1911 Encyclopædia Britannica/Veins (anatomy)
VEINS, in anatomy. The veins (Lat. vena) are b lood vessels which return the blood from the, capillaries toward the heart. As they approach that organ they join together to form larger and larger trunks. In man and other mammals three venous systems are recognized: (1) the general venous system; (2) the pulmonary system; and (3 the hepatic portal system. (See also Vascular System.)
The general venous system consists of superficial and deep veins; the former lie in the superficial fascia and are often visible through the skin. They are usually accompanied by lymphatic vessels though not as a rule by arteries, and, sooner or later, they empty their blood into the deep veins, often passing through special openings in~ the deep fascia to do so. Thedeep veins always accompan arteries, and are therefore known as venae comitesy With' small and medium-sized arteries-that is to say, arteries whose diameter is not much greater than that of an ordinary lead pencil-there are two of these venae comites, one on each side, connected by occasional cross communications, but arteries of a larger calibre have only'dne companion vein. In the scalp arid face the .superficial veins are remarkable for accompanying, more or less closely, corresponding arteries-more or less closely because the arteries in this re ion are very tortuous (see ARTERIES), and so are sometimes near their veins and sometimes far away, since the veins run a comparatively straight course. Frontal, superficial temgoral, posterior auricullir and occipital vqins are found in the scalp, t'e1r names indicating the areas they drain. Like all other superficial veins, they anastomose freely with one another and also at certain places communicate, through foramina in the skull, with the intracranial blood sinuses; these communications are known as emissary veins, and act as safety-valves to the sinuses. The frontal vein on the forehead passes down on the inner side of the eyelids, where it is known as the angular, and then becomes the facial vein, which runs down to an inch in front of the angle of the jaw, whence it passes into 'the neck to join the common facial. In the greater~'part'of its course it lies some distance behind the facial artery. The superficial temporal vein' runs down 'in front of the ear, where it joins the internal maxillary vein from the pterygoid plexus and so forms the te'mporomaxillary trunk, which passes down, embedded in the parotidgland, to about the angleof the 3aw. Here it divides into an anterior branch, 'which ] o1ns the facial vexn to form the common facial, and a posterior, which recexyes the posterior auricular vein and' in this wa forms the external Jugular.
The external jugular vein is easily reco nized through the skin and platysma muscle on the side 'of the neck, and eventually pierces the deep fascia above the middle of the clavicle to 'join the subclavian vein. The occipital' vein sinks deeply into the back of the neck and so forms the beginning of the vertebral vein. ' '-The intracranial blood sinuses lie between two layers of the dura mater and differ from the veins in having fibrous walls which do not contract or expand. g The superior longitudinal sinus runs7alon'g the upper, mar in of the falx cerebri (see BRAIN), while the in erior lon itudinal sinus'runs along the lower margin; these drain' the surface of the brain, and the blood passes backward in both. Where the falx meets'the' tentorium cerebelli, the inferior longitudinal sinus receives the veins of Galen from the interior of the brainfand then passes backward as the straight sinus to join the superior longitudinal sinus at the internal occipital protuberance (see SKULL). This meeting-place is known as the torcular Herophili, and fromjit the blood passes outward and downward through the right and left lateral sinuses, which groove the cranium (see SKULL) until they reach the posterior lacerated foramina, through which they 'pass to form the beginning of the internal jugular veins. Most of the blood from the base of the brain passes into the cavernous sinuses which lie in the middle cranial fossa, one on each side of the pituitary fossa. These receive the ophthalmic veins from the orbit in front and-, after running backward, for about an inch, divide into the superior and inferior petrosal sinuses, the former of which joins the ateral sinus within the cranium, but the latter runs to the posterior lacerated foramen, after passing through which it joins the lateral sinus, which is now becoming the internal' jugular vein; The internal 'ugular vein (fig. 5, I.];.) thus formed runs down at first behind and, then to the outer side: of the internal and common carotid arteries and at the root of the neck joins the subclavian vein of its own side to form the in nominate vein. In its course downthe neck it receives the common facial vein already -mentioned, as well as tributaries from the ton e, harynx, larynx and thyroid body; The deep veins of the heaciu audi face tend to form plexuses rather than venae comites; of these, pleriygoid, deep temporal, pharyngeal and .rub occipital plexuses are recogn zed.
