VEIN (OF., Fr. veine, from Lat. vow, blood vessel, from rebore, to carry, convey). In anatomy, a blood vessel which conducts the venous (or blue) blood from the capillaries back to the heart. The exceptions to this description are the pulmonary veins, which return the red, oxygenated blood from the lungs to the heart; the portal veins, which receive the blood from the pyloric, gastric, cystic, superior mesenteric, and splenic veins, and, entering the liver, break up into small branches which pass through all parts of that organ; and the umbilical veins, which convey the blood from the haus to the mother's placenta. The veins enlarge as they proceed, gathering blood from their tributaries, and filially pour it through the ascending and de scending lima carte into the right auricle of the heart. (See CIRCULATION.) Their coats are similar to those of the arteries, but much thinner, and even transparent. The internal coat consists of an epithelial layer, supported on several la minx of longitudinal elastic fibres. The middle or contractile coat consists of numerous alter nating 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 moue carw and pul
monary veins near the heart, striped muscular fibres may he detected, continuous with those in the auricles. The external or areolar fibrous coat consists of connective or areolar tissue, and of longitudinal elastic fibres; within some of the larger veins, as the inferior rrna cara, through its whole length, the external iliacs, the azygos, etc., there is also a longitudinal network of un striped muscular fibres. Valves are most numer ous 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 vend; taco;, the hepatic, portal, renal, pulmonary, and some other large veins, and in very small veins generally. The veins are nourished by nutrient vessels, or rasa vaso rum, like the arteries; but except in a few in stances (including the inferior vcna coca), nerves are not distributed to them.
The chief diseases of the venous system have been already sufficiently described in the articles PHLEBITIS, OR I NFLAMMATION OF TIIE VEINS ; P LEBOLITE ; PI I LEGMASIA ALBA DOLENS, OR Ai ILK- LEG ; THROMBOSIS; and VARICOSE VEINS.