Lymph vessels are often found running alongside the arteries and veins serving the area. Their walls are about the same thickness as those of small veins and have the same layers of tissue,ie.a fibrous covering,a middle layer smooth muscle and elastic tissue and an inner lining of endothelium. Like veins,lymph vessels have numerous cup-shaped valves to ensure that lymph flows in a one-way system towards the thorax. There is no ‘pump’,like the heart,involved in the onward movement of lymph,but the muscle layer in the walls of the large lymph vessels has an intrinsic ability to contract rhythmically(the lymphatic pump).
In addition,lymph vessels are compressed by activity in adjacent structures,such as contraction of muscles and the regular pulsation of large arteries. This ‘milking’ action on the lymph vessel wall helps to push lymph along.
Lymph vessels become larger as they join together,eventually forming two large ducts,the thoracic duct and right lymphatic duct,which empty lymph into the subclavian veins.
