In Lizards the iliac veins join the posterior cardinals, and so pour part of their blood into the kidneys; they also become connected by the epigastric veins with the system of the anterior abdominal or allantoic vein. The subclavian veins join the system of the superior venæ cavæ.
The venous system of Birds and Mammals differs in two important points from that of Reptilia and Amphibia. Firstly the anterior abdominal vein is only a fœtal vessel, forming during fœtal life the allantoic vein; and secondly a direct connection is established between the vena cava inferior and the veins of the hind limbs and posterior parts of the cardinal veins, so that there is no renal portal system.
Aves. The Chick may be taken to illustrate the development of the venous system in Birds.
On the third day, nearly the whole of the venous blood from the body of the embryo is carried back to the heart by two main venous trunks, the anterior ([fig. 125], S.Ca.V) and posterior (V.Ca) cardinal veins, joining on each side to form the short transverse ductus Cuvieri (DC), both of which unite with the sinus venosus close to the heart. As the head and neck continue to enlarge, and the wings become developed, the single anterior cardinal or jugular vein ([fig. 371], J), of each side, is joined by two new veins: the vertebral vein, bringing back blood from the head and neck, and the subclavian vein from the wing (W).
On the third day the posterior cardinal veins are the only veins which return the blood from the hinder part of the body of the embryo.
Fig. 371. Diagram of the venous circulation in the Chick at the commencement of the fifth day.
H. heart; d.c. ductus Cuvieri. Into the ductus Cuvieri of each side fall J. the jugular vein, W. the vein from the wing, and c. the inferior cardinal vein; S.V. sinus venosus; Of. vitelline vein; U. allantoic vein, which at this stage gives off branches to the body-walls; V.C.I. inferior vena cava; l. liver.
About the fourth or fifth day, however, the vena cava inferior ([fig. 371], V.C.I.) makes its appearance. This, starting from the sinus venosus not far from the heart, is on the fifth day a short trunk running backward in the middle line below the aorta, and speedily losing itself in the tissues of the Wolffian bodies. When the true kidneys are formed it also receives blood from them, and thenceforward enlarging rapidly becomes the channel by which the greater part of the blood from the hinder part of the body finds its way to the heart. In proportion as the vena cava inferior increases in size, the posterior cardinal veins diminish.
The blood originally coming to them from the posterior part of the spinal cord and trunk is transported into two posterior vertebral veins, similar to those in Reptilia, which are however placed dorsally to the heads of the ribs, and join the anterior vertebral veins. With their appearance the anterior parts of the posterior cardinals disappear. The blood from the hind limbs becomes transported directly through the kidney into the vena cava inferior, without forming a renal portal system[237].