THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



269 



is better developed than the more caudal portion. It is well to note here 

 that the liver receives purer blood than any other part of the body, and this is 

 undoubtedly correlated with the relatively enormous size of that organ in the 

 foetus. The rather impure blood which starts through the dorsal aorta is in 

 part distributed to the viscera, body walls, and lower extremities by the visceral 

 and segmental arteries, and thence is collected by the branches of the portal 

 vein and inferior vena cava to be returned as impure blood to the umbilical 

 current at the liver; in part it is carried by the umbilical arteries to the placenta, 

 there to be purified and collected by the branches of the umbilical vein (see 

 Fig. 244). 



Sup. vena cava 

 Lungs 



Pulmonary veins 

 Right atrium 



Right ventricle 

 Inf. vena cava 



Hepatic vein 

 Liver 



Inf. vena cava 



Ant. part of body 



Carotid and 

 subclavian arteries 



Pulmonary artery 



Left ventricle 



Hepatic artery 



Post, part of body 



FIG. 245. Diagram illustrating the circulation in the adult. Compare with Fig. 244. The 

 shading represents the relative impurity of the blood, the white being the purest blood. 



At birth, when the placental circulation is cut off, the proximal end of the 

 umbilical vein atrophies to form the round ligament of the liver; the ductus 

 venosus also atrophies and becomes merely a connective-tissue cord in the liver. 

 The hepatic portal circulation is still maintained by the portal vein. The 

 foramen ovale is closed and the impure blood from the inferior vena cava, as 

 well as that from the superior, passes from the right atrium into the right 

 ventricle and thence is forced out through the pulmonary artery to the lungs, 



