The course taken by the blood through the heart and vessels of the embryo differs essentially from that which persists in adult life.

Tracing the blood from the placenta, it passes along the umbilical vein toward the liver; here it may take either of two courses to reach the vena cava, one which follows the ductus venosus and avoids the liver, the other which passes by the venae advehentes (portal veins) to the liver, and proceeds by the venae revehentes (hepatic veins) to the inferior vena cava, which receives all the blood passing by both of these channels. From this the blood is emptied into the right auricle, and hence is guided by the Eustachian valve through the septum by the patent foramen ovale to the left auricle. From the left auricle it passes to the left ventricle,, which contracts and sends the blood into the aortic arch, where it is split up into two streams, one of which passes into the vessels of the head and neck, the other by the descending aorta to the trunk and lower extremities.

The blood from the head and neck is returned to the right auricle by the superior vena cava. The blood from this vein passes through the auricle to the right ventricle, which sends it through the pulmonary artery toward the lungs.

The pulmonary artery, in the embryo, has one very large branch, called the ductus arteriosus, which joins the aorta at a point just below the origin of the vessels of the head and neck; hence the main part of the blood passing from the right ventricle reaches the aorta by the ductus arteriosus, and only a very small part goes to the lungs, to be returned from them by the pulmonary veins to the left auricle.

The blood from the ductus arteriosus blends, therefore, with that in the aorta which is passing to the viscera and lower extremities. The main part of this blood travels by two large branches of the aorta (the hypogastric arteries) to the placenta, where it is aerated and purified, etc.

It is evident, if the placenta is the great renovating organ of the blood of the foetus, that the blood in the umbilical vein is the most arterial in the foetal circulation. The blood in the ascending vena cava and first part of the aorta is likewise fairly arterial, but the blood in the descending aorta is of a mixed character, as it contains blood which has nourished the head and neck, besides that which has come from the placenta by the inferior vena cava through the right auricle, foramen ovale, left auricle, and left ventricle.

As the foetal lungs are not called into play until after birth, but little blood passes to them in the foetus; this state of things is, however, completely altered at birth, when the lungs of the child expand, the pulmonary arteries increase in size, and the ductus arteriosus dwindles in a corresponding degree.

The liver, which in the foetus is of relatively greater size than in the adult, receives much blood coming from the placenta to the heart, and is thought to contribute to it several essential constituents.

The head and brain, which are largely developed in the foetus, receive well aerated blood; namely, the placental blood which has passed through the liver, and, in the inferior vena cava, is mixed with blood coming from the lower limbs. The rest of the foetus receives blood that is less aerated, as it is mixed with that which is returned from the head and neck to the right side of the heart, and which is sent through the ductus arteriosus to join the general blood current in the aorta going to the viscera and lower extremities.