THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



235 



ductus venosus. At this point the blood acquires some impurity from the 

 stream brought in by the vena cava itself and the portal vein. The slightly 

 impure blood then flows into the right atrium, is directed by the Eustachian 

 valve through the foramen ovale into the left atrium, thence flows into the 

 left ventricle and is forced out into the aorta. A part of the blood flows on 

 through the aorta, a part is carried to the upper extremities and head and 

 neck regions by the subclavian and carotid arteries. The latter part, then 

 becoming impure, is carried back to the right atrium by the subclavian 



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. 207. Diagram illustrating the circulation in the adult. Compare with Fig; 206. The 

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



and jugular veins and superior vena cava; from the right atrium the greater 

 portion flows into the right ventricle and thence is forced out into the large 

 pulmonary artery. But since the lungs are non-functional, this blood passes 

 through the ductus arteriosus to join the stream in the aorta. The blood 

 received by the more cranial portion of the foetus is but slightly impure, 

 for the impure blood from the ductus arteriosus joins the aortic stream distal 

 to the origin of tlie subclavian and carotid arteries. This accounts for 

 the fact that the more cranial portion of the body generally is better devel- 

 oped than the more caudal portion. It is well to note here that the liver 



