584 THE HUMAN EMBRYO. 



insignificant, there are at this stage only two vessels returning 

 blood to the right auricle : the anterior vena cava, which returns 

 venous blood from both sides of the head, and from both fore 

 limbs ; and the posterior vena cava, which brings back blood, 

 mainly arterial in character, from the placenta, and also from the 

 hinder part of the body of the embryo, and from the yolk-sac. 



During the fourth month the foramen ovale, which has 

 hitherto been a free opening, becomes partially blocked up by a 

 fold, which acts as a valve, allowing blood to pass from the right 

 to the left auricle, but obstructing its return in the opposite 

 direction. 



The Eustachian valve becomes larger at the same time ; and 

 partly owing to its increased size, and partly to slight changes in 

 the position and direction of the opening of the posterior vena 

 cava, the whole of the blood returned by this latter vessel is now 

 discharged through the foramen ovale into the left auricle. 



5. The Course of the Circulation during the Latter Half of Ges- 

 tation. 



During the latter four months or so of gestation the course 

 of the circulation is as follows : 



The right auricle receives blood from three sources 



(i) From the anterior vena cava. 



(ii) From the coronary sinus. 



(iii) From the posterior vena cava. 



The anterior vena cava returns venous blood from both sides 

 of the head, and from both fore-limbs. 



The coronary sinus, which is the persistent terminal portion 

 of the original left anterior vena cava, returns venous blood from 

 the walls of the heart itself. 



The posterior vena cava, which is much the largest of the 

 three, returns blood : (a) from the hinder part of the body, and 

 especially the kidneys and the hind limbs ; and (b) from the 

 placenta, the intestine and the liver. The latter of these two 

 streams requires further consideration. 



Of the two allantoic veins, by which the blood was returned 

 from the placenta in the earlier stages, the right one has dis- 

 appeared. The left allantoic vein, which is very large, enters 

 the body at the umbilicus, and runs forwards to the hinder 

 border of the liver ; here it is joined by the hepatic portal vein, 



