HEA
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HEA
feut this, While included in the Womb, fecms to have little more than a vegetative Life; and ought fcarce to be rcckon'd among the Number of Animals : And were it not for that fmall Share of mufcular Motion which it t-xercilcs in the Womb, it might, without Abfurdity, be accounted a Graft upon, or Branch of the Mother. See Foetus, Embryo, oc.
One Difficulty we muft not here conceal, which will lie againft the whole Doctrine, viz. That the Hearts of fe- deral Animals have been found to beat regularly and ftrongly, even in Vacuo ; not left fo than in the Air. Witncls the Hearts of Progs, tried by Mr, Soyle. 'Philo- fofh. ZtanfaB. N°. 6z.
Eftimate of the Force of the Heart.
The Quantity of the Force of the Heart has been va- riously citimated, and on various Principles ; by various Authors, particularly SorclH, Morland, Keill, Jurin, &c.
The Force of the Heart may be defined by the Motion wherewith the Heart contracts ; or by the Motion of a Weight which being oppofed to the Blood at its Exit out of the Hearty will jutt balance and {top the fame. — >This we have no Way of coming at a- priori ; the internal Structure of the Part, and the Nature and Power of the contracting Caufe being but imperfectly known : So that the only Means remaining, is to eftimate it by the Effeas.
All the Action of the Heart confifts in the Contraction of its Ventricles : The Ventricles contracting, ftrike or prefs upon the Blood, and communicating Part of their Motion thereto, drive it out with Vehemence where the Paf- fage is open. — The Blood thus protruded into the Aorta, and pulmonary Artery, preffes every Way ; partly againft the Coats of the Arteries, which by the laft Diaftole had been left in a collapfed, flaccid State ; and partly againft the anterior Blood moving on too (lowly before it. By fuch Means the Coats of the Arteries are gradually di- ftended, and the antecedent Blood has its Motion accele- rated. ' — By the Way it may be obferved, that the more flaccid the Arteries are, the lefs Refiftance will they make to a Dilatation ; and the more they are dilated, the more ftrongly will they refill: a further Diftraction ; fo that the Force of the Blood, at its Egrefs out of the Heart, is at firft fpent in diftending the Arteries, more than protruding the antecedent Blood ; but afterwards more in protruding the Blood, than in diftending the Arteries.
Sorelli, we have already obferved, in his Oecon. Animal, makes the Obftacles to the Motion of the Blood thro' the Arteries, equivalent to i8o,cco Pounds ; and the Force of the Heart it felf, only equal to 3,000 Pound, which is only ti of the Opposition it has to overcome. Then, deducting 45,000 Pound for the adventitious Help of the mufcular E'laftic Coat of the Arteries ; he leaves the Heart with a Force of 3,000 Pound, to overcome a Refiftance of 135,000 Pound; that is, with 1 to remove 45; which he fuppofes it enabled to do by Virtue of Percuffion.
But had he proceeded in his Calculation to the Veins, which he allows to contain Quadruple the Quantity of Blood as the Arteries do, and to which this Energy of Percuffion either docs not reach at all, or very languidly ; he would readily have feen the Syftem of Percuffion to b,e inefficient.
But his Calculus it felf is alfo found to be faulty ; the Fcrce afcribed by him to the Heart being immenfely too great. — ■ Dr. Jurin ftiews, that had he not made a Miftake in the Computation, the Refinance which the Heart has to overcome, muft have come out, on his own Principles, much greater ; and inftead of 135,000 Pound, would have been 1, 076,000 Pound, which tranfeends all Probability.
The great Faults in his Solution confift, according to Dr. ffurin, in his eftimating the motive Force of the Heart by a Pondus at reft ; in fuppofing the whole Weight fuftain'd by a Mufcle, in one of his Experiments, to be fuftain'd wholly by the contracting Force thereof; in a£- fuming Mufcles, equally heavy, to be of equal Force : In fuppofing the utmoft Force of the Heart exerted at every Syftole, &c
Dr. Keill, in his Eflays of the Anim. Oecon. firft ventured to fet afide Sorelli's Calculus ; and fubftituted another, almoft infinitely fmaller, in its Stead. His Method of eftimating the Force of the Heart is as follows : — ■ Hav- ing the Velocity wherewith a Fluid flows out at an Orifice, without meeting any Refiftance from an anterior Fluid ; the Force which produces that Motion is thus A determined. Let the Line AB be the heighth from which a falling Body will acquire a Velocity equal to that wherewith the Fluid flows out at the Orifice ; then is the Force which produces the Motion of this Fluid, equal to the Weight of a Cylinder of the fame Fluid, whofe B Bafe is equal to the Orifice, and Weight equal to
2 A B : Ccroll. z. "Prop. $6. Lib. II. Newton Princip.
Now, the Blood flowing out of the Heart, is refifted in its Motion by the anterior Blood in the Arteries and Veins, and therefore cannot flow with all the Velocity the Force of the Heart will give it ; part of that Force being fpent in overcoming the Refinance of the Mafs of Blood. If therefore we knew how much the Velocity of the Bloud is diminished by this Refiftance, or what Proportion the Velocity of the Blood refilled, has to the Blood driven out, and not refifted ; having already determined the Ve- locity of the Blood as it is refifted, we might eafily re- collect the Velocity, by which the Blood would flow were it not refifted ; and from thence, the abfolute Force or the Heart.
