Fig. 370. Anterior portion of the venous system of an embryonic Snake. (From Gegenbaur; after Rathke.)
vc. posterior cardinal vein; vj. jugular vein; DC. ductus Cuvieri; vu. allantoic vein; v. ventricle; ba. truncus arteriosus; a. visceral clefts; l. auditory vesicle.
While the front part of the posterior cardinal veins is undergoing atrophy, the intercostal veins, which originally poured their blood into the posterior cardinal veins, become also connected with two longitudinal veins—the posterior vertebral veins—which are homologous with the azygos and hemiazygos veins of Man; and bear the same relation to the anterior vertebral veins that the anterior and posterior cardinals do to each other.
These veins are at first connected by transverse anastomoses with the posterior cardinals, but, on the disappearance of the front part of the latter, the whole of the blood from the intercostal veins falls into the posterior vertebral veins. They are united in front with the anterior vertebral veins, and the common trunk of the two veins on each side falls into the jugular vein.
The posterior vertebral veins are at first symmetrical, but after becoming connected by transverse anastomoses, the right becomes the more important of the two.
The vena cava inferior, though considerably later in its development than the cardinals, arises fairly early. It constitutes in front an unpaired trunk, at first very small, opening into the right allantoic vein, close to the heart. Posteriorly it is continuous with two veins placed on the inner border of the kidneys[235].
The vena cava inferior passes through the dorsal part of the liver, and in doing so receives the hepatic veins.
The portal system is at first constituted by the vitelline vein, which is directly continuous with the venous end of the heart, and at first receives the two ductus Cuvieri, but at a later period unites with the left ductus. It soon receives a mesenteric vein bringing the blood from the viscera, which is small at first but rapidly increases in importance.
The common trunk of the vitelline and mesenteric veins, which may be called the portal vein, becomes early enveloped by the liver, and gives off branches to this organ, the blood from which passes by the hepatic veins to the vena cava inferior. As the branches in the liver become more important, less and less blood is directly transported to the heart, and finally the part of the original vitelline vein in front of the liver is absorbed, and the whole of the blood from the portal system passes from the liver into the vena cava inferior.
The last section of the venous system to be dealt with is that of the anterior abdominal vein. There are originally, as in the Anura, two veins belonging to this system, which owing to the precocious development of the bladder to form the allantois, constitute the allantoic veins ([fig. 370], vu).
These veins, running along the anterior abdominal wall, are formed somewhat later than the vitelline vein, and fall into the two ductus Cuvieri. They unite with two epigastric veins (homologous with those in the Anura), which connect them with the system of the posterior cardinal veins. The left of the two eventually atrophies, so that there is formed an unpaired allantoic vein. This vein at first receives the vena cava inferior close to the heart, but eventually the junction of the two takes place in the region of the liver, and finally the anterior abdominal vein (as it comes to be after the atrophy of the allantois) joins the portal system and breaks up into capillaries in the liver[236].