Veins, the name applied to four systems of blood vessels, differing in structure, course, and function, and having in common only the character of conveying blood toward and not from the heart. These systems are the common systemic, the portal, the pulmonary, and the umbilical, the first two circulating impure or venous, and the last two pure or arterial blood. As to the special anatomy of the general venous circulation, it will be sufficient to say here that all the veins from the lower limbs and the pelvic and abdominal organs carry. their contents into the inferior vena cava, and those of the head, upper limbs, and thorax into the superior vena cava; that these two great vessels pour their blood into the right auricle of the heart, whence it enters the right ventricle, to be sent by this through the pulmonary artery to the lungs for purification, returning arterial by the pulmonary veins to the left auricle, and thence by the left ventricle and aorta over the body. - The principal superficial vein of the side of the neck is the external jugular, in which venesection is occasionally performed; it is very conspicuous in some persons during violent agitation of body or mind.

The deep-seated internal jugular, by the side of the carotid artery, receives the blood from the sinuses of the brain; the median basilic at the bend of the elbow is the classical one for venesection, being very accessible and of considerable size; the longest vein in the body is the internal saphena, extending from below the ankle joint to within about an inch of the groin; the other veins as to their course generally follow the arteries; the heart has its own system of veins, not communicating with the venae cavae, but opening directly into the right auricle. These systemic veins, as they are called, correspond to the branches of the aorta, and grow larger and larger toward the heart. The portal veins collect the blood from the small vessels of the abdominal viscera into one, the vena portae, which subdivides like an artery within the liver. In the pulmonary circulation, by a contradiction in terms, the vessel called the artery carries venous blood, and the veins arterial blood. - Veins are generally thinner, less elastic, and of larger calibre than the corresponding arteries, and are provided with membranous folds or valves to prevent a backward flow of the blood.

In vertebrates generally they consist of an external fibrous and areolar coat, a middle or muscular, and an internal fibrous lined with fenestrated or striated membrane and epithelium. Venous capillaries do not essentially differ from arterial, consisting of tubes of homogeneous membrane, with a few oval nuclei; the veins of the brain have no muscular coat; at their junction with the heart they are more muscular, thicker, and red, from a prolongation into their structure of the muscle of the auricle, and they have also a partial investment of the serous layer of the pericardium; where the vena cava pierces the diaphragm it has a covering of fibrous tissue; the cerebral veins or sinuses are tubular excavations in the substance of the dura mater, lined with the usual internal membrane; the umbilical vein is smooth, without valves, lined with epithelium, and composed of a thick fibrous mass. Veins have their nutrient vessels, and a very few nerves. The venous system is far more extensive than the arterial, both in the size and number of the vessels and their branches.

The veins arise in the capillaries, increasing in size and diminishing in number toward the heart. (See Capillary Vessels.) The veins which return the blood from some of the erectile tissues were thought at one time to begin in little sacs, into which arteries much larger than capillaries open; but recent investigations have shown that the appearances were deceptive, and that, although the capillary system of erectile tissue is peculiar, having tortuous enlargements and lacuna, the ordinary arrangement of direct passage from arteries to veins through the capillaries exists. Veins intercommunicate very freely, forming networks and plexuses, the most remarkable of which in man are those about and within the spinal canal. The veins are passive organs, determining by the contraction of the muscles the course of the blood; they are also reservoirs for the circulating fluid, and active agents in absorption; as reservoirs, though very important in man, they are most remarkable in the lower animals, as in seals, whales, and many diving birds. - The valves in veins are interesting not only as specimens of animal mechanics, but as having in a great degree suggested to Harvey the discovery of the circulation of the blood; these are raised portions or pockets of fibrous membrane lined with epithelium, very delicate, and in the most perfect semilunar shape; there are generally two together in the larger vessels, opening toward the heart, and when in contact completely preventing the regurgitation of the blood; at the orifices of the smaller veins they are often single, and in the great vessels of the larger mammals frequently three; there are none in the capillaries, though they exist in veins of half a line in diameter; they vary from a mere linear elevation to a deep pocket.

Their situation is irregular, and their number not very great; in man they are found in veins subject to muscular pressure, and are therefore most numerous in the limbs; in the head and neck there are but two, in the external jugular, and these not very perfect; in the arm they are most numerous at the upper part, with none in the subclavian, innominata, and superior cava; in the legs they are most abundant at the lower part; there are none in the spinal veins, in those of the portal and hepatic systems, in the heart, kidneys, uterus, and lungs, as a general rule; they are few in cetaceans and birds, and almost absent in reptiles and fishes. - Veins are more subject to diffuse inflammation than arteries, and, from their active absorbent powers, morbid materials are carried rapidly and widely over the system from the heart. Phlebitis, or inflammation of their lining membrane, is a dangerous and common disease, sometimes leading to fatal purulent absorption, and frequently to obliteration of the vessel.

Varix or dilatation of a vein, from the comparatively small amount of circular fibres, is one of the most frequent of the morbid conditions of the body. (See Varicose Veins.) Small earthy concretions are not unfrequently deposited in the walls of veins from the blood; they are named phlebolithes or vein stones, and consist chiefly of phosphate and carbonate of lime. Entozpa are often found in the interior of the veins in the lower animals. A vein if wounded, either accidentally or in venesection, heals readily, without interference with its functions; but a wound in the axillary, subclavian, or lower part of the internal jugular, during a surgical operation, may prove suddenly fatal from the sucking in of air and a consequent instantaneous paralysis of the heart's action.