-seed.
The two bronchi arise at the tracheal bifurcation, the right bronchus being shorter and broader than the left. As they leave the bifurcation they make almost a right angle with each other, and each passes outward and downward to the hilus of the lung, where it subdivides into its ramifications. At the tracheal bifurcation a few bronchial lymphatic glands are not infrequently present.
The structure of the walls of the bronchi almost completely corresponds with that of the trachea. They are similarly provided with incomplete cartilaginous rings and possess a posterior membranous wall with bronchial glands. The shape of the cartilaginous rings is more irregular in the bronchi than in the trachea, and anastomoses between them are more frequent. At the bifurcation there is sometimes a single common cartilage which is connected with the last tracheal ring.
The cervical trachea is supplied with blood by the inferior thyreoid, the thoracic trachea by the bronchial arteries.
In the neck the veins empty into the inferior or middle thyreoid veins or into the thyreoid plexus. The lymphatics pass to the tracheal and bronchial lymphatic glands, and the nerves are furnished partly by the vagus and partly by the sympathetic.
From Human Anatomy (1909) by DR. Johannes Sobotta (1869-1945) Professor of Anatomy in the university of Wurzburg.