700 



MANUAL OIF PHYSIOLOGY. 



venae advehentes and the entrance of the venae revehentes is 

 given the name of the ductus venosus. 



Thus it may be seen that in theplacental circulation the blood 

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

 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 foetal 

 circulation will be described later on. 



Diagram of the heart and principal arteries of the chick. (Allen Thomson.) B. and 

 c. are later than A. 1, 1. Omphalo-mesenteric veins. 2. Auricle. 3. Ventricle. 4. 

 Aortic bulb. 5, 5. Primitive aortse. 6, 6. Omphalo-mesenteric arteries. A. United 

 aorta. 



The Arterial System. Around the pharynx are developed five 

 pairs of aortic arches. These commence anteriorly from the two 

 primitive aortse, and, passing along the side of the pharynx, end 

 in the aortse as they descend to become united in the dorsal region 

 of the embryo. The points of origin of the arches are termed 

 their anterior roots, and the points of termination their posterior 

 roots. 





