Lymph Vessels of the Lungs

All the lymph vessels of the lungs (Figure 20) drain to the bronchial lymph nodes (tracheobronchial and pulmonary lymph nodes). They are remarkably difficult to inject in dogs, at least much more difficult than those from cattle (specifically, calves). It is also difficult to separate the lymph vessels of the pleura from the lymph vessels of the superficial parenchymal layer during the injection. Because of the thinness of the pleura, it is not possible to decide with certainty whether the needle is only in the pleura or whether it has penetrated into the superficial lung layer as well.

Nevertheless, it is possible to inject the more obvious serosal or subserosal networks, which are relatively coarsely meshed. Occasionally, the lymph vessels arising from the subserosal (superficial) networks can be traced further; in some cases, it was possible to follow a subserosal lymph vessel arising from such a network to the lymph node(s) into which the deep lymph vessels of the respective lung lobe drained (see below). Examples of these lymph vessels are designated as 6 and 6’ in Figure 20. In other cases, such subserosal lymph vessels could only be traced for a short distance, before they descended and traveled deeply (as shown in Figure 20 in several places [e.g. at 7, 7, 7,]). A subserosal lymph vessel from one area of the lung may additionally run around the border of the lung to another area of the lung, most commonly from the costal area to the diaphragmatic area, as in the case of the lymph vessel labeled as 5 in Figure 20. In some of the cases in which the subserosal networks were filled by injection, it was not possible to identify lymph vessels arising from the networks and extending subserously, so it is assumed that the lymph vessels that arose from the networks immediately run deeply and join with the deep lymph vessels. On the other hand, the filling of a subserosal lymph vessel was often observed (although not nearly as often as in the cow) when deep injections of the lung were made – this lymph vessel could usually only be followed for a few centimeters before it returned to the depth.

The above findings demonstrate that superficial (subserosal) and deep lymph vessels cannot be clearly differentiated.  The deep lymph vessels merge within each individual lung lobe to form 2 to 4 trunks, which then leave the lobe of the lung accompanied by its bronchus and its blood vessel (a branch of the pulmonary artery and a pulmonary vein). Most of these lymph vessels are located on the dorsal wall of the bronchus and on its borders but occasionally are also present on its ventral side. The deep lymph vessels of the left apical lobe (the cranial part of the left cranial lobe)^[1] and left cardiac lobe (the caudal part of the left cranial lobe) emerge from each of the 2 lobes as 2 to 3 vessels, then merge to form 2 to 4 vessels, before draining into the left tracheobronchial lymph node. Most of the deep lymph vessels of the left diaphragmatic lobe (the left caudal lobe) drain to the middle tracheobronchial lymph node, and to a left pulmonary lymph node if present.

However, 1 of these lymph vessels usually travels further, over either the ventral edge of the left main bronchus or the end of the trachea, to drain to the left tracheobronchial lymph node. The deep lymph vessels of the right diaphragmatic lobe (the right caudal lobe) drain to the middle and right tracheobronchial lymph nodes and to the right pulmonary lymph node if one is present. The deep lymph vessels of the right cardiac lobe (the right middle lobe) go either under the right main bronchus and drain to the middle tracheobronchial lymph node, or under the bronchus of the right apical lobe (the right cranial lobe), draining to the right tracheobronchial lymph node. The deep lymph vessels of the right apical lobe additionally drain to the right tracheobronchial lymph node, while those of the accessory lobe drain to the middle tracheobronchial lymph node. The lymph vessels of the bronchi join the deep lung lymph vessels.