Lymph Vessels of the Nervous System
The lymph vessels and lymph pathways of the nervous system (the ventricular system of the brain and spinal cord, their sheaths, and the cerebrospinal nerves) behave in the dog exactly as I described for the cow in The Lymph Vessels of the Nervous System of Cattle . I have briefly summarized my results in this area in the cow, and have provided an excerpt below:
“I found that with careful injection of fluid (dye) into the subarachnoid space:
1. the subdural space does not fill;
2. the injected fluid passes from the subarachnoid space into the venous system;
3. the lymph vessels of the nasal cavity fill from the subarachnoid space;
4. the injected fluid continues from the subarachnoid space into the (subarachnoid) lymphatic clefts of all the cerebrospinal nerves, although the (subarachnoid) lymphatic clefts of all cerebrospinal nerves did not always fill in individual cases.
I was repeatedly able to trace the dye into the terminal branches of both the cerebral and spinal nerves after injections. For example, I was able to trace the hypoglossal nerve into the tongue, the infraorbital nerve into the upper lip, the cervical and intercostal nerves down to their cutaneous branches, the intercostal nerves along the entire ribs down to the sternum, and the sciatic nerve down to the stifle region. It is interesting to note that even the lymphatic clefts of the branches connecting the spinal nerves to the sympathetic nerve filled with dye, and the dye also continued into the sympathetic ganglia. Furthermore, I was also able to fill lymph vessels from the subarachnoid space, which, to my knowledge, has not been previously demonstrated. These lymph vessels, accompanied by the cerebrospinal nerves, leave the cranial cavity and the spinal canal and enter the corresponding lymph nodes (Ln./Lnn. retropharyngei, pteryoidea, mandibularis, cervicales profundae, cervicales superﬁciales, axillares primae costae, intercostales, mediastinales craniales and dorsales, lumbales aorticae and hypogastricae). However, the lymph vessels do not fill with dye directly from the subarachnoid space, but either from the lymphatic clefts of the cerebrospinal nerves, shortly after the nerves exit from the brain and spinal cord, or from the intervertebral foramen.”
In general, my observations in the dog were in complete agreement with these findings from the nervous system of the cow, with obvious differences resulting from the different behaviours of the lymph nodes. The lymph vessels of the cranial cavity in the dog drained only into the medial retropharyngeal lymph node, those of the cervical spinal canal drained to the mediastinal lymph nodes, those of the subarachnoid space of the thoracic spine drained to the mediastinal, intercostal, and cranial lumbar aortic lymph nodes, and those of the lumbar and sacral canal drained to the lumbar aortic, medial iliac, and hypogastric lymph nodes. As in the cow, I found that the subdural space is separated from the ventricular system, from the subarachnoid space, and from the cerebrospinal fluid pathways of the brain, and only connects with them peripherally; the injected dye passes from the subdural space into the venous system in the same way that it does from the subarachnoid space (for more details, see my article mentioned above ). In the dog, again as in the cow, I observed that the dye injected into the subdural space fills the (subdural) lymphatic clefts of all cerebrospinal nerves and that lymph vessels fill from these lymphatic clefts.
This is the same behaviour as described in detail above for the lymph vessels arising from the nerves after dye injection into the subarachnoid space. This behaviour is explained by the fact that the subdural and subarachnoid lymphatic clefts in the nerves are not separated, and instead communicate with each other, forming one lymphatic cleft system. Due to this, the lymphatic cleft system of a nerve has efferent lymph vessels.
The technique I used to inject the lymphatic spaces and lymph vessels of the nervous system in dogs was the same as that given in detail for cattle in the above referenced article . It should be emphasized that, with the small size of the dog, it is hardly possible to insert the cannula into the subdural space of the spinal cord and tie it in place without destroying the arachnoid. As a result, the injection fluid will probably always penetrate not only into the subdural space, but also into the subarachnoid space. If you wish to inject each of the two spaces separately, the only way to achieve this is to carefully insert the cannula into the subdural space when the front half of a dog is hung upside down, or the rear half of a dog is hung upside down (with the tail downwards), and to not tie in the cannula, but instead loosely attach it to the surrounding tissues, attach a glass funnel, and pour the injection liquid into the funnel. Because much lighter pressure is applied, the injection takes much longer. I also found that the injection fluid in the dog transferred into the venous system from both the subdural and the subarachnoid space much more easily and in relatively larger quantities than is the case with the cow – when such injections are done, large parts of the venous system easily fill, up to the finest capillaries.