Pathways of lymph flow through superficial inguinal lymph nodes in the pig

The pig lymph node has an unusual structure in that tissue containing lymph nodules generally occupies a central position. Our aim was to describe the lymphatic pathways through this node. We studied the structure of these pathways with light and electron microscopy, made casts of lymphatic vessels and sinuses with Microfil, and studied the distribution within the node of subcutaneously injected carbon particles. Most afferent lymphatics penetrate deeply within the node, where they give off several branches to peritrabecular sinuses that ramify through centrally located nodular tissue. However, where an afferent lymphatic enters the node there is a subcapsular sinus over an area of nodular tissue that occupies a conventional superficial position. Some lymph reaches this sinus from the central peritrabecular sinuses, but there can also be direct communications between this sinus and the afferent lymphatic. After flowing through sinuses in nodular tissue, lymph enters tissue that is analogous to medullary tissue in other species. This tissue is of two types, one consisting mainly of a diffuse network of reticular cells around spaces up to 10-12 microns across, and one that more closely resembles conventional medullary tissue. Lymph then flows to collecting ducts, which lack valves, and then to efferent lymphatics. Our findings do not support suggestions that a purely physical obstruction of lymphocytes in the lymph node accounts for the dearth of lymphocytes in efferent lymph of pigs.