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 vents care into the right auricle of the heart. See CIRCULATION. Their coats are similar to those of the arteries, but much thinner, and even transparent. They are, however, of 'considerable strength. The interanl coat consists of an epithelial layer, supported on several lamina3 of longitudinal elastic fibers. The middle or contractile coat consists of numerous alter nating layers of muscular and elastic fibers, the muscular fibers being disposed circularly round the vessel. The muscular fibers are wanting in some parts of the venous system, and specially developed in others (as, for example, the splenic and portal veins, where, perhaps from the physical character of the tissues which they pervade, there may be more than the ordinary resistance to the passage of the blood). In the yew cam and pulmonary veins near the heart, striped muscular fibers may be detected, con tinuous with those in the auricles. The external or areolar fibrous coat consists of con nective or areolar tissue, and of longitudinal elastic fibers; within some of the larger veins, as the inferior Dena caw, through its whole length, the external iliacs, the •izygos, etc., there is also a longitudinal net-work of unstriped muscular fibers. 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 vence cara, 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 vasorum, like the arteries; but except in a few instances (including the inferior vena care), nerves are not distributed to them.
The chief diseases of the venous system have been already sufficiently desciibed in the articles PHLEBITIS, OR INFLAMMATION OF THE VEINS; PHLEBOLITES; PHLEdMASIA ALBA DOLENS, OR MILIC-LEG; THROMBUS; and VARICOSE VEINS. We shall here merely
refer to two conditions of the venous system which must be regarded as the results of natural rather than morbid action : they are hypertrophy and atrophy. Hypertrophy is a natural and healthy change, which will be readily understood by one or two illustra tions. When the uterus enlarges during pregnancy, the quantity of blood in it.increases in at least a corresponding ratio, and so also Go the venous canals by which it is removed; while shortly after delivery, they return to their natural size; the being accompanied with a proportionate dilatation. This form of hypertrophy, with dilatation, often exerts a compensative action, one vein, or set of veins, taking additional work (and consequently requiring and increase of caliber), to make up for the partial or entire occlusion of another. When, for example, the ascending vena cava is diminished in size, or even entirely and permanently closed, it is well known that the lower portion of the vessel dilates in common with the branches entering into it, and that the superficial abdominal veins or azygos, or both, become enlarged, and thus carry to the heart the blood which ought to have reached the heart by the usual course. If the obstruction is only temporary, the enlarged veins return to their original state, except that cdditional transverse fibers are found in the middle coat. Atrophy of the veins accompanies the corresponding changes of other tissues, when a part is permanently diseased. Amputa tion above the knee soon reduces the femoral vein to less than one-third of its previous size. Mr. Callender, in his article on "diseases of the veins," in Holmes's System of Surgery, states that in the case in which a kidney became transformed into alarge cyst, the canal of the renal vein was impervious to a common probe; and this condition is daily • seen in the change which occurs in the umbilical vein shortly after birth.