The trachea begins where the larynx ends, that is, at the lower border of the cricoid cartilage which is on a level with the 7th cervical vertebra (Fig. 238). It is from 4 to 5 inches long, half of which is in the neck (cervical portion), and the other half in the thorax (thoracic portion). The trachea enters the thoracic inlet opposite the upper border of the manubrium sterni and ends at its lower border (Fig. 238 A and 238 B); the latter point is situated on a level between the 3rd and the 4th thoracic spines, where it divides into right and left primary bronchi. Its thoracic portion is opposite the manubrium and wholly within the superior mediastinum. It occupies the median plane except at its lower end where the aortic arch pushes it slightly to the right. About 15 or 20 horseshoe-shaped rings of hyaline cartilage keep its lumen open and support the lateral and anterior aspects, but they are deficient posteriorly where the wall is flattened. The posterior portion is closed by the trachealis muscle, which is made up of transverse plain fibers supplied by the recurrent laryngeal nerve (Fig. 238 C). Anteriorly, the trachea is crossed by the isthmus of the thyroid (2nd to 4th ring), and below that structure it is related to the inferior thyroid veins which pass downward upon it, frequently communicating with each other and opening into the innominate veins. In young children, the left innominate vein may appear above the level of the manubrium sterni, forming an anterior relation of the trachea.
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