The smallest lymph vessels are lymphatic capillaries, which originate in the peripheral tissues. They are larger in diameter than blood capillaries, but have a thinner wall. The lymphatic capillaries have a unique structure that allows interstitial fluid to flow into them, but not out. Lymph flows from the lymphatic capillaries into larger lymph vessels that lead toward the trunk of the body. The lymphatics continue to join together, finally forming two large ducts: the right lymphatic duct and the thoracic duct.