3. =The Trachea= (Fig. 105, 5; Fig. 106, _a_).--The trachea is that part of the air-passage which extends from the larynx to the bronchi (Fig. 106). It is a straight tube composed of a lining mucosa with ciliated epithelium, and a connective-tissue covering which encloses supporting cartilages. Each tracheal cartilage is incomplete dorsally where it lies against the œsophagus, the gap between the two free ends of each ring being filled with muscular and connective tissue. As a result of this the diameter of the trachea is not fixed, but can be increased and diminished. The first ring is broader than the others. Where the œsophagus leaves the median line, the dorsal surface of the trachea lies against the longus colli muscles (Fig. 72, _g″_). Its ventral surface is against the sternohyoid (Fig. 65, _e_) and sternothyroid (Fig. 65, _g′_) muscles. Its lateral surfaces are partly covered by the thyroid gland (Fig. 96, 6), and are in close relation with the carotid artery (Fig. 119, _a_, page 284), the vagus and sympathetic nerves (Fig. 156, _i_), and the internal jugular vein (Fig. 119, _b_). In the thoracic cavity the great vessels coming from the heart lie against the ventral surface of the trachea (Fig. 129, 2). At about the level of the sixth rib the trachea divides into the two main =bronchi= (Fig. 106). Each bronchus is supported by incomplete rings of cartilage like those of the trachea and has otherwise in general the structure of the trachea. In the lungs the bronchi become divided into many branches (Fig. 106), in the manner described in the account of the lungs.