Page:The American Cyclopædia (1879) Volume XVI.djvu/303

VEINS ing arteries, and are provided with membra- nous 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 par- tial investment of the serous layer of the peri- cardium ; where the vena cava pierces the dia- phragm 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 inves- tigations have shown that the appearances were deceptive, and that, although the capil- lary system of erectile tissue is peculiar, hav- ing tortuous enlargements and lacunae, the or- dinary 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 reser- voirs for the circulating fluid, and active agents in absorption ; as reservoirs, though very im- portant 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 cir- culation of the blood ; these are raised portions or pockets of fibrous membrane lined with epithelium, very delicate, and in the most per- fect semilunar shape ; there are generally two together in the larger vessels, opening toward the heart, and when in contact completely pre- venting the regurgitation of the blood ; at the orifices of the smaller veins they are often sin- gle, and in the great vessels of the larger mam- mals 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 situa- VEIT 283 tion 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 sys- tems, 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 in- flammation than arteries, and, from their ac- tive 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 com- mon disease, sometimes leading to fatal puru- lent absorption, and frequently to obliteration of the vessel. Varix or dilatation of a vein, from the comparatively small amount of cir- cular fibres, is one of the most frequent of the morbid conditions of the body. (See VARICOSE VEINS.) Small earthy concretions are not un- frequently 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. Entozoa are often found in the interior of the veins in the lower animals. A vein if wounded, either acciden- tally 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. VEIT, Philipp, a German painter, born in Ber- lin, Feb. 13, 1793. His mother was the daugh- ter of the philosopher Mendelssohn, who after the death of her first husband married Fried- rich Schlegel. He completed his studies in Eome, where he was associated with Corne- lius, Overbeck, and other German painters in the attempted revival of mediasval art; and he executed in the villa of the Prussian con- sul, J. S. ' Bartholdy, the fresco representing the "Seven Years of Plenty," as a companion piece to Overbeck's " Seven Years of Famine." In 1826 he became director of the Stiidel art institute at Frankfort, and produced works in oil and fresco, which are among the most char- acteristic productions of modern German art. His masterpiece is a fresco in the institute representing " Christianity bringing the Fine Arts into Germany." Such was his dislike of the new realistic school, that on the purchase in 1843 of Lessing's "Huss before the Council of Constance " for the institute, he resigned his office, and settled in Sachsenhausen, oppo- site Frankfort. Among his subsequent pro- ductions are an "Assumption of the Virgin" for the Frankfort cathedral, and " The Marys