A system of vessels in the vertebrate body, beginning in a network of exceedingly thin-walled capillaries in almost all the organs and tissues except the brain and bones. This network is drained by larger channels, mostly coursing along the veins and eventually joining to form a large vessel, the thoracic duct, which runs beside the spinal column to enter the left subclavian vein at the base of the neck.

The lymph fluid originates in the tissue spaces by filtration from the blood capillaries. While in the lymphatic capillaries, it is clear and watery. However, at intervals along the larger lymphatic vessels, the lymph passes through spongelike lymph nodes, where it receives great numbers of cells, the lymphocytes, and becomes turbid. The lymphatic vessels other than capillaries contain numerous valves preventing backflow of the lymph. The functions of the lymphatic system are to remove particulate materials such as molecular proteins and bacteria from the tissues, to transport fat from the intestine to the blood, and to supply the blood with lymphocytes.