Veins of the Upper Extremity.—On the dorsumof the hand and in front of the wrist superficial venous plexuses are easily seen through the skin. From these the blood passes up the forearm chiefly on its fiexor surface b the radial, median and anterior and posterior ulna veins. just belbw the bend of the elbow 'the median vein communicates with the deep veins and then divides into two branches like the limbs of a v . Of these the inner is the median basilic and is noticeable as the vein from which patients were usually bled, while the outer is the median cephalic. After a course of an inch or two the median basilic is joine by the anterior and posterior ulnar veins and the median cephalic by the radial. After thlsjunction the median basilic is continued up the inner side of the arm asthe basilu: which pierces the deep fascia about the middle of the arm and in the axilla ]oins the venae comites of the brachial artery to form the axillary vein, which lies on the inner side of its artery. The median cephalic vein after joining the radial runs up the outer side of the arm as the cephalic and a little below the cavicle passes through the custocoracoid membrane toenter the upper part of the axillary vein. At the outer border of the first rib the axillargevein becomes the subclavian (fig. 5, S.), which lies in front of and low its artery and is separated from it by the scalenuslanticus muscle. Thearrangement of the superficial veins, especially in front of the elbow, is liable to great variation and often differs on the right and left sides of the same body.
Veins of the Lower Extremity.-The superficial veins of the lower extremity begin in a venous arch on the dorsum of the foot. From the inner extremity of this the internal saphenous vein runs up, in front of the inner ankle along the inner side of the leg, and, passing behind the inner side of the knee, continues up the thigh, gradually working forward until it reaches the saphenous opening in the deep fascia of the thigh a little below the spine of t pubis. Here it pierces the deep fascia (fascia lata) to enter the common femoral vein. In this long course it has many valves and receives numerous tributaries, one of which, the saphenous collateral, runs up nearly parallel to it and on its outer side and joins it just below the saphenous opening. From the inner end of the dorsal arch of the foot the external saphenous vein runs up behind the outer, ankle along the mid line of the calf to pierce the deep fascia in the popliteal space behind the knee to open into the popliteal vein- Among the deep veins venae comites are found until the popliteal artery is reached, while above this superficial, deep and common femoral vein; accompany their respective arteries. n the groin the ' common femoral vein lies on the inner side of its artery. Veins of the Abdomen.-The common femoral vein, after passing deep to Poupart's ligament, becomes the external iliac (fig. 5, E.I. which runs along the brim of the true 'pelvis and, after a course of some three inches, joins the internal iliac (fig. 5, I.I.) which drains the pelvis and so forms the common iliac vein. In front of the body of the fifth lumbar vertebra the common iliac veins of the two. sides unite .to form the inferior vena cava (fig. 5, I.V.C.), a very large trunk which runs up on the right of the abdominal aorta to an opening in the diaphragm (q.v.). On its way it receives spermatic or ovarian veins from the genital glands, renal vein; (fig.d5, R.V.) from the kidneys, and lumbar veins (fig. 5, L.V.) from the abdominal walls. Before reaching the diaphragm it lies in a groove in the back of the liver (q.v.) and receives, the hepatic veins from that organ. The hepatic portal system which lies in the abdomen will be treated later. Veins of the Thorax.-The inferior vena cava, after piercing the diaphragm, has a very short thoracic course, and opens into the lower and back part of the right auricle of, the heart (q.v.). The right and left in nominate veins lg, R.§ I., and L.I.) are formed behind the sternal end of the clavicle by the union of the subclavian and internal jugulars of their own side. The left vein is much longer than the right and runs nearly horizontally behind the upper bali of the manubrium sterni to join its, fell'ow on therilght side, of that bone just below the, first rib. By the gunction o these the
- uierior venacava. (fig. 5, S.V.C.) is formed, , w ich runs down to the
