each side of the primitive fore-gut they pick u the mesenteric veins from the intestines as well as the splenic andp pancreatic veins as soon as these viscera are formed. The- liver, 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 (6 2, H.V.). 'While the vitelline veins'are lying oneach 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). Pt .
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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
