226 THE FOURTH DAY. [CHAP. 



which runs straight up from the bulbus arteriosus to 

 the head. 



FIG. 73. 



STATE OF ARTERIAL CIRCULATION ON THE FIFTH OR SIXTH 



DAY. 



E.G. A. external carotid. I.C.A. internal carotid. D. A. dorsal 

 aorta. Of. A. vitelline artery. U.A. allantoic arteries. 



In the same way the dorsal portions form a branch, 

 the internal carotid, which takes its origin from the 

 dorsal or far end of the third arch. 



In the venous system important changes also occur. 



As the liver in the course of its formation wraps 

 round the common trunk of the vitelline veins, or 

 meatus venosus, it may be said to divide that vessel 

 into two parts : into a part nearer the heart which is 

 called the sinus venosus (Fig. 74, S. V.), and into a part 

 surrounded by the liver which is called the ductus 

 venosus. Beyond, i.e. behind the liver, the ductus veno- 

 sus is directly continuous with the vitelline veins, or, as 

 we may now say, vein, for the right trunk has become 

 so small as to appear a mere branch of the left (Fig. 

 74, 0/0. 



