The Lymphatic System (Figure 10.1)
The mammalian body has a second system of organs and channels similar to the blood circulatory system; it is called the lymphatic system. The system carries an aqueous solution (lymph) containing lymphocytes that protect the body from infection.
The lymphocytes are generated in the bone marrow and are found in concentration in the spleen, thymus and tonsils as well as in numerous bean-like nodules called lymph nodes that act as filters of the lymph and centres of production of lymphocytes. Lymph nodes consist of a capsule surrounding a cortex and medulla. There are very many lymph nodes throughout the body; some of the lymph nodes are palpable (e.g. in the axilla, behind the stifle and ventral to the mandible). If large numbers of bacteria are filtered by a lymph node, it becomes enlarged and, depending on the location, palpable and the swelling visible. Lymph nodes are located along the abdominal aorta, adjacent to the kidneys and cranial to the external iliac arteries. There are also groups of lymph nodes corresponding to the territories of the three major arteries, viz., coeliac, cranial mesenteric and caudal mesenteric. In the pelvic region there are hypogastric nodes, deep and superficial inguinal nodes draining the reproductive organs and the mammary glands. Aggregations of lymph nodules are located in the small intestine; they are called Peyer's patches.Nutrients are available to the body tissues by leakage of fluid from the blood capillaries; a major function is the absorption of dietary fats. This interstitial fluid also collects waste products, bacteria and cellular debris, all of which drain into the lymphatic capillaries. In the small intestine the lymphatic capillaries within the villi are called lacteals; the lymph then enters lymphatic channels that are progressively of larger diameter, finally becoming a pair of lymphatic ducts. The left lymphatic duct is much larger than that on the right and is called the thoracic duct. In man both lymphatic ducts drain into the corresponding subclavian vein at its junction with the internal jugular vein, thereby returning the lymph to the blood circulation. At its origin the thoracic duct is dilated and receives lymph from the intestinal trunk; this dilated part of the thoracic duct is the cisterna (or receptaculum) chyli. The cisterna chyli is located dorsal to the aorta and ventral to the cranial lumbar vertebrae. The intestinal trunk collects lymph from the stomach, intestines, pancreas, spleen and part of the liver. This is the main drainage route for the transport of fatty acids and fats (chyle) from the digestive system.
In the domestic mammals there is considerable variation as regards the route by which lymph is returned to the blood circulation. The thoracic duct passes through the aortic hiatus (Figure 8.3) into the mediastinum and then usually empties either into the left jugular vein or the caudal vena cava.
10.2