Fig. 369. Diagram of the paired venous system of a Fish. (From Gegenbaur.)
j. jugular vein (anterior cardinal vein); c. posterior cardinal vein; h. hepatic veins; sv. sinus venosus; dc. ductus Cuvieri.
The cardinal veins appear in Fishes as four paired longitudinal trunks ([figs. 363] and [369]), two anterior (j) and two posterior (c). They unite into two transverse trunks on either side, known as the ductus Cuvieri (dc), which fall into the sinus venosus, passing from the body wall to the sinus by a lateral mesentery of the heart already spoken of (p. 627, [fig. 352]). The anterior pair, known as the anterior cardinal or jugular veins, bring to the heart the blood from the head and neck. They are placed one on each side above the level of the branchial arches ([fig. 299], a.cv). The posterior cardinal veins lie immediately dorsal to the mesonephros (Wolffian body), and are mainly supplied by the blood from this organ and from the walls of the body ([fig. 275], c.a.v). In many forms (Cyclostomata, Elasmobranchii and many Teleostei) they unite posteriorly with the caudal veins in the manner already described, and in a large number of instances the connecting branch between the two systems, in its passage through the mesonephros, breaks up into a capillary network, and so gives rise to a renal portal system.
The vein from the anterior pair of fins (subclavian) usually unites with the anterior jugular vein.
The venous system of the Amphibia and Amniota always differs from that of Fishes in the presence of a new vessel, the vena cava inferior, which replaces the posterior cardinal veins; the latter only being present, in their piscine form, during embryonic life. It further differs from that of all Fishes, except the Dipnoi, in the presence of pulmonary veins bringing back the blood directly from the lungs.
In the embryos of all the higher forms the general characters of the venous system are at first the same as in Fishes, but with the development of the vena cava inferior the front sections of the posterior cardinal veins atrophy, and the ductus Cuvieri, remaining solely connected with the anterior cardinals and their derivatives, constitute the superior venæ cavæ. The inferior cava receives the hepatic veins.
Apart from the non-development of the subintestinal vein the visceral section of the venous system is very similar to that in Fishes.
The further changes in the venous system must be dealt with separately for each group.
Amphibia. In Amphibia (Götte, No. [296]) the anterior and posterior cardinal veins arise as in Pisces. From the former the internal jugular vein arises as a branch; the external jugular constituting the main stem. The subclavian with its large cutaneous branch also springs from the system of the anterior cardinal. The common trunk formed by the junction of these three veins falls into the ductus Cuvieri.
The posterior cardinal veins occupy the same position as in Pisces, and unite behind with the caudal veins, which Götte has shewn to be originally situated below the postanal gut. The iliac veins unite with the posterior cardinal veins, where the latter fall into the caudal vein. The original piscine condition of the veins is not long retained. It is first of all disturbed by the development of the anterior part of the important unpaired venous trunk which forms in the adult the vena cava inferior. This is developed independently, but unites behind with the right posterior cardinal. From this point backwards the two cardinal veins coalesce for some distance, to give rise to the posterior section of the vena cava inferior, situated between the kidneys[233]. The anterior sections of the cardinal veins subsequently atrophy. The posterior part of the cardinal veins, from their junction with the vena cava inferior to the caudal veins, forms a rhomboidal figure. The iliac vein joins the outer angle of this figure, and is thus in direct communication with the inferior vena cava, but it is also connected with a longitudinal vessel on the outer border of the kidneys, which receives transverse vertebral veins and transmits their blood to the kidneys, thus forming a renal portal system. The anterior limbs of the rhomboid formed by the cardinal veins soon atrophy, so that the blood from the hind limbs can only pass to the inferior vena cava through the renal portal system. The posterior parts of the two cardinal veins (uniting in the Urodela directly with the unpaired caudal vein) still persist. The iliac veins also become directly connected with a new vein, the anterior abdominal vein, which has meanwhile become developed. Thus the iliac veins become united with the system of the vena cava inferior through the vena renalis advehens on the outer border of the kidney, and with the anterior abdominal veins by the epigastric veins.