FETAL CIRCULATION 



277 



affects but slightly the greater volume of pure placental blood. Entering 

 the right atrium, it mingles somewhat with the venous blood returned 

 through the superior vena cava. It is said that the blood from the 

 inferior vena cava is directed by the valve of this vein through the foramen 

 ovale into the left atrium (following the path of the sounds in Figs. 262 



Internal jugular vein 



Subclavian vein 

 Superior vena cava 



Right atrium 

 Valve of inf. vena cava 



Inferior vena cava 



Common carotid artery 



Subclavian artery 

 Arrow in ductus arteriosus 



Small intestine 



V 



Iliac art rev and vein 



Umbilical vein 



Umbilcal cord 

 Umbilical artery 



FIG. 286. Diagrammatic outline of the organs of circulation in the fetus of six months 

 (Allen Thomson). The arteries are conventionally colored red and the veins blue, irrespec- 

 tive of the nature of the blood conveyed by them. Arrows show the course of blood through 

 the heart, a, Aortic arch; dv, ductus venosus, hv, hepatic vein, vp, portal vein. 



to 264), which, before birth, receives little venous blood from the lungs. 

 This purer blood of the left atrium enters the left ventricle, and is driven 

 out through the aorta, to be distributed chiefly to the head and upper 

 extremities. 



The venous blood of the superior vena cava, slightly mixed, is sup- 

 posed to pass from the right atrium into the right ventricle, whence it 



