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The Septum, which feparates the two Auricles of the Heart, is picre'd thro' with an Aperture, call'd the Foramen Ovate ; and the Trunk of the Pulmonary Artery, a little af- ter it ha; left the Heart, fends out a Tube into the defend- ing Aorta, call'd the Communicating Canal.
The Fetus being born, the Foramen Ovale clofes, by de- grees, and the Canal of Communication dries up, and be- comes a fimple Ligament. See Foetus.
This Meehanifm once known, 'twas eafy to perceive its Ufe.
For while the Fetus is inclos'd in the Uterus, it receives no Air, but that little furnifh'd it by the Umbilical Vein : Its Lungs, therefore, can't fwell and fubfide as they do after the Birth, and after the free Admiflion of the Air. They continue almoft at reft, and without any Motion ; their Vef- fels are as it were full of themfelves, and don't allow the Blood to circulate, either in abundance, or with Eafe.
Nature, therefore, has excus'd the Lungs from the Paf- fage of the greateit Part of the Blood ; and has contriv'd the Foramen Ovale, by which Part of the Blood of the Ve- na Cava, receiv'd into the right Auricle, paffes into the left Auricle, at the Mouth of the Pulmonary Veins ; and by this means is found as far in its Journey as if it had pafs'd the Lungs.
But this is not all, for the Blood of the Cava, which, mif- fing the Foramen Ovale, paffes from the right Auricle into the right Ventricle ; being (fill in too great Quantity to pafs by the Lungs, whither 'tis driven thro' the Pulmonary Artery ; the communicant Canal intercepts part of it in the way, and pours it immediately into the descending Aorta. See Fo- ramen, &c.
This is the Doctrine of Harvey, Lower, and moft other Anatomifts ; but, M. ATery, of the Royal Academy, has made an Innovation in it.
He afligns another ufe for the Foramen Ovale ; and main- tains, that the whole Mafs of Blood brought from the Cava to the right Ventricle, paffes, as in Adults, into the Pulmo- nary Artery, whence Part of it is convcy'd by the communicant Canal into the Aorta ; and the reft brought from the Lungs by the Pulmonary Veins into the left Auricle, where 'tis di- vided into two Parts ; the one paffmg thro' the Foramen O- vale into the right Ventricle, without circulating thro' the Aorta and the reft of the Body ; the other Part pufh'd, as in Adults, by the Contraction of the left Ventricle, into the Aorta, and the whole Body of the Fetus.
The whole Queftion then turns upon this, viz. whether the Blood pafs thro' the Foramen Ovale from the right to the left Ventricle, or from the left to the right.
M. dlt Verney afferts the antient Opinion, againft M. ATe- ry, and maintains that the Foramen Ovale has a Valve fo difpos'd as to be open'd by the Blood driving into the right Ventricle, but fhut the more firmly by its pufhing into the left. M. ATery denies the Exigence of any fuch Valve.
Again, in an Adult, the Aorta being to receive all the Blood of the Pulmonary Artery, is found of the fame big- nefs. In a Fetus, the two Arteries are to receive unequal Quantities, which of the two Syftems foever be follow'd.
According to the common Opinion, the Aorta receiving more Blood than the Pulmonary, Ihould be bigger : accord- ing to the Opinion of M. Mery, the Pulmonary Artery Ihould be the bigger, as being efleem'd to receive a iarger Quan- tity of Blood.
To judge of the two Syflems thetefore ; it fhould feem there needed nothing but to determine which of the two Veffels were biggeft in a Fetus.
M. Mery always found the Pulmonary Artery half as big again as the Aorta ; and, on the other hand, M. T'aiivry, who feconded M. du Verney, produces Cafes where the Pulmonary is lefs than the Aorta : the Fafls on both Sides being cxamin'd by the French Royal Academy.
M. Tauvry adds, that tho the Pulmonary Artery Ihould be greater than the Aorta, yet this does not prove that more Blood paffes the firft than the fecond ; fince it may be ac- counted for from the Blood's preffing more (lowly towards the Lungs, which it finds fome difficulty to penetrate, and accordingly fwells, and is driven back.
M. Littrc, upon differing an Adult, in whom the Fora- men Ovale was (till open, and meafuring the Capacities of the Veffels on each fide, declares for M. Mery.
For the Source of the Circulation in the Fetus, Anatomifls are again divided.
The popular Opinion is, that during Geftation, the Arte- ries of the Uterus convey their Blood into the "Placenta, which is nourifh'd by it, and the Surplus convey'd into the Roots of the Umbilical Vein, which makes Part of the Na- vel-firing : thence 'tis carry'd to the Liver of the Fetus, where it enters the Vena Cava, and is thence convey'd to the right Ventricle of the Heart, and diftributed as before.
