268 TEXT-BOOK OF EMBRYOLOGY. 



advehent veins, is collected again by the revehent veins and poured into the 

 inferior vena cava; a part passes directly to the vena cava through the 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 ven- 

 tricle 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 



Sup. vena cava 

 Lungs 



Right atrium 



Right ventricle 



Inf. vena cava 



Liver 



Ductus venosus 



Placenta 



Inf. vena cava 



Umbilical vein 

 Umbilical artery 



Ant. part of body 



Carotid and 

 subclavian arteries 



Ductus arteriosus 



Pulmonary artery 

 Left ventricle 



Post, part of body 



FIG. 244. Diagram illustrating the foetal circulation. Compare with Fig. 245. 



Modified from Kottmann. 



The shading represents the relative impurity of the blood in different regions, the 

 darkest shading representing the most impure blood. 



subclavian and carotid arteries. The latter part, then becoming impure, is 

 carried back to the right atrium by the subclavian 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 the subclavian and carotid arteries. 

 This accounts for the fact that the more cranial portion of the body generally 



