794 DEVELOPMENT OF THE VASCULAR SYSTEM. 



which is devoted to the formation of the placenta. (Fig. 300.) As the 

 placenta soon becomes the only source of nutrition for the foetus, its 

 vessels increase in size, and preponderate over all the other parts of the 

 circulatory system. During the early periods of the formation of the 



Fig. 300. 



Diagram of the EMBRYO AND ITS VESSEI/S; showing the second or placental circu- 

 lation. The intestine has become further developed, and the mesenteric arteries have 

 enlarged, while the umbilical vesicle and its vascular branches are reduced in size. The 

 large umbilical arteries are seen passing out to the placenta. 



placenta, there are, as above mentioned, two umbilical arteries and two 

 umbilical veins. Subsequently one of the veins disappears, and the 

 whole of the blood is returned to the foetus by the other, which becomes 

 enlarged in proportion. For a long time previous to birth, there are, 

 therefore, in the umbilical cord two umbilical arteries, and but one 

 umbilical vein. 



Adult Circulation The placental circulation is exchanged, at the 

 period of birth, for the third or adult circulation. This is distinguished 

 by the disappearance of the placenta and the vessels connected with it, 

 and by the entrance into activity of the lungs and the alimentary canal, 

 as the organs of nutrition and aeration for the blood. A large propor- 

 tion of the blood is accordingly turned into different channels, and is 

 distributed to organs which were before but scantily supplied. This 

 change differs from that which preceded it mainly in its suddenness. 

 The transition from the first to the second form of circulation is a 

 gradual one ; the vitellus and umbilical vesicle diminishing as the pla- 

 centa enlarges, and the two organs, with their bloodvessels, coexisting 



