VIII.] THE VENA CAVA INFERIOR. 207 



9. The eveuts which we have thus briefly narrated are 

 accompanied by important changes in the arterial and 

 venous systems. 



The condition of the venous system at about the end 

 of the third day was fully described in Chap. V. § 16, and 

 the changes which have taken place between that date and 

 the latter days of incubation may be seen by comparing 

 the diagram Fig. 39 B with the diagrams Figs, 62 and 63. 



On the third day, nearly the whole of the venous blood 

 from the body of the embryo was carried back to the heart by 

 two main venous trunks, the superior (Fig. 39 B, Su. V.) and 

 inferior (Fig. 39 B, C) cardinal veins, joining on each side 

 to form the short transverse ductus Cuvieri, both which in 

 turn united with the sinus venosus close to the heart. As 

 the head and neck continue to enlarge and the wings become 

 developed, the single superior cardinal or jugular vein, 

 as it is usually called (Fig. 62, J), of each side, is joined 

 by two new veins : the vertebral vein {Su. V.), bringing 

 back blood from the head and neck, and the vein from the 

 wing {W). 



The inferior cardinal or vertebral veins have their roots 

 in the Wolffian bodies ; they become developed, pari passu, 

 with those organs, and may be called the veins of the 

 Wolffian bodies. On the third day they are the only veins 

 which bring the blood back from the hinder part of the body 

 of the embryo. 



About the fourth or fifth day, however, a new single 

 venous trunk, the vena cava inferior (Fig. 62, V. G. /.), makes 

 its appearance in the middle line, in a plane more dorsal 

 than that of the cardinal veins. 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 above the 

 Wolffian bodies. When the kidneys are formed it receives 

 blood from them, and thenceforward enlarging rapidly be- 

 comes the channel by which the greater part of the blood 

 from the hind limbs and the hinder part of the body finds its 

 way to the heart. In proportion as this vena cava inferior 

 increases in size, and the Wolffian bodies give place to the 

 permanent kidneys, the posterior cardinal veins diminish. 

 Communicating branches between them and it are established 



