Lymph Vessels of the Costal Pleura
The lymph vessels of the pleura were almost all filled by puncture injection, as this is the easiest and most convenient method, but they can also be injected by introducing coloured fluid into the pleural sacs and performing artificial respiration as described in The Lymphatic System of Cattle [6] on page 117.
The lymph vessels of the costal pleura (see Figure 19) drain to the sternal lymph node (Figure 19: 2), the cranial mediastinal lymph nodes (always the main cranial mediastinal lymph nodes in the examined cases) (Figure 19: 1, 1, 1’), the intercostal lymph node, and the cranial lumbar aortic lymph node. The behaviour of these vessels is very similar to the lymph vessels of the intercostal muscles.
Two to 4 lymph vessels arise from the pleurae of the 1st rib and the 1st intercostal space and drain into the main cranial mediastinal lymph node, as shown in Figure 19. One lymph vessel usually runs directly to the same lymph node from the middle of the intercostal space. Of the remaining lymph vessels, 1 to 2 curve sharply ventrally and 1 to 2 curve sharply dorsally. One of the lymph vessels arising from the pleurae of the 1st rib and the 1st intercostal space, however, may additionally drain to the sternal lymph node.
The lymph vessels of the ventral half of the pleura from the 2nd to 10th ribs and the 2nd to 9th intercostal spaces (Figure 19: d, d) turn ventrally and run under the M. transversus thoracis, reaching the internal mammary artery and vein and moving with them to drain into the sternal lymph node (Figure 19: 2), while the lymph vessels of the dorsal half of this region (Figure 19: a, a’) turn dorsally up to the aorta, and then turn cranially (Figure 19: b) to run along the aorta and the M. longus colli and drain into the main cranial mediastinal lymph node (Figure 19: 1, 1’), usually merging along the way to form 2 to 3 vessels. If an intercostal lymph node is present, some of these lymph vessels may drain to it (Figure 19: 11).
The pleural lymph vessels from the 11th to 13th ribs and 10th to 12th intercostal spaces also turn dorsally up to the vicinity of the aorta, but then run caudally (Figure 19: c) and drain to the cranial lumbar aortic lymph node. Only the lymph vessels from a small region at the insertion of the diaphragm at the 10th, 11th, and 12th intercostal spaces (Figure 19: d’) turn ventrally and enter the diaphragm (Figure 19: 5) to either merge with diaphragmatic lymph vessels or to merge directly to form 1 to 2 vessels that run cranioventrally at the insertion of the diaphragm, passing under the M. transversus thoracis and draining into the sternal lymph node.
The lymph vessels of the part of the pleura covering the M. transversus thoracis and the sternum penetrate the M. transversus thoracis and thus join the lymph vessels mentioned above, draining into the sternal lymph node. These lymph vessels are difficult to inject because the pleura is so thin that there is a risk of penetrating through it into the M. transversus thoracis. As can be seen in Figure 19, puncture injection mainly fills finely meshed lymph networks in the pleura of the intercostal spaces but, although to a lesser extent, in the pleura adjacent to the ribs, as well. I mention the pleural lymph networks adjacent to the ribs specifically because Dybkowsky [16] states that for the pleural lymph vessels of the dog, he was only able to fill lymph networks in the intercostal spaces. This statement was inconsistent with my findings in the dog and in the cow (see Baum [6], page 117), both when the lymph vessels were filled by puncture injection and by artificial respiration.
The lymph vessels arising from the lymph networks continue to run subpleurally. In each intercostal space and associated rib, there are usually 2 to 4 lymph vessels, of which 1 to 3 turn both ventrally and dorsally, as described above. Some of the lymph vessels run with the intercostal blood vessels, but some additionally run irregularly, i.e. not accompanied by blood vessels. It is difficult to say which of these two behaviours is more common. Occasionally, one of the lymph vessels of a specific intercostal space runs to its neighbouring intercostal space. I could not determine whether pleural lymph vessels, particularly those of the first intercostal spaces, pass through the intercostal musculature to drain to the axillary lymph node, which occurs in both the human and bovine species, even though I carefully examined 4 to 5 cases to look for this behaviour specifically.
