THE TRACHEA1
The trachea, or air-tube, is a cartilaginous and membranous cylindrical tube, flattened posteriorly, extending from the lower part of the larynx, on a level with the fifth cervical vertebra, to opposite the third dorsal, where it divides into the two bronchi, one for each lung. The trachea measures about four inches and a half in length; its diameter, from side to side, is from three quarters of an inch to an inch; being always greater in the male than in the female.
Relations. The anterior surface of the trachea is convex, and covered, in the neck, from above downwards, by the isthmus of the thyroid gland, the inferior thyroid veins, the arteria thyroidea ima, (when that vessel exists), the Sterno-hyoid and Sterno-thyroid muscles, the cervical fascia (in the interval between these muscles), and, more superficially, by the anastomosing branches between the anterior jugular veins: in the thorax, it is covered by the first piece of the sternum, the remains of the thymns gland, the left vena innominata, the arch of the aorta, the innominate and left carotid arteries, and the deep cardiac plexus. It lies upon the oesophagus, which is directed to the left, near the arch of the aorta; laterally, in the neck, it is in relation with the common carotid arteries, the lateral lobes of the thyroid gland, the inferior thyroid arteries, and recurrent laryngeal nerves; and, in the thorax, it lies in the interspace between the pleuras.
The Right Bronchus, wider, shorter, and more horizontal in direction than the left, is about an inch in length, and enters the right lung, opposite the fourth dorsal vertebra. The vena azygos arches over it, from behind; and the right pulmonary artery lies below, and then in front of it.
The Left Bronchus is smaller, more oblique, and longer than the right, being nearly two inches in length. It enters the root of the left lung, opposite the fifth dorsal vertebra, about an inch lower than the right bronchus. It crosses in front of the oesophagus, the thoracic duct, and the descending aorta; passes beneath the arch of the aorta, and has the left pulmonary artery lying at first above, and then in front of it.
The trachea is composed of imperfect cartilaginous rings, fibrous membrane, muscular fibres, longitudinal yellow elastic fibres, mucous membrane, and glands.
The Cartilages vary from sixteen to twenty in number: each forms an imperfect ring, which surrounds about two-thirds of the cylinder of the trachea, being imperfect behind, where the tube is completed by fibrous membrane. The cartilages are placed horizontally above each other, separated by narrow membranous intervals. They measure about two lines in depth, and half a line in thickness. Their outer surfaces are flattened, but, internally, they are convex, from being thicker in the middle than at the extremities. The cartilages are connected together, at their margins, by an elastic fibrous membrane, which covers both their surfaces; and in the space between their extremities, behind, forms a distinct layer. The peculiar cartilages are the first and the two last.
The first cartilage is broader than the rest, and sometimes divided at one end: it is connected by fibrous membrane with the lower border of the cricoid cartilage, with which, or with the succeeding cartilage, it is sometimes blended.
The last cartilage is thick and broad in the middle, in consequence of its lower border being prolonged downwards, and, at the same time, curved backwards, at the point of bifurcation of the trachea. It terminates on each side in an imperfect ring, which encloses the commencement of the bronchi. The cartilage above the last, is somewhat broader than the rest at its centre. Two or more of the cartilages often unite, partially or completely, and are sometimes bifurcated at their extremities. They are highly elastic, and seldom ossify, even in advanced life. In the right bronchus, the cartilages vary in number from six to eight; in the left, from nine to twelve. They are shorter and narrower than those of the trachea.
The Muscular Fibres are disposed in two layers, longitudinal and transverse. The longitudinal fibres are the most external, and arise by minute tendons from the termination of the tracheal cartilages, and from the fibrous membrane.
The transverse fibres, the most internal, form a thin layer, which extends transversely between the ends of the cartilages, at the posterior part of the trachea. The muscular fibres are of the involuntary class.
The Elastic Fibres are situated beneath the mucous membrane, enclosing the entire cylinder of the trachea; they are most abundant at its posterior part, where they are collected in longitudinal bundles.
