Note n — BOOK IV.

Mr. Cheselden, an ingenious and most accurate Anatomist, having somewhat particular in his Observations about the Circulation of the Blood through the Heart of the Fœtus, I shall present the Reader with some of his Observations, which he favoured me with the Sight of. The Blood (saith he) which is brought to the Heart by the ascending Cava, passes out of the right Auricle into the left, through a Passage called Foramen Ovale, in the Septum [common to them both] without passing through the right Ventricle (as after the Birth) while the Blood from the descending Cava passeth through the right Auricle and Ventricle into the pulmonary Artery, and thence into the Aorta through the Duct, betwixt that and the pulmonary Artery, called Ductus Arteriosus, whilst a small Portion of the Blood, thrown into the pulmonary Artery passeth through the Lungs, no more than is sufficient to keep open the pulmonary Vessels. Thus both Ventricles are employed in driving the Blood through the Aorta to all Parts of the Fœtus, and to the Mother too. But after the Birth, the Blood being to be driven from the Aorta through the Fœtus alone, and not the Mother too, one Ventricle becomes sufficient, whilst the other is employed in driving the Blood through the Lungs, the Ductus Arteriosus being shut up by means of the Alteration of its Position, which happens to it from the raising the Aorta by the Lungs when they become inflated. After that the Blood is thus driven into the Lungs, in its return it shuts the Valve of the Foramen Ovale against the Foramen it self, to whose Sides it soon adheres, and so stops up the Passage. The Ductus Arteriosus, or Ductus Arteriosus in Ligamentum versus, is seldom to be discerned in adult Bodies, but the Figure of the Foramen Ovale is never obliterated.