On the third day the course of the vessels from the yolk-sack is very simple. The two vitelline veins, of which the right is already the smaller, form the ductus venosus, from which, as it passes through the liver on its way to the heart, are given off the two sets of venæ advehentes and venæ revehentes ([fig. 371]).
With the appearance of the allantois on the fourth day, a new feature is introduced. From the ductus venosus there is given off a vein which quickly divides into two branches. These, running along the ventral walls of the body from which they receive some amount of blood, pass to the allantois. They are the allantoic veins ([fig. 371], U) homologous with the anterior abdominal vein of the lower types. They unite in front to form a single vein, which becomes, by reason of the rapid growth of the allantois, very long. The right branch soon diminishes in size and finally disappears. Meanwhile the left on reaching the allantois bifurcates; and, its two branches becoming large and conspicuous, there still appear to be two main allantoic veins. At its first appearance the allantoic vein seems to be but a small branch of the vitelline, but as the allantois grows rapidly, and the yolk-sack dwindles, this state of things is reversed, and the less conspicuous vitelline appears as a branch of the larger allantoic vein.
Fig. 372. Diagram of the venous circulation in the Chick during the later days of incubation.
H. heart; V.S.R. right vena cava superior; V.S.L. left vena cava superior. The two venæ cavæ superiores are the original ‘ductus Cuvieri,’ they open into the sinus venosus. J. jugular vein; Su.V. anterior vertebral vein; In.V. inferior vertebral vein; W. subclavian; V.C.I. vena cava inferior; D.V. ductus venosus; P.V. portal vein; M. mesenteric vein bringing blood from the intestines into the portal vein; O.f. vitelline vein; U. allantoic vein. The three last mentioned veins unite together to form the portal vein; l. liver.
On the third day the blood returning from the walls of the intestine is insignificant in amount. As however the intestine becomes more and more developed, it acquires a distinct venous system, and its blood is returned by veins which form a trunk, the mesenteric vein ([fig. 372], M) falling into the vitelline vein at its junction with the allantoic vein.
These three great veins, in fact, form a large common trunk, which enters at once into the liver, and which we may now call the portal vein ([fig. 372], P.V). This, at its entrance into the liver, partly breaks up into the venæ advehentes, and partly continues as the ductus venosus (D.V) straight through the liver, emerging from which it joins the vena cava inferior. Before the establishment of the vena cava inferior, the venæ revehentes, carrying back the blood which circulates through the hepatic capillaries, join the ductus venosus close to its exit from the liver. By the time however that the vena cava has become a large and important vessel it is found that the venæ revehentes, or as we may now call them the hepatic veins, have shifted their embouchment, and now fall directly into that vein, the ductus venosus making a separate junction rather higher up ([fig. 372]).
This state of things continues with but slight changes till near the end of incubation, when the chick begins to breathe the air in the air-chamber of the shell, and respiration is no longer carried on by the allantois. Blood then ceases to flow along the allantoic vessels; they become obliterated. The vitelline vein, which as the yolk becomes gradually absorbed proportionately diminishes in size and importance, comes to appear as a mere branch of the portal vein. The ductus venosus becomes obliterated; and hence the whole of the blood coming through the portal vein flows into the substance of the liver, and so by the hepatic veins into the vena cava.
Although the allantoic (anterior abdominal) vein is obliterated in the adult, there is nevertheless established an anastomosis between the portal system and the veins bringing the blood from the limbs to the vena cava inferior, in that the caudal vein and posterior pelvic veins open into a vessel, known as the coccygeo-mesenteric vein, which joins the portal vein; while at the same time the posterior pelvic veins are connected with the common iliac veins by a vessel which unites with them close to their junction with the coccygeo-mesenteric vein.
Mammalia. In Mammals the same venous trunks are developed in the embryo as in other types ([fig. 373] A). The anterior cardinals or external jugulars form the primitive veins of the anterior part of the body, and the internal jugulars and anterior vertebrals are subsequently formed. The subclavians ([fig. 373] A, s), developed on the formation of the anterior limbs, also pour their blood into these primitive trunks. In the lower Mammalia (Monotremata, Marsupialia, Insectivora, some Rodentia, etc.), the two ductus Cuvieri remain as the two superior venæ cavæ, but more usually an anastomosis arises between the right and left innominate veins, and eventually the whole of the blood of the left superior cava is carried to the right side, and there is left only a single superior cava ([fig. 373] B and C). A small rudiment of the left superior cava remains however as the sinus coronarius and receives the coronary vein from the heart ([figs. 373] C, cor and [374], cs).