CIRCULATION.
The successive modifications through which the embryo passes may be characterized by three successive stages.
A first stage, marked by the development of the blastoderm and the existence of the umbilical vesicle. A second stage in which the allantois and umbilical vessels appear and develope. And a third stage—after expulsion, characterized by the development of the lungs and the organs of social life.
These three stages correspond to three successive circulations. The first is the stage of the blastodermic or umbilical circulation. The second is the stage of the allantoid or placental circulation. The third is the stage of the pulmonary or definitive circulation.
We will follow Kolliker in our description of the development of the heart and vessels.
First Circulation.—The first trace of blood and vessels is visible, as we have seen, a few hours after the appearance of the primitive line, and the formation of the first circulatory apparatus occurs at the same time at the centre and at the circumference, in the embryo and in the blastoderm, at the heart, capillaries, veins and arteries. That is to say, that, while the terminal sinus is being developed at the periphery of the vascular area, the heart is being formed at the centre, and between the heart and the terminal sinus there appear the capillaries, arteries and veins. Then the heart begins to beat, the puizetum saliens of Aristotle: it is then only a simple tube, terminating at each end in two branches.
The superior branches are the two first aortic arches, the inferior are the omphalo-mesenteric veins. The blood runs through the two aortas, passes into the body of the embryo, and thence goes to the omphalo mesenteric or vitelline arteries, which carry it to the vascular area. Thence it goes into the terminal sinus, and so into the branches of the omphalo mesenteric or vitelline veins, which conduct it back to the heart. Such is the first circulation. (V. Fig. 171.) Second appearance of the allantois and the umbilical vessels, with the development of the placenta, completely change these circulatory conditions, and determine the second circulation.
The contracting heart sends the blood from the left ventricle into the aorta, and from the right ventricle into the pulmonary artery. In the aorta the blood then divides into two streams; one, the larger, is directed towards the head and arms by the brachio-cephalic trunk of the vessel, with the left primitive carotid, and the left sub-clavicular artery. The other stream passes into the descending aorta, and fills all the branches that arise from it, carrying blood to the organs of the trunk, the lower limbs, and the placenta. But this aortic blood is not pure, since it is
mixed with blood from the right ventricle. The right ventricle does indeed direct the blood to the pulmonary artery, but the lungs are not functionating, and but a small portion of the blood of the right ventricle goes into them. The larger portion of it goes through the ductus arteri osus into the aorta. The aortic blood is thus a mixture of the blood of the left and of the right ventricle, of arterial and venous blood; and it is this mixed blood that goes through the umbilical arteries to the placenta. There the blood receives the oxygen necessary for the fcetal respiration, and is collected again into the umbilical vein. The umbilical vein brings the blood back to the embryo, where it divides into two currents. One, the larger, goes through the ductus venosus into the inferior vena cava, while the smaller one goes to the liver and joins the divisions of the portal vein. Thus brought to the liver, the blood returns to the inferior vena cava by the hepatic veins. The inferior cava thus receives the blood from the lower limbs, the kidneys, and the pelvic organs as well as the blood of the umbilical vein and of the vena porta; it then goes to the right auricle, but a portion only passes thence into the ventricle; the major part goes through the open ductus Botalli into the left auricle, thence to the left ventricle, and meantime the blood that has gone through the brachio cephalic, the left primitive carotid and the left subclavicular arteries, is gathered again into the superior vena cava, and, goingto the right auricle, passes into the right ventricle, and thence through the pulmonary artery and ductus arteriosus again into the aorta. Thus no part of the fcetus receives absolutely pure blood. (V. Fig. 172.) Third soon as the fcetus has made its first inspiration, the third circulation, the pulmonary or definitive circulation, is established..
The umbilical vein, its function gone, is obliterated, and becomes the round ligament of the liver; the umbilical arteries and the ductus venosus also atrophy. The ductus Botalli disappears, as does the ductus arteriosus, after persisting for one or two days. The pulmonary circulation replaces the placental circulation.
The two auricles no longer communicate, and the entire blood of the pulmonary artery passes into the lungs. There it becomes oxygenated, and is returned by the pulmonary veins to the left side of the heart and so to the systemic circulation. Thus, in its turn, the third circulation is established. (V. Fig. 173 )