rig t auricle of the' cart; The chief tributaries of, the in nominate veins are the vertebral, the internal mammary and the ingerior thyroid. The intercostal veins open into the aaygos veins, whic begin in the abdomen sometimes by a 'vertical trunk joining, the, lumbpr veins known as the ascending lumbar, sometimes on the right side by a communication with the inferior vena cava. ' The right azygos vein is known as the vena azygas major (fig. E A.M'.) and passes through theYaortic opening, of the diaphragm. =ntering the thorax, it runs nip in front of the thoracic vertebrae, to the right of the aorta and toracic duct, and receives the intercostal veins pf the right side, At the level of the fourth thoracic vertebra it arches forward to open into the posterior'surface of the superior vena cava. ' On the left side, the up r intercosta veinsjointo form 'the left superior intercostnl vein 5, L, S.I.), .which opens into-the-left in nominate. Lower downpthe intercostal veins from the fourth to the seventh spaces form the superior hemiazygos vein or hemiazygps accessoria (fig. '5, H.A.), which 'runs down on the left of the spinal column and, crossing it about the level of the ei hth or ninth thoracic vertebra, opens into the vena az gos major. gfhe lower intercostal veins on the left side join the in erior hemiazygos vein (fig. 5, H.V.), which runs up and olpens either into the superior hemiazygos or into the' azygos maipr be ow the opening of that vein. " Pulmonary erwns System.-The veins emerging from the lungs bring back the oxygenated blood from those organs to the lleft ventricle of the heart and also the greater art, if not all, of, theblood carried b the bronchial arteries to nourish the lungs. The existence of bronchial veins is asserted, but they are extremely difficult to demonstrate, and if present are quite incapable of returnin all the blood which the bronchial arteries car? to thelungs. There are three, pulmonary veins{ coming out of the right lung, while on the left there, are only two. On the right side(however, two of the three veins usually unite in the root of the lung, so that there are as a rule -two pulmonary veins entering the left auricle of the heart on each side, but it is not uncommon to find three' on the ri ht side or one on the left. The pulmonary veins have no valves ami return the blood carried to the lungs by the ulmonary arteries as well as most, ifnot all, of that carried by the 'bronchial arteriesQ Hepatic Portal System.-The veins which drain the blood from the stomach, intestines, Zpleen and pancreas unite to form-aflarge vein, which beginsbehin, the head of the pancreas:and ends(by dividin into right andleft branches in the transverse fissure of the liver. gfhis is the 'pirtal vein which'lies in front of the inferior vena cava and is a ut-three inches long. Its' formative tributaries are the superior and inferiormesentericand the splenic veins: These accompany the arteries of the same inamepand their most usual method of termination is that the inferior mesenteric runs up and joins 'the splenic to the left of the middle line of the body, and this, after running' horizontally' to a point a little to the right of-the middle -line, § oinsthé'superior mesenteric, and so the portal vein is formed. ° There are two marked characteristics of the portal system; one is that it has no valves and the other that it begins and ends in capillaries, since the two terminal 'branches of' the 'portal', vein branch and rebranch in 'a manner already described 'in'the article LINER; In the lower part of the- rectum the veins run partly into the portal and partly into the general system, and in this de ndent; position they are liable to become varioosex and to form iiiemorrhoids or piles;
The histology of the' veins corresponds very closely to'that of the arteries (q.v.).-; their walls are, however, much thinner and there is less muscular and elastic tissue. At certain places, especially where tributaries come in, the endothelial linin is raised to form semi lunar pocket-like valves. In most cases tgere are two cusps to each valve, but three or one are sometimes found. Theopening of the pocket is of course arranged so that it shall only be hlled when there is a tendency to regurgitation of the blood., g Embryology,
° A The vitelline or omphalo-mesenteric veins, returning the blood from the yolk- sac, are the first to appear, and later on, with the formation oéjtheplacenta, the umbilica veins develop. Both these open into 1; e hinder, (caudal) part of the heart, which is already being con-1