Again, the Blood brought from the Iliac Arteries of the Fetus, enters the Navel-ftring by the Umbilical Arteries ; thence paffes into the 'Placenta, where it is refum'd by the Veins of the Uterus, which carry it back again to the Mo-
ther; and perhaps alfo by the Roots of the Umbilical Vein which mix it afrefh with tho Blood of the Mother.
According to this Syilem therelbre, 'tis the Blood of the Mother that fupplies the Child; which is here only regarded as a diftincl Member, or Part of her Frame.
The beating of her Heart fends it a Portion of her Blood - and fo much of the Impulfe is preferv'd, as fuffices to main- tain that languid Circulation which a Fetus enjoys ; and in all probability, gives that feeble Pulfation obferv'd in the Heatt.
Other Anatomifls maintain, that the Fetus is only fup. ply'd with Chyle from the Glands of the Uterus ; which is further elaborated, and turn'd into Blood in the Veffels of the Fetus ; and circulates therein, without any further Com- munication with the Mother.
They allow of no reciprocal Circulation, excepting between the Placenta and the Fetus.
But the former Opinion is beft fupported : For the Pla- centa being feparated from the Uterus, during the time of Geflation, neither yields any Chyle, nor any thing but Blood. Befides, M. ATery has fhewn, that the Uterus has no Glands to furnifh any Chyle.
Two other Obfcrvations of the fame Author, confirm the popular Syftem : The inner Surface of the Uterus is lin'd with Veins ; and the outer Surface of the Placenta is not lin'd with any Membrane. Now as 'tis by thefe two Sur- faces that the two feem in fome meafure glu'd together; it looks as if they were only left without Membranes, for an immediate Communication between their Biood Vefiels.
Add to thefe a Fact whereof M. Mery was an Eye-wit- nefs : A Woman big with Child was kill'd by a Fall ; in the Cavity of her Belly were found feven or eight Pints of Blood ; all the Blood- Veffels being empty'd : the Child too was found dead ; but without the leafb Appearance of any Wound or Contufion : all its Blood-Veffels being empty of Blood, like thofe of the Mother. The Body of the Placenta itill adher'd to the whole inner Surface of the Uterus ; nor was there any extravafared Blood.
Now the Blood here had no other way to difcharge it felf, but by the Veins of the Uterus : whence it follows, that thofe Veins bring back to the Mother the Biood of the Fetus ; which alone eftablimes the whole Syftem. If the Circula- tion were only from the Fetus to the Placenta, and not alfo to the Mother; the dead Child would have had all its Blood.
Upon the whole, the Blood in the Lungs of the Fetus has none of the Advantages of Air or Refpiration ; which yet being neceffary, Nature, 'tis fuppos'd, takes care that it re- ceive a Portion of Air, mix'd together with its Mother's Blood, and rranfmitted to it by the Umbilical Veffels, to be diffus'd thro' the Body.
This is confirm'd hence, that by conftringing the Navel- firing very tight, the Child dies like a "Man (Irangled : which appears to be owing to nothing bur the want of Air. Add to this, that as foon as the Mother ceafes to refpire, the Fetus expires.
As to the Velocity of the circulating Blood, and the Time wherein a Circulation is compleated ; feveral Compu- tations have been made.
By Dr. Kelt's Account, the Blood is driven out of the Heart into the Aorta, with a Velocity which would carry it 52 Feet in a Minute. But this Velocity is continually abated in the Progrefs of the Blood thro' the numerous Sec- tions, or Branches of the Arteries : fo that e'er it arrive at the Extremities of the Body, its Motion is infinitely dimi- nifh'd.
The fame Author, upon a moderate Ratio of the Bran- ches of the Arteries to the Trunks, fhews, that the greateit Velocity of the Blood is to the leaft, in a greater proportion than of iooo,ooooo,ooooo,ooooo,ooooo,coooo,ooooo,ooooo,
to I.
The fpace of Time wherein the whole Mafs of Blood may ordinarily circulate, is varioufly determin'd.
Some of the lateft Writers ftate it thus : Suppofing the Heart to make 2000 Pulfcs in an Hour, and that at every Pulfe there is expell'd an Ounce of Blood ; as the whole Mafs is not ordinarily computed to exceed 24 Pounds, it mull be circulated 7 or 8 times over in the fpace of an Hour.
Circulation of the Sprits, or the Nervous Juice. That the Spirits circulate, is evine'd in the fame manner as fom e Authors chufe to prove the Circulation of the Shod ; viz. That as the Heart drives out every Hour three or four thou- fand Ounces of Blood, whereas, ordinarily, there is not above two thoufand in the whole Body ; there is a Neceffity for the Blood driven out, to return to the Heart, in order to fupply a Fund to be expell'd.
In like manner, 'tis Ihewn, that there is form'd each Hour a large quantity of Spirits, which are nothing but the more fubtil Parts of this Blood driven out from the Heart ' whence 'tis inferr'd, that thefe too mud circulate. See Spirit.
Tho Courfe they are fuppos'd to take is this.
The
