The left tracheal duct (Figures 16 and 2-7: f) and right tracheal duct (Figures 8-12: f) are two larger lymph vessel trunks, around 2.5 to 4 mm thick in large dogs when filled, which lie on each side of the trachea in the neck region. They arise on each side from the union of the efferent vessels of the medial retropharyngeal lymph node (Figures 2, 4, 6; 8-12; 16: a, a’), which usually occurs near the lymph node at the initial part of the trachea, though may less commonly occur further caudally (Figure 10), at times even as far as the cranial edge of the caudal one-third of the neck (Figure 9).
The left tracheal duct runs on the left side of the trachea and esophagus (Figure 16: 6, 7), while the right tracheal duct runs on the right side of the trachea, first together with the common carotid artery and the internal jugular vein on the dorsomedial border of the thyroid gland (Figure 16: 1) (though the right tracheal duct may be somewhat separated from the vessels on the thyroid gland) and then continue together with or near the aforementioned vessels to close to the thoracic inlet. On each side, the tracheal duct receives the efferent vessels of the cranial cervical lymph node (Figures 2, 6; 8, 9, 11; 16: b, b’), occasionally one of the efferent vessels of a middle cervical lymph node (Figure 12: c’) and of a caudal cervical lymph node (Figure 8: c), and the lymph vessels draining the thyroid gland (see lymph vessels of the thyroid gland and Figure 2: 1). The tracheal duct may divide and form an island with a smaller branch (Figure 16: i, 4) that later merges back with the main duct. In some cases, there may be several smaller branches.
The terminal part of the tracheal duct behaves differently on the right and left sides. The terminal part of the left tracheal duct usually receives 1 to 2 efferent vessels from the superficial cervical lymph node (see superficial cervical lymph centre and Figures 2, 3, 4, 6; 16: d, d’) and may receive an efferent vessel from the left axillary lymph node (Figure 16: e and axillary lymph nodes). The left tracheal duct opens somewhat cranially from the left 1st rib (in large dogs 1 to 3 cm from the rib) into the terminal part of the thoracic duct, though a branch of the left tracheal duct may open directly into the venous system. This drainage pattern is highly variable. Of the numerous variations, only the patterns that occur most frequently are listed:
1. The left tracheal duct opens into the end of the thoracic duct (Figure 16);
2. The terminal portion of the left tracheal duct bifurcates, and one branch opens into the end of the thoracic duct, with the other opening into the common jugular vein (Figure 4); the end of the left tracheal duct may even divide into 3 branches (Figure 7);
3. The left tracheal duct opens into a branch of the terminal portion of the thoracic duct (Figures 2, 3, 5, 6).
The terminal part of the right tracheal duct (Figures 8-11: f) merges with the efferent vessel(s) of the right superficial cervical lymph node (Figures 8-11: g) near the thoracic inlet, 2 to 3 cm cranial from the 1st right rib in large dogs (Figures 8-12: 5). The union of the two results in a short lymphatic trunk, only 1.5 cm long, even in large dogs, which is approximately twice as thick as the right tracheal duct before the union (4 to 6 mm). This short lymphatic trunk is commonly called the right lymphatic trunk, although there is no specific reason for this separate name. The right lymphatic trunk (Figures 8-11: h) lies on the right side of the trachea and the right subclavian vein in front of the right 1st rib and opens either into the right subclavian vein, cranial to its connection to the cranial vena cava, or in the angle between the internal and external jugular veins. The branching and merging of the right lymphatic trunk may form one or several islands. Occasionally, branches of the right lymphatic trunk may enter the venous system directly; the most common variations are shown in Figures 8-11: h, h’. An efferent vessel from the right axillary lymph node usually opens into the right lymphatic trunk or into one of its terminal branches (see axillary lymph nodes and Figures 9, 10: e).