A blood vessel conveying blood towards the heart. Only the pulmonary veins carry oxygenated blood. Small veins (venules) drain blood from the capillaries and unite to form a vein. The walls of a vein consist of three layers: an inner endothelium ( tunica interna, tunica intima), a middle smooth muscle layer ( tunica media), and an outer fibrous tissue layer ( tunica adventitia). The walls are thinner and less elastic than those of arteries. Unlike arteries, the larger veins contain valves which assist the flow of blood back to the heart. Veins, especially those in the legs, can become enlarged and twisted ( varicosed). See also Appendix E for the principal veins of the head and neck Veins of the head and neck.