To find this, the Author made the following Experi- ment. — Having uncovcr'd the Iliack Artery and Vein in the Thigh of a Dog, near his Body; and paired convenient Ligatures under them ; he open'd the whole Diameter of the Veflels, and received the Blood which run from it in the Space of ten Seconds : The fame was afterwards done by the Artery, for the fame Space of Time ; and both the Quan- tities of the Blood were exactly weighed. The Experiment was repeated for the greater Security : Till, the Quantity of Blood from ihe Artery, was found at a Medium, to that from the Vein in the fame Space of Time, nearly as 7 ± to 3.
Now, the Velocity of the Blood in the Iliack Artery, -'fo near the Aorta, mull be nearly the fame with that in the Aorta it felf ; and confequently the Velocity with which it flows out of the Iliack Arrery cut afunder, is the fame with which it would flow out of the Heart unrefifled : Or, the Blood runs thro' a Wound in the Iliack Artery, with all the Velocity it received from the Heart. Now all the Blood which runs along the Iliack Artery, returns again by the Iliack Vein ; and confequently, the Quantities of Blood which pafs thro' both in the fame Space of Time, are equal. The Quantity of Blood, therefore, which runs out of the Iliack Vein cut afunder, is the fame which ran thro' the Iliack Artery, before it was cut, in that Space of Time. Having, therefore, the Quantity which runs thro' the Iliack Artery when it is cut, and when it is not cur, we have their Velocities ; for the Velocity of a Fluid, running thro' the fame Canal, in equal Spaces of Time, is directly as their Quantities : But the Velocity of the Blood, when the Artery is cut, is equal to that it receives by the full Force of the Heart ; and the Velocity when it is not cut, is that Velocity with which the Blood moves thro' the Aorta, refifted by the anterior Blood : And therefore thefe two Velocities are to one another as 7 ~ to 3.
If now the Heart throws out two Ounces of Blood every Syftole, which is a probable Affumption, then the Blood moves thro' the Aorta at the Rate of 156 Feet in a Minute ; and therefore the abfolute Velocity, wherewith the Blood would be forced into the Aorta, did it find no Refiftance, is fuch as would make it move 390 Feet in a Minute, or 6 \ Feet in a Second.
We now proceed to enquire, what is the Height from which a falling Body will acquire this given Velocity - for this Heighr doubled, gives the Length of the Cylinder whole Bafe is equal to the Orifice of the Aorta, and. Weight equal to the abfolute Force of the Heart.
It is known by Experiment, that the Force of Gravity will make a Body move 50 Feet in a Second, which is the Velocity it acquires in falling thro' 15 Feet; and there- fore this Velocity is to the Velocity of the Blood flowing without Refiftance into the Aorta, as 30 to 6. 5 : But becaufe the Heights from which Bodies acquire given Ve- locities, are as the Squares of the Velocities, that is, as 900 to 42.25 ; theretbre, as 900 to 42. 25, fo is 15 to 0.74. > — This Height doubled, gives 1.48, or in Inches 17. -j6, which is the Height of a Cylinder of Blood, whofe Bafe is equal to the Aorta, which we have fuppofed to be equal to o. 4187 ; and therefore the folid Content is 7. 436112, the Weight of which is equal to the abfolute Force of the Heart.
This Weight is five Ounces, whence the Force of the Heart is found eq?ial to the Weight of five Oimces.
The fame Author, by another Method .of Calculating, from the Laws of Projectiles, finds the Force of the Heart almoft equal to 8 Ounces, which tho' fomewhat more than was before determined, yet the Difference is of no fmall Mo- ment in Rcfpect of Sorelli's Account ; the great Failing in whofe Calculation Dr. Keill takes to arife from his not diftingui filing between the Blood at reft, and already in Motion. — The Force of the Heart, 'tis certain, is not employ'd in giving Morion to any Quantity of Blood at reft, but only to continue it in Motion ; how that Motion firft arofc, ftems out of human Capacity to determine. 'Tis demonftrable, that if the Refiftance of the Blood al- ways bore the fame Proportion to the Force of the Heart
