Lymph Vessels of the Thyroid Gland
The lymph vessels of the thyroid gland (Figures 2, 6, 8, 16: 1) drain to the medial retropharyngeal lymph node, the cranial, middle, and caudal cervical lymph nodes, and the cranial mediastinal lymph nodes, and additionally are frequently found to drain directly into either the tracheal duct or the internal jugular vein.
The thyroid lymph vessels were injected in 17 dogs, most often on both sides.
The lymph vessels that drain to the medial retropharyngeal lymph node (Figures 2, 6, 8, 16: a, a’) and to the cranial cervical lymph node (Figures 2, 6, 8 and 16: b, b’) arise near the cranial end of the thyroid gland, while the lymph vessels that drain to the caudal (and middle) cervical lymph node(s) (Figures 8, 16: c, c’) and the cranial mediastinal lymph nodes arise from both surfaces of the caudal half of the thyroid gland.
However, there are frequent exceptions to this rule: in Figure 8, for example, a lymph vessel originating from the caudal part of the thyroid gland was found to drain to a cranial cervical lymph node (k’). Occasionally, all lymph vessels arising from the caudal end of the thyroid gland may drain to both the medial retropharyngeal and cranial cervical lymph nodes (Figure 6). If there are two medial retropharyngeal lymph nodes, then the lymph vessels of the thyroid gland usually open into both of them (Figures 8, 16: a, a’).
During injection, rich, coarse lymphatic networks fill in the capsule, but soon coalesce. The lymph vessels draining to the medial retropharyngeal and cranial cervical lymph nodes usually merge to form 3 to 4 small vessels, after which either some or all drain to the cranial cervical lymph node when it is present, while the lymph vessels that do not enter the cranial cervical lymph node drain directly to the medial retropharyngeal lymph node. If the cranial cervical lymph node is absent, all lymph vessels drain directly to the medial retropharyngeal lymph node. In 1 case in which a cranial cervical lymph node was present, none of the cranial lymph vessels of the thyroid gland were observed to drain to it (Figure 2).
Likewise, the lymph vessels running along the trachea and esophagus and draining to the caudal cervical and cranial mediastinal lymph nodes merge to form 1 to 3 trunks. If a caudal (or middle) cervical lymph node (Figure 8, 16: c’) is present, then either all or some of these lymph vessels drain to it, while the rest (Figure 16: l) drain directly into a cranial mediastinal lymph node. If the caudal cervical lymph node is absent, all these lymph vessels drain directly to the cranial mediastinal lymph nodes, with a preference on the left side for the cranial mediastinal lymph node that lies on the left side of the cranial vena cava.
The thyroid lymph vessels frequently cross the median plane, even in those cases where an isthmus is not present. If there is an isthmus between both thyroid glands, then the injected fluid passes from one thyroid gland through the isthmus into the other thyroid gland, and the lymph vessels on both sides fill up even though only one side was injected.
An interesting example of a pronounced median crossing of thyroid lymph vessels was seen in one of the cases mentioned above, in which lymph vessels draining caudally from the left thyroid gland to the caudal cervical and cranial mediastinal lymph nodes could not be injected. In this case, close examination revealed that some of the lymph vessels passed directly over the ventral aspect of the trachea to the right side then ran down the right side of the trachea and drained to a caudal cervical lymph node located on the ventral aspect of the trachea.
The thyroid lymph vessels also have a strong tendency to form networks.
It is evident from the above description that, while one can generally consider there to be both cranial and caudal drainage areas of the lymph vessels of the thyroid gland, these two areas cannot be clearly demarcated, and the division into lateral and medial drainage areas is even less evident.
It is striking how often lymph vessels of the thyroid gland open directly, i.e. without having passed a lymph node, into either the tracheal duct or into the venous system (Figure 2: k). I was able to demonstrate this finding in approximately one-third to one-half of all 17 dogs examined. In all cases, it was the caudal lymph vessels of the thyroid gland that merged to form a small trunk which, near the thoracic inlet, opened either into the tracheal duct (right or left) or into the internal jugular vein, even though a caudal or middle cervical lymph node was present in some of these cases. For more details see Baum [8]. It is also remarkable that the blue injection fluid often appeared in the internal jugular vein during injections into the thyroid gland, similarly to the transfer of injection fluid into veins that can be observed during the injection of bone lymph vessels.
