VIII.] THE VEN.E CAV^E. 209 



in the substance of the Wolffian bodies, so that soon the 

 blood of those organs, as well as that from all the rest of the 

 hind body, with the exception of the alimentary canal and its 

 appendages, passes into the vena cava inferior. 



The diminished trunks of the cardinal veins remain for 

 some time ; their anterior ends unite to form the small azygos 

 vein. 



At its first appearance the vena cava inferior may be con- 

 sidered 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 the ductus venosus may 

 now be said to join the vena cava instead of being prolonged 

 into the sinus. 



While this growth of the vena cava is going on, the points 

 at which the ductus Cuvieri enter into the sinus venosus, or, 

 as we may now say, vena cava inferior, are drawn in towards 

 the heart itself, and finally these trunks fall directly and 

 separately into the auricular cavities, and are henceforward 

 known as the right and left vena cava superior (Fig. 63, V.S.R., 

 V.S.L.}. There are therefore, when these changes have been 

 effected, three separate channels, with their respective orifices, 

 by which the blood of the body is brought back to the heart, 

 viz. the right and left superior and the inferior venae cavae. 



While the auricular septum is as yet unformed, the blood 

 from these veins falls into both auricles, perhaps more into 

 the left than into the right. As the septum however grows 

 up, the three vessels become connected with the right auricle 

 only while the left receives the two pulmonary veins coming 

 from the lungs. (Compare Chap. vn. 7.) 



On the third day the course of the vessels from the yolk- 

 sac is very simple. The two omphalo-mesaraic veins, of which 

 the right is already the smaller, form the meatus venosus 

 from which, as it passes through the liver on its way to the 

 heart, are given off the two sets of vence advehentes and venae 

 revehentes. 



With the appearance of the allantois on the fourth day, 



a new feature is introduced. From the .meatus venosus, 



a short distance behind the liver, there is given off a vein 



which quickly divides into two branches. These, running 



E. 14 



