VEIN

in human physiology, any of the vessels that, with four exceptions, carry oxygen-depleted blood to the right upper chamber (atrium) of the heart. The four exceptionsthe pulmonary veinstransport oxygenated blood from the lungs to the left upper chamber of the heart. The oxygen-depleted blood transported by most veins is collected from the networks of microscopic vessels called capillaries by thread-sized veins called venules. As in the arteries, the walls of veins have three layers, or coats: an inner layer, or tunica intima; a middle layer, or tunica media; and an outer layer, or tunica adventitia. Each coat has a number of sublayers. The tunica intima differs from the inner layer of an artery: many veins, particularly in the arms and legs, have valves to prevent backflow of blood, and the elastic membrane lining the artery is absent in the vein, which consists primarily of endothelium and scant connective tissue. The tunica media, which in an artery is composed of muscle and elastic fibres, is thinner in a vein and contains less muscle and elastic tissue, and proportionately more collagen fibres (collagen, a fibrous protein, is the main supporting element in connective tissue). The outer layer (tunica adventitia) consists chiefly of connective tissue and is the thickest layer of the vein. As in arteries, there are tiny vessels called vasa vasorum that supply blood to the walls of the veins and other minute vessels that carry blood away. Veins are more numerous than arteries and have thinner walls owing to lower blood pressure. They tend to parallel the course of arteries. See also artery; capillary. in geology, ore body that is disseminated within definite boundaries in unwanted rock or minerals (gangue). The term, as used by geologists, is nearly synonymous with the term lode, as used by miners. There are two distinct types: fissure veins and ladder veins. Fissure veins, the earliest described bedrock deposits, occupy one or more fissures; they are tabular, with two dimensions much greater than the third. Fissure veins are formed in two stages, sometimes greatly separated in time; first the fissure is formed, and then it is filled with ore. There are several varieties: simple, with relatively straight, parallel walls; chambered, with irregular, fragmented walls; dilation, or lenticular, with fat lenses in a string or roughly parallel in schists; sheeted, with several distinct, closely spaced, parallel fractures; and composite, with several roughly parallel fissures and connecting diagonals in partially replaced rock. Ladder veins are short, rather regularly spaced, roughly parallel fractures that traverse dikes (tabular bodies of igneous rocks) from wall to wall. Their width is restricted to the width of the dike, but they may extend great distances along it. Ladder veins are not as numerous or important as fissure veins.