Henry Gray (18251861). Anatomy of the Human Body. 1918.
septum in which the foramen ovale is situated; shortly after birth it fuses with the primary septum and the foramen ovale is obliterated.
The valve of the inferior vena cava serves to direct the blood from that vessel through the foramen ovale into the left atrium.
The peculiarities in the arterial system of the fetus are the communication between the pulmonary artery and the aorta by means of the ductus arteriosus, and the continuation of the hypogastric arteries as the umbilical arteries to the placenta.
The ductus arteriosus is a short tube, about 1.25 cm. in length at birth, and of the diameter of a goose-quill. In the early condition it forms the continuation of the pulmonary artery, and opens into the aorta, just beyond the origin of the left subclavian artery; and so conducts the greater amount of the blood from the right ventricle into the aorta. When the branches of the pulmonary artery have become larger relatively to the ductus arteriosus, the latter is chiefly connected to the left pulmonary artery.
The hypogastric arteries run along the sides of the bladder and thence upward on the back of the anterior abdominal wall to the umbilicus; here they pass out of the abdomen and are continued as the umbilical arteries in the umbilical cord to the placenta. They convey the fetal blood to the placenta.
The peculiarities in the venous system of the fetus are the communications established between the placenta and the liver and portal vein, through the umbilical vein; and between the umbilical vein and the inferior vena cava through the ductus venosus.
Fetal Circulation (Fig. 502).The fetal blood is returned from the placenta to the fetus by the umbilical vein. This vein enters the abdomen at the umbilicus, and passes upward along the free margin of the falciform ligament of the liver to the under surface of that organ, where it gives off two or three branches, one of large size to the left lobe, and others to the lobus quadratus and lobus caudatus. At the porta hepatis (transverse fissure of the liver) it divides into two branches: of these, the larger is joined by the portal vein, and enters the right lobe; the smaller is continued upward, under the name of the ductus venosus, and joins the inferior vena cava. The blood, therefore, which traverses the umbilical vein, passes to the inferior vena cava in three different ways. A considerable quantity circulates through the liver with the portal venous blood, before entering the inferior vena cava by the hepatic veins; some enters the liver directly, and is carried to the inferior cava by the hepatic veins; the remainder passes directly into the inferior vena cava through the ductus venosus.
In the inferior vena cava, the blood carried by the ductus venosus and hepatic veins becomes mixed with that returning from the lower extremities and abdominal wall. It enters the right atrium, and, guided by the valve of the inferior vena cava, passes through the formen ovale into the left atrium, where it mixes with a small quantity of blood returned from the lungs by the pulmonary veins. From the left atrium it passes into the left ventricle; and from the left ventricle into the aorta, by means of which it is distributed almost entirely to the head and upper extremities, a small quantity being probably carried into the descending aorta. From the head and upper extremities the blood is returned by the superior vena cava to the right atrium, where it mixes with a small portion of the blood from the inferior vena cava. From the right atrium it descends into the right ventricle, and thence passes into the pulmonary artery. The lungs of the fetus being inactive, only a small quantity of the blood of the pulmonary artery is distributed to them by the right and left pulmonary arteries, and returned by the pulmonary veins to the left atrium: the greater part passes through the ductus arteriosus into the aorta, where it mixes with a small quantity of the blood transmitted by the left ventricle into the aorta. Through this vessel it descends, and is in part distributed