VEIN (0.4, Gr.; Vena, Lat.; Veins., Fr.; Blutader, Ger.; Vena, Ital.; Vena, Span.). — In general anatomy the term vein, in the higher animals, includes four sets of blood-vessels, differing in so many respects from each other as to render it difficult to give a general defini tion which shall include all under one head, either as it regards structure, course, or func tion : indeed, so unlike are the different sys tems of veins, that the only remark which can be said to apply to them all is, that they convey blood in a direction towards the heart ; in this respect, as well as in their want of uniformity of character, being directly opposed to the arteries.
The four systems of veins are the systemic, the portal, the pulmonary, and certain veins peculiar to foetal lye — the ductus venosus and umbilical vein ; and it will be found, upon contrasting them, that they differ much from each other, considering that all enjoy the common appellation vein. Thus, the systemic veins, which correspond to the branches of the aorta, excepting those of the abdominal viscera, collect the blood from small and nu merous vessels into larger and fewer in its progress towards the heart, constituting a course of circulation of a truly venous cha racter ; while, on the other hand, that portion of the blood, which is conveyed to the ab dominal viscera by the branches of the ab dominal aorta, is first collected from numerous branches into one vessel — the vena porta thus far the circulation being venous, and is then again broken up into smaller and re dividing vessels, after the manner of ar teries, and forming, as regards this particular, an arterial circulation, though the vessels are nevertheless veins. In both these in stances the blood conveyed by the vessels in question has a similar quality, which is characteristic of veins, and is called venous ; it is blood saturated with carbon. In the two other systems of venous circulation— the pulmonary and umbilical — the current is venous while the fluid is arterial, these vessels being channels by which the blood is returned to the heart after it has left the arterialising organs. Again, as regards the structure and calibre of veins, our definition is necessarily loose ; for, while veins are, in a general way, thin, less elastic, and distensible, as contrasted with arteries, they are nevertheless very varied in these particulars in different regions, and though, for the most part, the calibre of veins is greater than that of the corresponding arteries, still this is not always the case, as is found upon contrasting the area of sections of the pulmonary arteries and veins. It is there fore imi ossible to give a succinct, and, at the same time, a comprehensive definition of vein ; and are one which appears to me to be most applicable and to include the members of each venous system is this — that a vein is a blood-vessel, neither artery nor capillary, convey ing blood in a direction towards the heart, having walls of greater tenuity and extensibility and, at the same time, less elasticity t/zan an artery.
The literary history of this subject is chiefly interesting as referring to veins, in their rein tion to the general circulation. Until the time of Harvey's great discovery, the general physiological relationship of the veins was not understood, and the most discrepant notions were entertained.
Hippocrates, in none of his writings, draws distinction between arteries and veins. Plato, in his Timmus, describes the veins as con nected with the heart, and receiving blood from it. He regards the veins as the messen gers, transmitting to the whole body the orders coming from the soul : he attributes to them the functions of sentient and motor nerves. He considers that the veins have two centres — the heart and the liver ; and he makes no distinction between them and arteries. Praxa goras taught that the veins were blood-vessels, in contradistinction to the arteries which he considered air-vessels. Aristotle drew no distinction between arteries and veins. philus describes both arteries and veins as blood-vessels : but he expresses himself in doubt, as to whether the veins arise from the heart or the livers Erasistralus, holding the same general doctrines as Praxagoras, further described the texture of veins very minutely, and, accordino. to Marx, he noticed the ex istence of valves. Ce/sus and Aretceus made no advance in this subject. Galen distin guished between. arteries and veins — both blood-vessels : he also observed the anasto moses of each : he stated the origin of veins to be from the liver, and of the arteries from the heart, and that both were destitute of sensa tion. Avicenna described the veins " veme quieten," in contradistinction to the arteries, which he styled " vet= pulsantes et audaccs." According to Baulinus, Avicenna described the valves of veins under the name of "cel lul w." The European anatomists of the 15th, 16th and the early part of the 17th century, re mained in a state of great confusion and ig norance, as to the bearing of the veins upon the circulating system. Vesalius endeavoured to establish the doctrine, that the vena cava takes its origin from the heart, and not the liver ; but in this he was opposed by Sylvius, Columbus, Eustachius, and Fallopius.