Lymph Vessels of the Pericardium, Heart, and Aorta
A. Lymph Vessels of the Pericardium and Mediastinum
The lymph vessels of the pericardium cannot be separated from those of the cardiac part of the mediastinum. Therefore, the description of the lymph vessels of the cardiac part of the mediastinum also applies to the lymph vessels of the pericardium (see lymph vessels of the mediastinum).
B. Lymph Vessels of the Heart
Most of the lymph vessels of the heart drain to all 3 lymph nodes in the tracheobronchial group, while the rest drain to the cranial mediastinal lymph nodes.
i. Lateral Wall of the Left Ventricle
The lymph vessels of the lateral wall of the left ventricle (Figures 18: a; 21: b) drain to the left tracheobronchial lymph node (Figure 21: l), the middle tracheobronchial lymph node (Figures 18: 1; 21: k), and the cranial mediastinal lymph nodes (Figures 18: 3; 21: m). Some of the lymph vessels run from the puncture sites to the left longitudinal groove, while others run to the right longitudinal groove, and the rest run upwards to the coronary groove. In general, the lymph vessels of the left part of the left ventricular wall (Figure 21) run towards the left longitudinal groove (Figure 21: o), the lymph vessels of the caudal part of the left ventricular wall (Figure 21) rise towards the coronary groove (Figure 21: n, n’), and the lymph vessels of the right part of the left ventricular wall (Figure 18) run towards the right longitudinal groove (Figure 18: e). However, these individual areas are by no means sharply demarcated from each other; these lymph vessels fill towards both sides from most sites of injection (e.g. towards the left longitudinal groove and the coronary groove, or towards the right longitudinal groove and the coronary groove). From some sites of injection near the cardiac apex, the lymph vessels even filled in all 3 directions upon injection. Lymph vessels also pass from the right side of the cardiac apex to the left side, and from the left side to the right side, as shown in Figure 21.
The lymph vessels running to the right longitudinal groove merge within it to form 2 to 3 small vessels, some of which rise above the left atrium to drain to the middle tracheobronchial lymph node (Figure 18: 1), while others join those in the coronary groove and curve with them to the left (see below and Figure 18). In rare cases, all the lymph vessels of the right longitudinal groove curve to the coronary groove. The lymph vessels entering the coronary groove also merge, forming 1 to 3 vessels, which then run in the left part of the coronary groove (Figure 21: n’), more or less hidden by the left atrial appendage (auricle), to the pulmonary artery where they either join or merge with the 2 to 3 vessels ascending in the left longitudinal groove (Figure 21: o). The resulting lymph vessels pass between the left atrial appendage and the pulmonary artery, running over the right side of the pulmonary artery to drain to either the left tracheobronchial lymph node (Figure 21: l) or one of the cranial mediastinal lymph nodes (Figure 21: m), or to both groups of lymph nodes. The lymph vessels usually drain to the cranial mediastinal lymph node that lies on the cranial vena cava.
ii. Lateral Wall of the Right Ventricle
The lymph vessels of the lateral wall of the right ventricle (Figures 18: b; 21: a) behave similarly to the lymph vessels of the left ventricular wall, draining to the left and middle tracheobronchial lymph nodes and to the cranial mediastinal lymph nodes. From the left part of the right ventricular wall, the lymph vessels usually run towards the left longitudinal groove (Figure 21: o), and, from the cranial part of the right ventricular wall, they run directly upwards to the coronary groove (Figures 18: d’; 21: n). From the right part of the right ventricular wall, they run towards the right longitudinal groove (Figure 18: e), either joining the corresponding lymph vessels from the right part of the left ventricular wall and running with them via the left atrium upwards to drain to the middle tracheobronchial lymph node (Figure 18: 1), or turning caudally in the coronary groove and joining the lymph vessels of the left ventricular wall.
The lymph vessels entering the coronary sulcus run in the left part of the sulcus to the caudal border of the pulmonary artery (see Figure 21), merging here with the lymph vessels ascending in the longitudinal sinus (Figure 21: o) (see above) and with the lymph vessels coming from the left ventricular wall (see above), before travelling deeply between the pulmonary artery and the left atrial appendage, as described above, and draining to the left tracheobronchial lymph node (Figure 21: l) and the cranial mediastinal lymph nodes (Figure 21: m).
The following deviations from this behaviour have been observed:
1. The lymph vessels that meet in the coronary groove do not merge with the ascending lymph vessels in the left longitudinal groove, rather forming a small trunk that extends over the left side of the pulmonary artery and aorta to drain to one of the cranial mediastinal lymph nodes (this type of lymph vessel is also drawn in Figure 18).
2. The lymph vessels usually running from the right part of the right ventricular wall to drain to the middle tracheobronchial lymph node do not drain to this lymph node, but instead join the lymph vessels running in the coronary groove, as described for the lymph vessels of the left ventricular wall. In this case, all the lymph vessels of the two ventricular walls eventually merge to form 2 to 3 larger vessels, which run deeply between the pulmonary artery and the left atrial appendage.
