PORTAL SYSTEM. 



703 



After a time the right umbilical and right omphalo-mesenteric 

 veins disappear, whilst from the trunk formed by the junction of 

 the left umbilical and left omphalo-mesenteric veins, branches 

 are given off to the liver (the vence advehentes), and at a point 

 nearer the heart, vessels are received from the liver (the vence 

 revehentes). 



To the part of the vessel intervening between the origin of the 

 vense advehentes and the entrance of the venae revehentes is given 

 the name of the ductus venosus. 



Thus it may be seen that in the placental circulation, the blood 

 is conveyed from the aorta, by the umbilical arteries, to the foetal 



\i> iR /R 



t iii 



Diagram of the heart and principal arteries of the chick. (Allen Thom- 

 son.) B. and c. are later than A. 1,1. Omphalo-mesenteric veins. 2. Au- 

 ricle. 3. Ventricle. 4. Aortic bulb. 5, 5. Primitive aortse. 6, 6. Om- 

 phalo-mesenteric arteries. A. United aorta. 



placenta, and here it undergoes changes, owing to its close rela- 

 tionship to the maternal blood. From the placenta it is returned 

 by the umbilical vein, which sends a part through the liver and 

 a part direct to the heart. The more minute details of fcetal cir- 

 culation will be described later on. 



