IX.] THE VENOUS SYSTEM. 285 



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. 89, V.G.I.\ 

 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 thencefor- 

 ward enlarging rapidly eventually becomes 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. The 

 blood originally coming to the posterior cardinals from 

 the posterior part of the spinal cord and trunk is trans- 

 ported into two posterior vertebral veins; which are 

 placed dorsal to the heads of the ribs and join the 

 anterior vertebral veins. With the appearance of these 

 veins the anterior part of the posterior cardinals dis- 

 appears. 



At its first appearance the vena cava inferior may 

 be considered as a branch of the trunk which we have 

 called the sinus venosus, but as development proceeds, 

 and the vena cava becomes larger and larger, the sinus 

 venosus assumes more and more the appearance of being 

 merely the cardiac termination of the vena cava, and 