The individual drainage areas of the right ventricular wall are not sharply demarcated as described for the left ventricular wall.
iii. Lymph Vessels of the Left Atrium
Some of the lymph vessels of the left atrium (Figure 21: d), specifically those from the atrial appendage and the part of the atrium adjacent to it, run over both surfaces of the atrium towards the coronary groove where they join with the lymph vessels of the left ventricular wall running in the coronary groove, draining with them to the left tracheobronchial lymph node (Figure 21: l) and usually into a cranial mediastinal lymph node (Figure 21: m) as well; other lymph vessels of this area of the left atrium run over the dorsal surface of the left atrium and drain into the same lymph nodes (see Figure 21). The lymph vessels from the posterior (caudal) and from the right (adjacent to the right atrium) parts of the left atrium drain to the middle tracheobronchial lymph node (Figure 21: k). Most pass directly upwards over the left atrium to drain to the lymph node (as shown in Figure 21), while some merge with the lymph vessels of the left ventricular wall, which rise upwards from the right longitudinal groove over the left atrium to drain to the middle tracheobronchial lymph node (Figure 18). If the lymph vessels of the left ventricular wall do not take this path, and instead join the lymph vessels which run to the left side in the coronary groove as described previously, then the lymph vessels of the left atrium that join them also take the same path.
iv. Lymph Vessels of the Right Atrium
Some of the lymph vessels of the right atrium (Figure 18: c) drain to both the middle (Figure 18: 1) and right (Figure 18: 2) tracheobronchial lymph nodes, while others drain to the cranial mediastinal lymph nodes (Figure 18: 3). The lymph vessels of the part of the right atrium adjacent to the left atrium join with the lymph vessels from the right part of the left atrium, running upwards with them to drain to the middle tracheobronchial lymph node (Figure 18: 1). Rarely, these lymph vessels may also run caudally in the coronary groove and merge with the lymph vessels of the left ventricular wall (see above). From the remaining part of the right wall of the right atrium, some of the lymph vessels ascend to drain to the right tracheobronchial lymph node (Figure 18: 2), some merge to form a lymph vessel that runs cranially along the cranial vena cava to drain to one of the cranial mediastinal lymph nodes (Figure 18: 3), and some run to the coronary groove (Figure 18: d’) where they join with the lymph vessels of the right ventricular wall (see above). The lymph vessels of the left wall of the right atrium drain to the middle tracheobronchial lymph node as well as into the right tracheobronchial lymph node, occasionally. The lymph vessels of the left atrial appendage (Figure 21: c) and the part of the right atrium adjacent to it run over both surfaces of the right atrium to the coronary groove, where they merge with the lymph vessels of the right ventricle and drain with them to the left tracheobronchial lymph node (Figure 21: l) and possibly to one of the cranial mediastinal lymph nodes (see above and Figure 21: m).
v. Lymph Vessels of the Ventricular Septum
The lymph vessels of the ventricular septum emerge in both longitudinal grooves and join the lymph vessels of the ventricular wall.
vi. Lymph Vessels of the Papillary Muscles
The lymph vessels of the 2 papillary muscles of the left ventricle behave as follows: the lymph vessels of the papillary muscle adjacent to the left longitudinal groove pass from the base of this papillary muscle through the left part of the ventricular wall, joining with the lymph vessels ascending in the left longitudinal groove (Figure 21: o), and those of the papillary muscle adjacent to the right longitudinal groove join with the lymph vessels in the right longitudinal groove (Figure 18: e) in the same manner.
The 2 to 3 papillary muscles of the right ventricle behave as follows: the lymph vessels of the posterior right papillary muscle near the right longitudinal groove pass through the ventral part of the lateral wall of the right ventricle, then enter the right longitudinal groove (Figure 18: e), joining with the other lymph vessels of the groove. The lymph vessels of the left anterior or 2 left anterior papillary muscles pass through the ventral part of the lateral wall of the right ventricle near the left longitudinal groove, joining with the lymph vessels rising in the left longitudinal groove (Figure 21: o). The lymph vessels emerge from the base of the papillary muscles and run to the lateral wall, at least some running on the transverse muscles.
The lymph vessels of each papillary muscle merge to form 2 to 4 larger lymph vessels.
An extremely rich, fine network of lymph vessels fills with dye under the endocardium of the papillary muscles.
vii. Lymph Vessels of the Endocardium and Epicardium
The lymph vessels of the endocardium and the epicardium behave exactly as they do in the cow (pages 148 and 149 of The Lymphatic System of Cattle [6]): the endocardial lymph vessels form abundant networks under the endocardium, which may be multilayered at thick endocardial sites, such as at the tip of the papillary muscles. The lymph vessels that emerge from the endocardium will enter the myocardium and gradually penetrate obliquely, joining with the lymph vessels of the myocardium.
The lymphatics of the epicardium cannot be clearly demarcated from the subepicardial lymphatics of the myocardium.
c. Lymph Vessels of the Aorta
The lymph vessels of the aorta can be injected in dogs with a high degree of certainty. The lymph vessels from the aortic arch and the adjacent part of the thoracic aorta were injected (Figure 17: 3) and were observed to be very small lymph vessels that drained into a cranial mediastinal lymph node (specifically, into the lymph node located on the left side of the cranial vena cava near the aortic arch) (Figure 17: a1).
From the middle part (Figure 17: 31) of the thoracic aorta, some of the lymph vessels drain to the cranial mediastinal lymph nodes (Figure 17: a1), and some drain to the middle tracheobronchial lymph node (Figure 17: b1). If an intercostal lymph node is present, some of the lymph vessels draining to the cranial mediastinal lymph nodes will also drain to it.
The lymph vessels of the caudal part of the thoracic aorta (Figure 17: 32) merge to form 1, 2, or 3 larger vessels and drain to the 2 cranial lumbar aortic lymph nodes, while the lymph vessels of the abdominal aorta drain to both the lumbar aortic and medial iliac lymph nodes.