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The left umbilical vein on reaching the liver now branch of the portal vein and establishes a new communication with the left hepatic vein. This is the ductus venosus and, as soon as it is formed, there is no longer any the blood' returnin 'from the"placenta shbuld';pass liver capillaries. The 'development of the cardinal velnsimusrnow be returned to. 'As the heart moves from the neck into the thorax the primitive jugulars elongate and it is now rxecog the internal jugu ars in the greater part of their extent. When the arms bégin to bud out subclavian veins are develop and an oblique connecting vein (figs. 4 and' 5, L.I.) fi . . D.V.,
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between the point.of junction of the left subclavian with the primitive jugular* and the hinder part of the primitive jugular of the right side. This Connexion becomes the left in nominate vein, while the hinder part of the primitive 'ugular persists as the left superior intercostal vein (fig. 5, L.S.I.). 611 the right side that part of the primitive jugular between the subclavian and the- junction with the left innommate becomes the right- in nominate (figs. 4 and* 5, R.I.) while the hinder (caudal) part of the right primitive ju ular and the right duct of Cuvier become the superior vena' cava (és: 4 and 5. S.V.C.). The' external jugular is a later formation. The right and left posterior cardinal veins receive the inter coital and lumbar segmental veins andl are! continued into the lower limbs as the internal iliac and eventually the sciatic veins (figs. 4 and 5, I.I.), the primitive blood path from thew thighs. The veins from 'the primitive kidneys .open»> into the segmental veins, and. when the permanent kidney is formed- (see URINARY SYSTEM) a large renal vein on each side is established; There are, however, many cross communications (fig. 4, T'.C.) between the right and left posterior cardinal veins, some ofvwhich become very~ important later on, though most of them are transitory. The probable origin-of the ingenor vena cava is to be sought in a pair of veins called sub cardinals w ich have been found int eirabbit embryo lying arallel and a little. ventral to the posterior cardinals (fig. 4, R.S.CF:-L.S.C.) and effecting a. junction with the renal sand .transverse communications (T.C.) asthey cross these. Posteriorly (caudal) they join the cardinals, but anteriorly the right one establishes a communication with the ductus venosus (fig. 4, D.V.) a little below the point at which that vessel joins the left hepatic. It -is from the ri ht one of these that the greater part of the inferior vena cava is formed. It will now be seen that the adult vena cava is formed by contributions -from four embryonic veins, most anteriorly .the hepatic, then the duttus venosus, thenthe right sub cardinal and posteriorly the right posterior cardinal (F. T. Lewis, Am. J. of Anatyvol. I, 229, IQ02).' The anterior (cephalic) part of':the right posterior cardinal forms the vena azygos major, and an inspection of fig. 4 will show that in the adult this may rise from the renal, from an ascending lumbar vein or, byia cross communication above the renal, from the inferior vena cava. The left .posterior cardinal becomes obliterated below and its segmental tributaries find their way by cross communications to thevena cava (fig. 5). 'Above'(cepha1ad) the left renal vein the left cardinal forms the hemiazygos (fig., H.V.)'and, higher still, the hemiaiygos accessoria (fig, 5, These open into the azygtgs major by persistent cross communications which liewdorsal to te heart when: that organ reaches its permanent position. = It must be mentioned infthisfconnexion that some modern authorities doubt Whetherthe azygos veins of mammals are really persistent cardinals ence t quite in their anterior parts, justbeforevthey join the ducts of guvier., The, left duct of Cuvier is only represented in the human adult by the oblique vein of Marshall on the dorsunr of the left auricle. The external iliac veins (figs. 4 and 5, EJ.) become fully developed, .like their arteries, when the blood changes its course from the, ack .to the front of the thigh. After birth the umbilical vein fand thelductus venosus become converted ' into fibrous cords and the circulation in the pulmonary veins is established. (For further details see Development of the Human Body, by J. P. McMurrich, London, 1906. .In this will be found the literature; of the- subject up tothat date, the writings of F. Hochstetter being the most important., Seealso Quain's Anal. vol. i., 1908.) 'A »fCa1qpara!ive Anatomy., . .
