VEINS, ANATOMY, in anatomy. The veins (Lat. vena) are blood 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: (i) 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. The deep veins always accompany arteries, and are therefore known as venae comites. 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 one companion vein. In the scalp and face the superficial veins are remarkable for accompanying, more or less closely, corresponding arteries more or less closely because the arteries in this region 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 temporal, posterior auricular and occipital veins are found in the scalp, their 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 temporomo-xttlary trunk, which passes down, embedded_in the parotid gland, to about the angle of the jaw. Here it divides into an anterior branch, which joins the facial vein to form the common facial, and a posterior, which receives the posterior auricular vein and in this way forms the external jugular.
The external jugular vein is easily recognized 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. The superior longitudinal sinus runs' along the upper margin of the falx cerebri (see BRAIN), while the inferior longitudinal 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 brain and 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 from it 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 lateral 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 jugular vein (fig. 5, I.J.) 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 innominate vein. In its course down the neck it receives the common facial vein already mentioned, as well as tributaries from the tongue, pharynx, larynx and thyroid body. The deep veins of the head ana face tend to form plexuses rather than venae comites; of these, pterygoid, deep temporal, pharyngeal and suboccipital plexuses are recognized.
Veins of the Upper Extremity. On the dorsum of 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 flexor surface by the radial, median and anterior and posterior ulna veins. Just below the bend of the elbow the median vein communicates with the deep veins and then divides into two branches like the limbs of a Y 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 joined by the anterior and posterior ulnar veins and the median cephalic by the radial. After this junction the median basilic is continued up the inner side of the arm as the basilic which pierces the deep fascia about the middle of the arm and in the axilla Joins 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 clavicle passes through the costocoracoid membrane to enter the upper part of the axillary vein. At the outer border of the first rib the axillary vein becomes the subclavian (fig. 5, S.), which lies in front of and below its artery and is separated from it by the scalenus anticus muscle. The arrangement 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 the 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 veins accompany their respective arteries. In 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 veins (fig. 5, 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 innominate veins (fig. 5, 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 half of the manubrium sterni to join its fellow on the right side of that bone just below the first nb. By the junction of these the superior vena cava (fig. 5, S.V.C.) is formed, which runs down to the right auricle of the heart. The chief tributaries of the innominate veins are the vertebral, the internal mammary and the inferior thyroid.
The intercostal veins open into the azygos veins, which begin in the abdomen sometimes by a vertical trunk joining the lumbar 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 azygos major (fig. 5, A.M.) and passes through the aortic opening of the diaphragm. Entering the thorax, it runs up in front of the thoracic vertebrae, to the right of the aorta and thoracic duct, and receives the intercostal veins of 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 upper intercostal veins join to form the left superior intercostal vein (fig. 5, L.S.I.), which opens into the left innominate. Lower down the intercostal veins Irom the fourth to the seventh spaces form the superior hemiazygos vein or hemiazygos accessoria (fig. 5, H.A.), which runs down on the left of the spinal column and, crossing it about the level of the eighth or ninth thoracic vertebra, opens into the vena azygos major. The lower intercostal veins on the left side join the inferior hemiazygos vein (fig. 5, H.V.), which runs up and opens either into the superior hemiazygos or into the azygos major below the opening of that vein.
Pulmonary Venous System. The veins emerging from the lungs bring back the oxygenated blood from those organs to the left ventricle of the heart and also the greater part, if not all, of the blood carried by 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 cmite incapable of returning all the blood which the bronchial arteries carry to the lungs. 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 right side or one on the left. The pulmonary veins have no valves and return the blood carried to the lungs by the pulmonary arteries as well as most, if not all, of that carried by the bronchial arteries.
Hepatic Portal System. The veins which drain the blood from the stomach, intestines, spleen and pancreas unite to form a large vein which begins behind the head of the pancreas and ends by dividing into right and left branches in the transverse fissure of the liver. This is the portal vein which lies in front of the inferior vena cava and is about three inches long. Its formative tributaries are the superior and inferior mesenteric and the splenic veins. These accompany the arteries of the same name, and 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, joins the 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 LIVER. In the lower part of the rectum the veins run partly into the portal and partly into the general system, and in this dependent position they are liable to become varicose and to form haemorrhoids 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 lining is raised to form semilunar pocket-like valves. In most cases there are two cusps to each valve, buf three or one are sometimes found. The opening of the pocket is of course arranged so that it shall only be filled when there is a tendency to regurgitation of the blood.
The vitelline or omphalo-mesenleric veins, returning the blood from the yolk sac, are the first to appear, and later on, with the formation of the placenta, the umbilical veins develop. Both these open into the hinder (caudal) part of the heart, which is already being constricted off as the sinus venosus (see fig. i).
