THE DEVELOPMENT OF THE VASCULAR SYSTEM 



265 



as a continuation of the proximal part of the primitive fibular. The anterior 

 tibial and primitive fibular together thus constitute the sciatic vein (Fig. 

 243). Another vessel appears in embryos of fifteen days, which represents 

 the beginning of the femoral vein and opens into the cardinal, cranial to the 

 opening of the sciatic (Fig. 243). From this time on the femoral, with its 

 branches, enlarges at the expense of the other veins and becomes the principal 

 vein of the lower extremity. In the human embryo the femoral anastomoses 

 with the sciatic near the knee and the proximal portion of the sciatic then 

 atrophies, the distal portion persisting as the small scphenous vein. 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 cf 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 Kollmann. 



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

 darkest shading representing the most impure blood 



large saphenous vein and the posterior tibial vein possibly are derivatives of 

 the femoral, but this question has not been settled. 



CHANGES IN THE CIRCULATION AT BIRTH. During fcetal life the course of 

 the blood is adapted to the placental circulation, since the placenta is the 

 only means by which the blood is purified and from which the foetus derives 

 its nutriment. The pure blood from the placenta passes through the umbil- 

 ical vein to the liver; there a part of it is distributed to the liver by some of 

 the 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 



