1st. All the blood that the trunk of the inferior vena cava receives, either from the capillary system of the inferior extremities, or from that of the abdomen, or from the placenta by the umbilical vein, instead of stopping in the right auricle, as in the adult, passes entire into the left through the foramen ovale, the superior edge of which is so arranged, that nothing can mix with the blood of the superior vena cava; so that when we examine attentively, we see that it is really with the left auricle that the inferior vena cava is continued. Hence why this auricle is in proportion as much dilated as the right; for it would be very contracted, if it had only to receive the blood of the pulmonary veins, the quantity of which is merely nothing in the first periods of life. From this auricle the blood passes to the left ventricle, which transmits it to the aorta, where it meets the carotids and subclavians, which, by numerous ramifications, carry it to the capillary system of the head and the superior extremities. 2d. After having remained in this system, the blood returns by the different branches of the superior vena cava to the right auricle, where the superior edge of the foramen ovale prevents it from communicating with the other blood; from this auricle it passes to the ventricle, which transmits it to the pulmonary artery, which sends a small part of it that returns to the left auricle by the veins of the same name, but transmits almost the whole of it by the ductus arteriosus to the descending aorta, below the origin of the carotids and the subclavians, which carry the other blood. This is carried by the branches and ramifications of the aorta to the capillary system of the abdomen and inferior extremities; the remainder is afterwards carried by the umbilical artery and lost in the placenta. It follows from what we have just said, that notwithstanding the continuity of the two great sanguineous systems in the fœtus, there is in the first months after conception, a kind of separation of the blood they contain; that there is even if we may so say, two systems wholly different from those which will afterwards exist in a separate manner in the adult. The first of these systems has, 1st, for origin all the capillaries of the abdomen, of the inferior extremities, and even those of the placenta; 2d, for common trunks, below the inferior vena cava, above the quadruple branch called the aorta; 3d, for agent of impulse the left side of the heart; 4th, for termination all the capillaries of the head and the superior parts. The second commences in these last capillaries, and is composed, 1st, for its trunks, of the superior vena cava and the descending aorta; 2d, for its agent of impulse, of the right side of the heart; 3d, for its termination, of the capillaries of the inferior parts.