They that from the Placenta supply the Body of the Fœtus with Air, are as much distress'd as t'other; for they are forc'd to beg the Question twice, which, even when granted, will not answer their Ends. First, they suppose, that an intimate mixture or confusion of Air with the Blood, is necessary for the support of Animal Life, a Postulatum, which perhaps the former part of this Discourse may have render'd unnecessary; and next, that the Fœtus is supply'd with Air from, and its Blood mix'd with it in the Placenta.
But here again they fetch a Compass without necessity or proof. For if a mixture of Air were necessary to a Fœtus, why should it be separated from the Mother's Blood, and not rather both communicated together, since it is so much more easie and commodious? But neither does the Placenta seem to be instructed and provided for the separation of Air, but of a much grosser Fluid, destin'd to some other use, which Autopsie confirms: Yet, were both these Opinions true, they are however defective, and the Circular Motion of the Blood unprovided for.
By the way of Transfusion, this great Phænomenon is naturally accounted for, and the Ends, for which the other two Hypotheses were devis'd, might both be answer'd with more ease. For the Hysterick Arteries transmitting their Blood immediately to the Umbilical Vein, may very easily transmit such Nutricious Juices or Aerìal Particles, as are contain'd in the Blood, along with it, without depositing them by the way. By this means so much of the Impulse of the Mother's Blood is preserv'd, as suffices to maintain that languid Circulation which a Fœtus enjoys. For the Blood being driven through the Arteries of the Uterus into the Umbilical Vein, is convey'd directly to the Sinus of the Porta, and thence by a short and direct Passage through the Cava to the Heart; where passing through the Foramen Ovale to the Left Ventricle, and through the Canalis Arteriosus from the Right and Pulmonary Artery, it is all deliver'd without coming at the Lungs to the Aorta, and from thence again by the Umbilical Arteries to the Veins of the Uterus, making a sort of Epicycle to the main Circulation in the Mother.
As this Opinion is Favour'd by the Structure and Disposition of the Blood-Vessels on both Parts, so there is nothing in it difficult to be conceiv'd, or repugnant to Experience. Late Discoveries have made it appear, that the Arteries and Veins are continu'd Tubes, and that the latter contain nothing but what they receive from the former, and no Reason appears why we shou'd think this Method to be varied in the Placenta. On the other hand, if the Arteries of the Uterus were continued to the Veins of the same part, and those of the Fœtus in like manner, without communicating with each other, their Confluence in the Placenta seems to be altogether impertinent, and of no use, and the Umbilical Arteries and Vein fram'd for no other Service or Purpose, than to give the Blood room for an idle Sally.
Thus the Reasonableness of this old Opinion may be vindicated, but the Certainty of it rests upon stronger Proof. Mr. Cowper, to whose happy Industry we owe the Confirmation of many ancient Discoveries, and the Benefit of some new ones, has the Honour to re-establish this old, but long exploded Truth. For by pouring Mercury into a Branch of the Uterine Arterie of a Cow, that went into one of the Cotyledones of the Uterus, he fill'd those Branches of the Umbilical Veins, which went from that Cotyledon to the Navel of the Fœtus; which, with a part of the Uterus, he keeps prepared by him.
It would be a weak Objection, to alledge, That the Observation and Experiment being made on the Uterus of a Cow, the Inference would not hold from thence to a Woman, the one being Glanduliferous, and the other Placentiferous; since every one of these Cotyledones, or Uterine Glandules, is in all respects a little Placenta, and all the difference between them is in number, name, and magnitude. Why Ruminants differ in this Particular from other Viviparous Animals, is beside the Subject of our present Enquiry. But the great Flux of Blood, which constantly follows upon drawing the Placenta from Women (which is frequently so great as to cost them their Lives) is as plain a demonstration to Reason of the Continuity of the Vessels, as Mr. Cowper's Experiments is to the Eye.
I have heard it objected by very Learned Men, that if there were such a Continuity of Vessels, and such Transfusion of Blood, the Fœtus must necessarily perish through loss of Blood, upon the separation of the Placenta from the Uterus; but that, on the contrary, no visible Flux of Blood does follow while the Fœtus continues wrapt in the Membrane, in which Condition it may be kept alive some Hours. To this it may be answer'd, that the Circulation in the Fœtus, being deriv'd from the Mother, may be suppos'd wholly to cease upon the cutting off the Communication between them, till it is again renew'd more forcibly by Respiration. But if we allow the motion already impress'd upon the Blood to be sufficient to keep it going a little while; yet it must needs be so exceeding languid, that the meer resistance of the External Air must be more than enough to hinder any Efflux of Blood from a Fœtus before Respiration. How long Life may be preserv'd without an actual Circulation of the Blood, is a Question not of this place. But we have been convinc'd by many and notorious Observations and Experiments, that Life has been recover'd a long time after all tokens of Respiration, Circulation, or even Life it self, have disappear'd; so that we can't think the first Solution either impossible or improbable.
I expect to be told, that in the early Days of Gestation in Viviparous Animals, there is no Placenta, or any Adhesion of the Umbilical Vessels to any part of the Mother, and consequently no such Transfusion; and that in Oviparous there is no continuity, or communication of Vessels of any kind, during the whole time of Incubation.
But these Objections carry neither the Weight nor Difficulty along with them, that they may be suppos'd to do; for in those Days there is neither Blood or Blood-Vessels, and consequently there can be no Circulation of the Blood; and the Embryo, of what Species soever, is no more than a Vegetable at that time; nor does the Fœtus of any Viviparous Creature enjoy any Circulation, or shew any signs of Animal Life, till after those Vessels, as well as others requisite to the Circulation, are compleated.
It must be confess'd, that Oviparous Animals are denied the benefit of this Communication; but that want is sufficiently compensated by a peculiar Mechanism, which directly answers the ends of Respiration, and the pressure of the Atmosphere upon the Fœtus. There is at the obtuse end of an Egg a small Cavity fill'd with Air, which is the succedaneous Instrument to the Respiratory Organs. For as soon as the Contents begin to be warm'd by the Incubation of the Hen, or any analogous Heat of Furnace or Dunghill, the several Humours of the Egg require a fermentative motion, and the Air contain'd in the Cavity or Vesicle, at the obtuse end of the Egg, is rarefied, and the Vesicle extended and enlarg'd, and consequently the other Contents are comprest; to which the fermentative motion naturally resists. But both Bodies being as well compressible as dilatable, and both having an expansive motion imprest upon them by Incubation, the Compression and Renitency will be mutual, but varied in degree, according as either, through the variation of Circumstances, shall prevail. By this means, an Alternation of Compression and Dilatation will be produc'd in both, answering the respiratory motion, by which a motion will be communicated, which, as soon as the Organs by which it should be regulated are compleated, will in the Body of the Pullus be regular and circulatory.