While this is going on the veins from the different body segments are received into two longitudinal trunks on each side, the anterior (cephalic) of which is the primitive jugular or anterior cardinal (fig. i, S.V. P.J.), and the posterior (caudal), the posterior cardinal or simply cardinal vein (fig. i, P.C.). As the heart is at first situated in the region which will later be the neck of the embryo, the primitive jugular receives very few segmental veins and the cardinal very many. These, two trunks join one another on each side and open into the side of the sinus venosus (S.V.) by a transverse communication which is called the duct of Cuvier (D.C.). The condition of the venous system at this stage is shown in the accompanying diagram (fig- i)- As the vitelline veins run from the yolk sac to the heart along 1zz s.v.
each side of the primitive fore-gut they pick up the mesenteric veins from the intestines as well as the splenic and pancreatic veins as soon as these viscera are formed. The Jiver, however, is developed right across their path, and both they and the umbilical veins break up into a mass of capillaries in it,' leaving that part of them which lies between the liver and the heart to form the primitive hepatic veins (fig. 2, H.V.)! While the vitelline veins are lying on each side of the fore-gut (future duodenum) they are connected by three transverse channels, the anterior and posterior of which appear on the ventral side of the gut, the middle on the dorsal side (see fig. 2).
This figure of eight does not persist, however, because the anterior (cephalic) part of it on the left and the posterior (caudal) part on the right become obliterated, and what is left forms the portal vein (fig. 3, P.V.). The two umbilical veins unite at the umbilicus (fig. 3) and soon all the blood from the placenta passes through the left one, the right becoming rudimentary.
The left umbilical vein on reaching the liver now joins the left branch of the portal vein and establishes a new communication with the left hepatic vein. This is the ductus venosus (fig. 3, D.V.), and, as soon as it is formed, there is no longer any need that all the blood returning from the placenta should pass through the liver capillaries. The. development of the cardinal veins must now be returned to. As the heart moves from the neck into the thorax the primitive jugulars elongate and it is now recognized become the internal jugulars in the greater part of their extent. When the arms begin to bud out subclavian veins are developed (fig. 4, S.) and an oblique connecting vein (figs. 4 and 5, L.I.) is established E.I.- 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 innominate vein, while the hinder part of the primitive jugular persists as the left superior intercostal vein (fig. 5, L.S.I.). On the right side that part of the primitive jugular between the subclavian and the junction with the left innominate becomes the right innominate (figs. 4 and 5, R.I.) while the hinder (caudal) part of the right primitive jugular and the right duct of Cuvier become the superior vena caya (figs. 4 and 5. S.V.C.). The external jugular is a later formation. The right -and left posterior cardinal veins receive the intercostal and lumbar segmental veins and are continued into the lower limbs as the internal iliac and eventually the sciatic veins (figs. 4 and 5, 1. 1.), the primitive bloodpath from the 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 of which become very important later on, though most of them are transitory. The probable origin of the inferior vena cam is to be sought in a pair of veins called subcardinals which have been found in the rabbit embryo lying parallel and a little ventral to the posterior cardinals (fig. 4, R.S.C.-L.S.C.) and effecting a junction with the renals and transverse communications (T.C.) as they 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 right 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 ductus venosus, then the right subcardinal and posteriorly the right posterior cardinal (F. T. Lewis, Am. J. of Anat. vol. I, 229,, 1902). 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, by a 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 the vena cava (fig. 5). Above (cephalad) the left renal vein the left cardinal forms the hemiazygos (fig. S, H.V.) and, higher still, the hemiazygos accessoria (fig. 5, H.A.). These open into the azygos major by persistent cross communications which He dorsal to the heart when that organ reaches its permanent position. It must be mentioned in this connexion that some modern authorities doubt whether the azygos veins of mammals are really persistent cardinals except. quite in their anterior parts, just before they join the ducts of Cuvier. The left duct of Cuvier is only represented in the human adult by the oblique vein of Marshall on the dorsum of the left auricle. The external iliac veins (figs. 4 and 5, E.I.) become fully developed, like their arteries, when the blood changes its course from the back to the front of tne thigh. After birth the umbilical vein and the ductus 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 to that date, the writings of F. Hochstetter being the most important. See also Quain's Anat. vol. i., 1908.)
In the Acrania (Amphioxus), although there is no heart, the blood vessels returning the blood to the subpharyngeal region are distinctly of a vertebrate type. There is a subintestinal vessel or vein bringing the blood from the intestine to the liver and breaking up into capillaries in that organ just as the portal vein does in the higher forms. From the liver a hepatic vein carries the blood forward to the region below the pharynx where the heart is formed in Vertebrata. There is no renal portal system. In the Cyclostomata (lampreys and hags) the cardinal veins are formed and the blood from the caudal vein passes directly 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 kidney and ends by giving numerous branches to that viscus. The blood returning from the kidney passes into the beginning of its own posterior cardinal vein or sinus, 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 constant in this and all other vertebrates.
In the Dipnoi (mud-fish) a pulmonary vein from the lung-like swim-bladder is formed and an inferior vena cava or postcaval vein carries the blood from the kidneys to the heart. This is its first appearance in the vertebrate phylum. In the lower fishes there is a 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 Amphibia (Urodela) the postcaval and posterior cardinal veins are well developed, the former vessel running from the right cardinal vein a little in front of (cephalad) the kidney to the hepatic vein, in this way closely foreshadowing man's embryology. In the Anura (frogs and toads) the posterior cardinals are usually suppressed, but these are very specialized animals. The anterior abdominal vein in amphibians jeins the portal vein close to the liver.
In the Reptilia the renal portal circulation persists, but is rudimentary in birds and disappears in mammals. The anterior abdominal or epigastric vein of amphibians and reptiles returns the Dlood 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 a 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 Anal. vol. 2, 1903, and vol. 5, 1906, also Anal. Anzeiger, Bd. 29, 1906).
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.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)