VEIN, in anatomy, one of a number of thin ramifying elastic tubes arising in the extremities of the body, and pro ceeding by a more or less direct course to the heart, to which they carry back the blood sent forth by the arteries and transferred to them by the capillaries connecting the two kinds of vessels. They fall under three great divisions: the pulmonary, the systemic veins, and those constituting the portal system. The pulmonary veins consist of four short venous trunks which carry the red blood back from the lungs to the left side of the heart, and which are found two on each side in the root of the correspond ing lung. The systemic veins arise by small branches, which receive the blood from the capillaries throughout the body, and uniting to form larger vessels and then two large venous trunks, the su perior and inferior venal cavie, finally enter the right auricle of the heart, into which the coronary veins also conduct the blood which nourishes that organ itself. These systemic veins are natur ally divided into two groups according to the channel by which they enter the heart. The veins of the head, the neck, the upper limbs, the spine, the heart, and part of the walls of the thorax and ab domen, make their entrance into the right auricle by the superior vena cava, while those of the lower part of the trunk and the abdominal viscera do so by the ineferior vena cava. The veins of the portal system bring back the blood from the stomach, the intestines, the spleen, and the pancreas; then joining, they form the great portal vein which ramifies in the surface of the liver, after the manner of an artery, before finally entering the heart by the inferior vena cava.
In geology, a crack in a rock filled up by substances different from the rock. These may be either earthy or metallic. In very many cases the fissures have been produced by volcanic or earthquake action, and they often coincide with faults. Water descending by these fis sures to unknown depths has been raised to so high a temperature that it has be come capable of holding in solution vari ous metallic and other mineral sub stances. As the water has cooled it has gradually deposited these matters held in solution, not doing so simultaneously, but in succession. Metalliferous veins vary greatly in width, being sometimes a few inches, frequently three or four feet, and sometimes much more. The thinner portions often branch off into innumerable slender ramifications like the veins of an animal, whence their name. Sometimes part of the material filling veins has fallen in from above or been segregated from the rocks con stituting the sides of the fissure. They are often parallel, are associated with dykes, and are more common in the palwozoic than in more modern strata. They vary in age, and not infrequently one crosses another. In mining, a lead or lode of ore-bearing rock, alive or dead; that is, containing ore or not; also a seam of metalliferous matter filling up a former fissure in rock.