Veins

Veins, in Anatomy, if we except the pulmonary, the portal, and the umbilical veins, are the vessels which carry back venous blood from the capillaries, and, enlarging as they proceed, finally pour it through the ascending and descending venæ cavæ into the right auricle of the heart; see CIRCULATION. Their coats are similar to those of the arteries, but much thinner, and may be even transparent. They are, however, of considerable strength. The internal coat consists of an epithelial layer, supported on several laminae of longitudinal elastic fibres. The middle or contractile coat consists of numerous alternating layers of muscular and elastic fibres, the muscular fibres being disposed circularly round the vessel. The muscular fibres are wanting in some parts of the venous system, and specially developed in others. In the venæ cavæ and pulmonary veins near the heart striped muscular fibres may be detected, continuous with those in the auricles. The external coat consists of connective or areolar tissue, and of longitudinal elastic fibres; within some of the larger veins, as the inferior vena cava, through its whole length, the external iliacs, the azygos, &c., there is also a longitudinal network of unstriped muscular fibres. The existence of valves in the veins is mentioned in the article CIRCULATION. These valves are most numerous in the veins of the extremities, especially the lower ones, these vessels having to act against the force of gravity more than most others. They are absent in the venæ cavæ, the hepatic, portal, renal, pulmonary, and some other large veins, and in very small veins generally. The veins are nourished by nutrient vessels, or vasa vasorum, like the arteries; but except in a few instances (including the inferior vena cava) nerves are not distributed to them. For the chief diseases of the venous system, see the articles PHLEBITIS, THROMBOSIS, and VARICOSE VEINS.