592 EMBRYOLOGY. 



31. Since through these cross anastomoses more and more of the 

 blood, and finally the whole of it, is conveyed from the trunks of the 

 left half of the body into those of the right half, the proximal part 

 of the left superior vena cava, except a small portion, which lies in 

 the coronary groove of the heart, degenerates, receives the cardiac 

 veins, and becomes the sinus.coronarius cordis. Likewise the cardiac 

 end of the left cardinal vein disappears. 



32. From the paired fundaments of the venous trunks are formed 

 the single superior vena cava, the sinus coronarius cordis, and the 

 vena azygos and hemiazygos. 



33. The vitelline veins, which afterwards become unpaired, give 

 rise, when the liver is developed, to the portal circulation (the venae 

 hepaticse advehentes and revehentes). 



34. The umbilical veins, of which the right early degenerates, origi- 

 nally run in the abdominal wall above the liver to the sinus reuniens ; 

 then the left forms an anastomosis with the vitelline vein under the 

 liver, whereby its current shares in the portal circulation. 



35. There arises out of an anastomosis between the umbilical vein 

 and the cardiac end of the inferior vena cava on the under surface 

 of the liver the ductus venosus Arantii, which results in the division 

 of the blood of the umbilical vein into two currents. 



36. After birth the umbilical vein degenerates into the ligamentum 

 teres hepatis, and the ductus venosus Arantii is obliterated ; the venae 

 hepaticse advehentes now receive their blood from the terminal part 

 of the original vitelline vein or the portal vein only, which collects 

 the blood from the intestinal canal. 



37. The septum transversum, in which run the venous trunks on 

 their way to the heart, is the starting-point for the development of 

 the diaphragm and the pericardial sac, and forms at first an incom- 

 plete partition between the abdominal cavity and pleuro-pericardial 

 cavity, which still communicate with each other on either side of 

 the vertebral column. 



38. The pericardial sac is separated off from the thoracic cavity 

 as follows : (1) the Cuvierian ducts or future superior venae cavae, 

 instead of running transversely, run more and more obliquely from 

 above downward, detach themselves from the septum transversum, 

 and elevate the pleura into pericardial folds, which run from above 

 downward and project inward ; (2) the margin of the pericardial fold 

 fuses with the mediastinum posterius, in which are enclosed oesophagus 

 and aorta, whereby the superior venae cavae are transferred to the 

 mediastinum. 



