PLACEXTAL CIRCULATION. 



667 



extremity of the foetus. These arteries are called the vertebral 

 arteries, on account of their course and situation, running parallel 

 with the vertebral column. They give of throughout their course, 

 many small lateral branches, which supply the body of the foetus, 

 and also two larger branches the omphalo-mesenteric arteries 

 which pass out, as above described, into the area vasculosa. The 

 two vertebral arteries remain separate in the upper part of the body, 

 but soon fuse with each other a little below the level of the heart ; 

 so that, below this point, there remains afterward but one large 

 artery, the abdominal aorta, running from above downward along 

 the median line, giving off the omphalo-mesenteric arteries to the 

 area vasculosa, and supplying smaller branches to the body, the 

 walls of the intestine, and the other organs of the foetus. 



The above description shows the origin and formation of the first 

 or vitelline circulation. A change, however, now begins to take 

 place, by which the vitellus is superseded, as an organ of nutrition, 

 by the placenta, which takes its place; and the second or placental 

 circulation becomes established in the following manner : 



Second Circulation. After the umbilical vesicle has been formed 

 by the process already described, a part of the vitellus remains in- 

 cluded in it, while the rest is retained in the abdomen and inclosed 

 in the intestinal canal. As these 



two organs (umbilical vesicle and Fl s- 254< 



intestine) are originally parts of 

 the same vitelline sac, they remain 

 supplied by the same vascular 

 system, viz : the omphalo-mesen- 

 teric vessels. Those which remain 

 within the abdomen of the foetus 

 supply the mesentery and intes- 

 tine ; but the larger trunks pass 

 outward, and ramify upon the 

 walls of the umbilical vesicle. 

 (Fig. 254.) At first, there are, 

 as we have mentioned above, 

 two omphalo-mesenteric arteries 

 emerging from the body, and two 

 omphalo-mesenteric veins return- 

 ing to it ; but soon afterward, the two arteries are replaced by a 

 common trunk, while a similar change takes place in the two veins. 

 Subsequently, therefore, there remains but a single artery and a 



Diagram of YOCNG EMBRYO AJTD ITS 

 VESSELS, showing circulation of umbilical 

 vesicle, and also that of allantois, beginning to 

 be formed. 



