From what has been said, it appears, that the two portions of the blood of the fœtus are almost wholly separate in the first months; all that comes from the inferior vena cava goes into the ascending aorta; all from the superior passes into the descending, the lungs receiving scarcely any except by the bronchial arteries for their nutrition. But as the period of birth approaches, these two portions of blood begin to mix, and the circulation then has an arrangement between that of the adult and that of the first months. At birth even, the foramen ovale and the ductus arteriosus are much contracted, the circulation goes on in the mother's womb almost in the same way as it does after birth; the whole difference is that the fluid is of the same nature, because respiration has not taken place. The sudden change of the circulation at birth, arises particularly from the introduction of red blood into the economy. As to the mechanical phenomena, they are gradually produced by the gradual contraction of the two openings of communication. The blood gradually ceases to move from the inferior to the superior capillaries; it then goes from both of these to those of the lungs and reciprocally. In considering the circulating phenomena, we err in supposing that their change is sudden at birth. It is sufficient to examine the foramen ovale and the ductus arteriosus at different periods of pregnancy, to see that they contract successively, and that consequently these phenomena are successive, so that if the fœtus should remain in the womb a long time beyond its period, and the contraction should continue in the foramen ovale and the ductus arteriosus, the blood would circulate as in the adult, from the pulmonary to the general capillary system exclusively, and reciprocally. The only difference would be in the uniformity of its colour, because it would pass through the first system, without coming in contact with the air. I do not say that the entrance of the air does not suddenly bring to the lungs the remainder of the blood which passed by the ductus arteriosus; but this kind of sudden turn takes place only in a part of the blood of the pulmonary artery; a part already passed through the lungs before birth, though the air cells were empty. In general, there is a constant relation between the quantity of blood that the right ventricle sends to the lungs, and that which the left sends to the inferior parts. The more the first increases, the more abundant is the second; this last evidently exceeds that which goes to the superior parts. These three things, 1st, the quantity of the blood of the inferior vena cava which mixes with that of the superior, and passes with it into the right auricle; 2d, that which from the right ventricle goes through the lungs and returns to the left auricle; 3d, that which from the left ventricle goes to the descending aorta, constantly increase as the period of accouchment approaches.