A COMPARATIVE STUDY OF THE CIRCULATORY SYSTEM 261 



placenta to the fetus. Very soon after it enters the fetus through the 

 umbiHcal perforation, it divides into two channels, one of which 

 unites directly with the inferior vena cava, and the other with the 

 portal vein in the immediate vicinity of the liver. The portal branch 



Fig. 129. — The Fetal, Cibculation. 

 P, placenta; UV, umbilical vein carries oxygenated blood and unites with the 

 vena cava inferior (JVC) and portal vein {PV). This blood mixes with the venous 

 blood and enters the right atrium, (RA) being here diverted largely through the fora- 

 men ovale into the left auricle {LA). From here it passes into the left ventricle {LV), 

 aorta (A) and either into head circuit or abdominal aorta {A A). Here it may be 

 diverted into the portal organs (PO) or continue onward into the common iliac {CJA), 

 external iliac (EJA) or hypogastric arteries (HA). In the latter case the blood again 

 reaches the placenta by way of the umbilical arteries (UA). The blood from the head 

 enters the superior vena cava (SVC) and right auricle (RA), where it is diverted into 

 the right ventricle (RV) and pulmonary artery (PA). From here it passes chiefly 

 through the ductus arteriosus (DA) into the aorta. A small portion of its traverses 

 the lungs proper (L) to be returned to the left auricle (LA) by way of the pulmonary 

 vein (PV). The striated vessels contain venous blood and the dotted vessels, mixed 

 blood. 



is known as the ductus venosus. Whichever course the placental 

 blood selects, it eventually reaches the right auricle. It is to be noted, 

 however, that it is immediately mixed with the blood of the inferior 



