How is it, that the instant the black blood enters the system of red blood in the adult, alarming consequences follow, soon asphyxia, then death, take place, whilst in the fœtus, the black blood circulates with impunity in the arteries? It is a difficult question to resolve, and yet these two contradictory facts are equally true. The difference of the nature of the blood of the fœtus might perhaps serve to remove this difficulty, if we better understood this difference. In fact, though the colour assimilates this blood with that of the veins of the adult, yet it does not appear to be the same; it has an unctuous feel, unlike the other. It is never found in the dead body coagulated like it, but always fluid, like the blood of those who have died of asphyxia. Fourcroy discovered no fibrin in it; he observed that it did not take the vermilion colour by the contact of the air; that it contained no phosphoric salts, &c. It is then very probable that if the black blood is fatal in the arteries of the adult, whilst it circulates with impunity in those of the fœtus, that it arises from the difference of the nature of the one and the other. Besides, observe that there is a very great difference in the functions of the fœtus and the adult. The first scarcely has animal life; it wants many functions of organic. The relation of the organs with each other, is of a nature wholly different from what it will be after birth. No kind of analogy ever can be established between the fœtus and the infant in this respect. Thus we have observed that the experiments upon life and death give a result wholly different in animals with red and warm blood, and in those with red and cold, which approximate nearly the organization of the fœtus in some respects. We cannot then establish any kind of parallel in respect to the injury of the respiratory phenomena, between the fœtus and the infant, an injury, the causes of which I have sought in my experiments, since the organization relative to these phenomena differ so essentially in the one and the other. Although, as I have said, the blood of the two vascular systems is confounded in the fœtus, yet there is, especially in the first periods, a kind of separation in the general mass of blood, a separation that was first accurately observed by Sabatier, and which is the result of the arrangement of the foramen ovale and the ductus arteriosus. This separation divides the mass of blood into two. The following is the manner in which the circulation of the blood is performed in this respect.