~, In~ the .Acrania (Amphgiaxus), although (there, is no heart, the blood vessels returning the blood to the sub pharyngeal region are distinctly of a vertebrate type. There is a subirxtestinal vessel or vein bringing the blood 'from the intestine to tl-ierliver and breaking up into capillaries in that organ just as the portal vein does in.the higher forms. From the, liver a hepatic vein fcarries the blood forward to the re ion below the pharynx where the heart is formed in Vertebrata. There is no renal portal system. In the'C 'clostomata (lampreys and hags) the cardinal veirrsare formed and the blood from the caudal vein passes .directl into the posterior cardinals without, any renal portal system. In fishes the single-caudal vein divides into two branches, each of which runs forward to the outer, side of its respective ikidne, and ends by givingnumeruus b anches to that viscus. The blood(returning from the -kidney pasdes into the beginning of its .own posterior cardinal vein or sinus, i which lies on the inner side of the kidney; This constitutes a renal portal system.. The cardinal veins and ducts of Cuvier' closely. resemble the arrangement already detailed, in the human foetus, 'while the hepatic portal system from 'the intestine to' the liver is Constantin this and all other vertebrates(» - = ", " In the Dipnoi (mud-iish), a pulmonary vein from, the lun -like swim-bladder is formed and an inferior vena cava or postcavalgvein carries the blood from the kidneys to the heart. This is its first appearance in the vertebrate phylum. In the lowerffishes there isai vein of the lateral line on each side, but in the Dipnoi these coalesce and form a median anterior (ventral) abdominal, vein which is; constant in the Amphibia. Subclavian and iliac veins return the blood from the fins and 'open respectively into the junction of the anterior and posterior cardinals and into the caudal vein. ' » In the tailed Amfnhibia (Urodela) the postcaval and posterior cardinal veins are we l developed, the'former'vessel running from the right cardinal veiin a little in front of "(c?halad) the kidney to the hepatic vein, in this wa 'closely foresha owing"man's embryology., In the Anura (frogs and/ toads) theiposterior cardinals are usually suppressed, 'but' these are very specialized'animals.f The anterior abdominal vein' in amphibians joins the portal vein 'close to the lVEl'.,
In the Re tilia the renal portal circulation persists, butisrudimentary in hires andf 'disappears in mammals. The anterior abdomina or epigastric vein ofiamphibians -and reptiles returns the blood from the allantois in the embryo and in higher forms becomes the umbilical veins returning 'the blood from the placenta; there is, therefore, a continuous line of ascent from the lateral line veins of the fish to the umbilical vein of man. In reptiles, birds, monotremes, marsupials and many rodents, insectivores, bats and .ungulates, a left superior vena cava (precaval vein) is present as well as 5. right; it passes ventral. to the root' of the left lung and then dorsal to the left auricle of the heart until it reaches the coronary sinus to open into the right auricle. Its course is indicated in man by the left superior intercostal vein, the vestigial fold of Marshall (see Coelom and Serous Membranes) and the oblique vein of Marshall. It can be readily reconstructed from figs. 4 and 5 if the transverse communication (L.I.) is obliterated. In some mammals the postcaval vein is double, especially, in its hinder (caudal) part, and this sometimes occurs as a human abnormality (see F. W.. McClure, Am. Journ. of Anat. vol. 2, 1903, and. vol. 5, 1906, also Anat. Anzeiger Bd. 29, 1906). ' ' i-Except
in Cetacea, one or both azygos veins are always present in mammals. When there is only:one it is usually the right, though a. few forms among the marsupials, rodents and ungulates have only the left (F. E. Beddard, P.Z.S., 1907, p. 181). In many of the lower mammals the external jugular vein is much larger than the internal and returns most of the blood from the brain through an opening called the postglenoid foramen. For. this reason it was formerly regarded as the representative of the primitive jugular. It.is now, however, thought that the internal jugular is that representative, and that the arrangement of man, in which the internal jugular drains the interior of the cranium, is the more generalized and primitive.
For further details and literature see R. Wiedersheim's Comparative Anatomy of Vertebrates, translated by W. N. Parker (London, 1907). (F. G. P.)