Lymphatic system
It would be reasonable to discuss the lymphatic system with either the circulatory system or the immune
system, because in many ways, it is a link between the two. The fundamental parts include the lymph nodes and nodules, the spleen, the thin vessels that link them together, and, of course, the fluid (tissue fluid or interstitial fluid) or lymph that flows in the vessels.
In addition to circulation in blood, lymph provides routes for passage of lymphocytes and ready links between clusters for lymphoid tissues. Because of the manner in which interstitial fluid drains from many organs, localized lymph nodes are often one of the first sites that are impacted by localized tissue infections. A prime example are the paired Supernumery lymph nodes that are located on the dorsal aspect of the two rear mammary glands between the upper portion of the udder and the body wall of the cow. In older animals, it is common to see these two lymph nodes greatly enlarged, as a consequence of the proliferation and activation that occurred as a consequence of mastitis and the drainage of lymph-containing antigens. The consequences of poor drainage and osmotic changes induced by the presence of leaking milk proteins in lymph is frequently a cause for the udder edema that is relatively common in first-calf heifers.The lymph vessels are part of a one-way pathway for fluids that roughly parallels the venous circulation. These vessels with branches coming from throughout the body progressively coalesce and empty into the general circulation in the cranial vena cava. The beginnings of this circulatory pathway are essentially thin, blind-ended pouches that collect interstitial fluid that drains to localized tissue lymph nodes and the regional nodes and the large lymph vessels that parallel the trunk of the body. Lymph drainage from the forelimbs, neck, and head also empties at the thoracic duct. The fluid flow through the lymphatic vessels is one-way because of the presence of valves.
Moreover, because it is not pressurized (as in the sense of the circulatory system), the lymph is dependent on passive movement and some impact of changes in the diaphragm to aid emptying of lymph into the thoracic duct.The microscopic structure of lymphoid tissue is somewhat variable, but common features are clusters or clumps of lymphocytes essentially trapped within strands of connective tissue, especially reticular fibers. In some regions (the submucosa of the GI tract), the tissues also appear in diffuse scattered clumps or more organized nodules (Peyer's patches). Other lymphoid tissues (spleen, tonsils, thymus, and nodes) are much more highly structured and encapsulated.
A typical node has an outside capsule with multiple entering or afferent lymphatic vessels that provide fluid that essentially percolates through the organ as well as efferent vessels that typically exit from an indentation called the medullary sinus. Bands of connective tissue called trabeculae pass from the outer capsule in the internal structure of the nodes to help organize the clusters of lymphocytes and to provide some structural integrity. The internal structure contains many clusters of lymphocytes as well as sinuses and vessels that converge in the medullary sinus where the efferent vessels exit the node. The gross structure is somewhat reminiscent of kidney or adrenal gland. There is an outer cortex, paracortex, and medulla. The afferent vessels provide fluid to a sub- capsular space of sinus that connects with smaller internal sinuses and progressively slows drainage to the medulla and the hilus of the node where the efferent lymphatic vessels and blood vessels exit the node.