The Trachea

The trachea extends from the larynx to the roots of the lungs, where it divides into the right and left bronchi. It is kept permanently open by a series of about fifty to fifty-five incomplete cartilaginous rings embedded in its wall. It occupies a median position, except near its termination, where it is pushed a little to the right by the arch of the aorta. It is approximately cylindrical, but its cervical portion is for the most part depressed dorso-ventrally by contact with the longus colli muscle, so that the dorsal surface is flattened. The average caliber is about two to two and one-half inches (ca. 5 to 6 cm.), but in the greater part of the neck the transverse diameter is greater and the dorso-ventral smaller. It is inclosed by a fascia propria.

In its cervical part the trachea is related dorsally to the œsophagus for a short distance, but chiefly to the longus colli muscle. Laterally it is related to the thyroid gland, the carotid artery, the jugular vein, the vagus, sympathetic, and recurrent laryngeal nerves, and the tracheal lymph ducts and cervical lymph glands. The œsophagus lies on its left face from the third cervical vertebra backward. The sterno-cephalicus muscles cross it very obliquely, passing from the ventral surface forward over its sides, and diverging to reach the angles of the jaw. The omo-hyoidei also cross it very obliquely, passing over the lateral surfaces of the tube, and converging ventrally to the body of the hyoid bone. The sterno-thyrohyoideus lies on the ventral surface. The scaleni lie on either side near the entrance to the thorax.

The thoracic part of the trachea passes backward between the pleural sacs and divides into the two bronchi over the left atrium of the heart. It is related dorsally to the longus colli for a short distance, and beyond this to the œsophagus. Its left face is crossed by the aortic arch, the left brachial artery, and the thoracic duct. Its right face is crossed by the vena azygos, the dorso-cervical and vertebral vessels, and the right vagus nerve. Ventrally it is related to the anterior vena cava, the brachiocephalic and common carotid trunks, and the left recurrent nerve.

The bifurcation of the trachea (Bifurcatio tracheæ) is situated opposite to the fifth intercostal space, and about five or six inches (ca. 12 to 15 cm.) below the seventh and eighth thoracic vertebræ.

The trachea is composed of—(1) A fibro-elastic membrane in which are embedded (2) the cartilaginous rings; (3) a muscular layer; (4) the mucous membrane. The elastic membrane is intimately attached to the perichondrium of the rings. In the intervals between the latter it constitutes the annular ligaments (Ligamenta annularia). The rings of the trachea (Cartilagines tracheales) are composed of hyaline cartilage. They are incomplete dorsally, and when their free ends are drawn apart, resemble somewhat the letter C. In the cervical part the thin wide free ends overlap, while in the thoracic part they do not meet; here the deficiency is made up by thin plates of variable size and form, embedded in a membrana transversa. Ventrally the rings are about one-half inch (ca. 1 to 1.2 cm.) wide and much thicker than dorsally. The first ring is attached to the cricoid cartilage by the crico-tracheal membrane, and is usually fused dorsally with the second ring. Various irregularities, such as partial bifurcation or partial or complete fusion with an adjacent ring, are common. The arrangement in the terminal part is very irregular. The muscular layer (Musculus trachealis) consists of unstriped fibers which stretch across the dorsal part of the tube. It is separated from the ends of the rings and the membrana transversa by a quantity of areolar tissue. When it contracts the caliber of the tube is diminished. The mucous membrane is pale, normally, and presents numerous fine longitudinal folds, in which are bundles of elastic fibers. The epithelium is stratified columnar ciliated. Numerous tubular mucous glands (Glandulæ tracheales) are present.

Blood-supply.—Common carotid arteries.

Nerve-supply.—Vagus and sympathetic nerves.