Fig. 373. Diagram of the development of the paired venous system of Mammals (Man). (From Gegenbaur.)
j. jugular vein; cs. vena cava superior; s. subclavian veins; c. posterior cardinal vein; v. vertebral vein; az. azygos vein; cor. coronary vein.
A. Stage in which the cardinal veins have already disappeared. Their position is indicated by dotted lines.
B. Later stage when the blood from the left jugular vein is carried into the right to form the single vena cava superior; a remnant of the left superior cava being however still left.
C. Stage after the left vertebral vein has disappeared; the right vertebral remaining as the azygos vein. The coronary vein remains as the last remnant of the left superior vena cava.
The posterior cardinal veins form at first the only veins receiving the blood from the posterior part of the trunk and kidneys; and on the development of the hind limbs receive the blood from them also.
As in the types already described an unpaired vena cava inferior becomes eventually developed, and gradually carries off a larger and larger portion of the blood originally returned by the posterior cardinals. It unites with the common stem of the allantoic and vitelline veins in front of the liver.
Fig. 374. Diagram of the chief venous trunks of Man. (From Gegenbaur.)
cs. vena cava superior; s. subclavian vein; ji. internal jugular; je. external jugular; az. azygos vein; ha. hemiazygos vein; c. dotted line shewing previous position of cardinal veins; ci. vena cava inferior; r. renal veins; il. iliac; hy. hypogastric veins; h. hepatic veins.
The dotted lines shew the position of embryonic vessels aborted in the adult.
At a later period a pair of trunks is established bringing the blood from the posterior part of the cardinal veins and the crural veins directly into the vena cava inferior ([fig. 374], il). These vessels, whose development has not been adequately investigated, form the common iliac veins, while the posterior ends of the cardinal veins which join them become the hypogastric veins ([fig. 374], hy). Owing to the development of the common iliac veins there is no renal portal system like that of the Reptilia and Amphibia.
Posterior vertebral veins, similar to those of Reptilia and Birds, are established in connection with the intercostal and lumbar veins, and unite anteriorly with the front part of the posterior cardinal veins ([fig. 373] A)[238].