316. CHAPTER IV. ON THE CIRCULATION OF THE BLOOD IN THE FOETUS; AND ON THE FORAMEN OVALE AND DUCTUS ARTERIOSUS BELONGING TO THE HEART IN EMBRYOS AND INFANTS.
VERHEYEN. "The blood of the forms not only circulates through its own proper parts, as in adults, but it is also sent through the umbilical arteries to the placenta, a small portion being distributed to the membranes on the way: and it returns from the placenta through the umbilical vein to the vena cava, and so to the cavities of the heart. This is the first difference between the circulation of the blood in the foetus, and the circulation after birth, when the placenta is separated, and the blood no longer circulates beyond the limits of its own bodily system; and thus it is that at birth the umbilical vessels are cut away and thrown aside, as being of no further use. The blood returning through the umbilical vein, is mixed with the rest of the blood in the vena ports, from the sinus of which a large portion of it is carried to the vena cava through a short venous tube [the ductus venosus], which is peculiar to the foetus, and falls into the vena cava immediately under the diaphragm...."
"Another peculiarity in the foetal circulation is observable about the heart, and as in the former case, involves the existence of special channels or passages; by means of which the larger part of the sanguineous mass passes in fact through the left ventricle of the heart and through the aorta, without previously going through the lungs. These special passages are the foramen between the vena cava and the pulmonary vein, and a short arterial tube [the ductus arteriosus], between the pulmonary artery and the descending sorts. The foramen is situated between the inferior vena cava and the pulmonary vein; opening into the former immediately below a tubercle that lies adjacent to the first division of the vena, cava; and into the pulmonary vein near the left ventricle. It is called from its figure the foramen ovale. It is so large at first towards the vena cava its to exceed the aorta itself in size, but where it terminates in the pulmonary vein it is somewhat narrower. In the upper part, towards the right ventricle, it has a raised border to prevent the blood from easily passing it by; but at the lower part it is comparatively flat and even, to make the entrance of the blood more easy. Before the outlet of the foramen in the pulmonary vein, there is a large valve, which towards the lungs is attached to the wall of the vein, but towards the ventricle of the heart is entirely free. This valve serves principally to prevent the blood which passes through the foramen from the vena cava, and the blood which passes from the pulmonary vein towards the left ventricle, from impeding each other in their course. Through this foramen then a large portion of the blood of the vena cava that would otherwise pass to the right ventricle, runs of necessity to the left: immediately, when the left ventricle is open, and when it is not open, mediately through the corresponding auricle, from which the foramen almost directly opens. I say a large portion of the blood, this being sufficiently indicated by the size of the foramen; although what the actual quantity is, I believe it to be impossible to determine. But perhaps a doubt may exist ... as to how the blood of the vena cava can pass into the pulmonary vein through a simple foramen, for it may be thought that the two veins are distant from each other, as being adjacent to different cavities of the heart; hence that in order for the blood to pass from the cava to the pulmonary vein, a tube must be required to effect a communication between them. But this doubt will cease, if we only consider that these two veins are inserted into the ventricles on the posterior part, so near together that their coats join, and form as it were a complete wall perforated by the foramen ovale."
"The ductus arteriosus arises from the pulmonary artery, near its commencement and immediately before its division into a right and left branch: in fact, we may say that this artery divides into three branches, namely, into the two already mentioned, and into the very canal of which we are speaking. This duct, canal, or tube, terminates in the descending aorta not far from its commencement. It is of much larger calibre than the great branch of the pulmonary artery; in fact, nearly one half of the whole artery is expended upon it. A great portion of the blood sent from the right ventricle of the heart into the pulmonary artery, passes immediately through this tube into the descending aorta, and is distributed by the twigs of the latter through the lower parts of the body, and through the secundines."
"The circulation in the foetus is therefore performed as follows: A great part of the blood passes from the inferior vena cava near the heart, through the foramen ovale into the trunk of the pulmonary vein; thence into the left ventricle, immediately if the ventricle be open at the time, but through the left auricle if the ventricle be not open. But that portion of the blood of the inferior vena cava that does not pass through the foramen ovale, runs together with that which comes through the superior vena cave, into the right ventricle, in part immediately, and in part mediately through the auricle, just as in ourselves: from the right ventricle it is expelled into the pulmonary artery, and a great part of it passes thence immediately through the ductus arteriosus into the descending aorta; the other part pursues its course through the pulmonary arteries and veins till it comes to the left ventricle, which it enters, sometimes immediately, and sometimes mediately through the auricle; and then, together with the blood that arrives at the same moment through the foramen ovale, it is driven into the aorta by the constriction of the ventricle. But since the descending aorta is much occupied by the blood conveyed through the ductus arteriosus, it is evident that it cannot receive much blood from the left ventricle, and therefore that nearly the whole, or at least by far the greater portion of the blood of this ventricle is distributed through the superior parts of the body. The blood that passes through the descending aorta, circulates not only through the inferior parts of the foetus, but also through the secundines, as we before observed.... And although the pulmonary arteries after their division are collectively somewhat more capacious than the ductus arteriosus, it does not therefore follow that a less quantity of blood passes through the latter than through the pulmonary arteries; but rather a larger; for the transit of the blood through the lungs of the foetus is exceedingly difficult, inasmuch as they are collapsed by reason of having no respiration, and therefore considerably compress their blood vessels...." (Corp. Hum. Anat., lib. ii., tr. v., cap. xix.)
"The foramen ovale remains longer open towards the ventricle and auricle than on the other side, as I have sometimes observed in the human subject, and very frequently indeed in brutes.... Now, as this foramen is closed on the side of the pulmonary vein, but not on the side of the vena cava, therefore we find that in the latter situation there is always a large sinus left to mark the place of the foramen; and this sinus is found even in old subjects; nay, even in the bodies of those who have died of old age." (Ibid., cap. xx.) Corp. Hum. Anat., lib. i., tab. xx., fig. 11, represents the vena cava opened, showing the foramen ovale, and beside it a tubercle (seen also in some animals) near the orifice of the coronary vein, which orifice in the sheep is said to be common to the coronary and the bronchial vein."
