IX.] THE VENOUS SYSTEM. 287 



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. VII. p. 228). 



On the third day the course of the vessels from the 

 yolk-sac is very simple. The two vitelline 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 venae 

 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 along the ventral side of the body from 

 the walls of which they receive some amount of blood, 

 pass to the allantois. They are the allantoic or um- 

 bilical veins. The single vein which they unite to form 

 becomes, by reason of the rapid growth of the allantois, 

 very long ; and hence it is perhaps better to speak of it 

 as the allantoic vein (Fig. 90, U}. The right branch 

 soon diminishes in size and finally disappears. Mean- 

 while the left on reaching the allantois bifurcates ; and, 

 its two branches becoming large and conspicuous, there 



