HEA
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HEA
Some late Authors, upon considering the Structure and Difpofition of the fpiral Fibres, chufe rather to make the Heart a double Mufcle, or two Mufcles tyed together. — In Effect, the two Ventricles, with their refpective Auricles, are found two diftinct Bodies, two Veflcls, or Cavities, which may be feparated, and yet ftill remain Veflcls 5 the Septum, which was fuppofed to belong only to the left, being now found to confift of Fibres derived from them both. — To fay no more, the two Ventricles, according to M. Wiii/tow, are two fevcral Mufcles united together, not only by the Septum, but by feveral Plans of Fibres ari- sing from the exterior Bafe of the Heart, and meeting at the Apex, which entering the left Ventricle, lines the Parietes, £?c.
The Heart has alfo its proper Blood Veffels, viz. two Arteries fpringing from the Entrance of the Aorta ; and one larger Vein, with one or two leffer ; all which, from their encompaffmg the Heart, are called Coronaria, See Coronary.
The Nerves of the Heart and its Auricles, come from a Plexus of the Tar Vagum, called by Willis, "Plexus Cardiacus. See Nerve and Plexus.
It has alfo Lymphce duffs, which carry the Lymph from the Heart to the "Thoracic Duct. See Lymph deduct.
The TJfe of the Heart, and its appendant Auricles, is to circulate the Blood through the whole Body : In Order to which, they have an alternate Motion of Contraction and Dilatation. — By the Dilatation, call'd the 2)iafole, their Cavity is open'd, and their internal Dimeniions en- larged to receive the refluent Blood from the Veins 5 and by their Contraction, call'd the Syftole, their Cavity is flirunk, and their Dimeniions leffen'd, to expel the Blood again into the Arteries. See Systole and Diastole.
It mufl be added, that thefe alternate Motions of the Heart and Auricles, are oppolite in Time to each other; the Auricles bein« dilated whilit the Heart is contracted again ; and contracted whilft it is dilated, to drive the Blood into it. See Auricle.
By Means of the right Ventricle, the Blood is driven through the Pulmonary Vein, from which being received into the Pulmonary Artery, it is returned to the left Ven- tricle ; from which by the Aorta, it is difkibuted all over the reft of the Body, and thence returned to the right Ventricle by the Vena Cava ; fo making an entire Circu- lation through the whole Body. See Circulation.
Scheuchius fpeaks of a Man who had no Heart ; but Molinetti denies it : As alfo that there ever were two Hearts found in any Man, tho' the Cafe be common in divers Infects, which have naturally feveral Hearts 5 wit- nefs Silk-Worms, which have a Cham of Hearts running the whole length of their Bodies. But we have incontestable Inftances of two Hearts, even in the fame Man. — There have been Hearts found all gnawed and devoured with Worms : Muretus open'd the Hearts of fome "Banditti, which were all Hairy, or at leaft invefted with a To- raentura, or Down. — ■ And what is Hill more extraordinary, we have Accounts of Perfons in whom the Heart has been found inverted, or turned upfide down 5 particularly in a Woman hang'd fome Time ago in Saxony, and a Man hang'd at 1>aris. Journ, des Scav.
Timorous Animals have always larger Hearts than cou- rageous ones ; as we fee in Deer, Hares, Affes, &c. • — ■ There is a Bone found in the Bafis of the Hearts of certain Animals, particularly Deer ; which feems to be no other than the Tendons of the Fibres of the Heart indurated and ofiified.
Hiftorians relate, that Pope Urban VIII. being open'd after his Death, was found to have fuch a Bone in his Heart. The Cafe is very ufual in the Trunk of the Aorta, which fprings from the Heart. See Aorta.
Many amphibious Creatures, as the Frogs, &c. have but one Ventricle in the Heart. — The Tortoife is laid by the French Academifts to have three Ventricles : M. Sujjiere charges this on them as a Miftake 5 and maintains it has but one. The Point is fcarce yet afcertain'd. Mem. de VAcad. An. and <Philofoph. Tranfatl. N°. 528. ^theory of the Motion of the Heart.
The Principle of Motion in the Heart, or the Power from which its alternate Contraction and Dilatation arifes, has been greatly controverted among the late Phyficians and Anatomifts.
The Expulfion of the Blood out of the Ventricles, argues a very considerable Motion in the Part. — The motive Powers 'tis certain, mull furmount the Refinance made to it; and according to Sort's Computation, the Reiiftance made to the Motion of the Blood thro' the Arteries, is equal to 180000 Pounds, which therefore are to be re- moved by the Heart ; or elfe the Circulation mufi ceafe. Now, whence comes the Machine of the Heart to have fuch a Power ? And after the Expuliion, what other Power
is it that furmounts the former, and reftores the Part to its State of Dilatation, to produce a reciprocal JEJius*
The whole Affair remain'd in the utmoft Darkncfs and Uncertainty, till Dr. Lower's excellent Treatife 2)e Corde, where the Mcchanifm, whereby the Contraction, or Syftole, is effected, was admirably explain'd. — The Caufe of the Di- latation, or Diaftole, which Dr. Lower had in great Meafure ovcr-look'd, has fince been happily fupplicd by Dr. 'Drake. That the Heart is a Mufcle, furnilhed and initructed for Motion like other Mufcles, is abundantly demonftrated by Dr. Lower and others; and as it is a folitary Mufcle, without any proper Antagonist, and not directly Under the Power of the Will, nor excrciiing voluntary Motion, it ap- proaches ncareft to the fphinclcr Kind, which has thefe Conditions in common with it. See Sphincter.
But in conttant, and regular Alternations of Contraction and Dilatation, the Heart differs exceedingly from all the other Mufcles of the Body. See Muscle.
This Alternation has given the Learned Abundance of Trouble, who finding nothing peculiar in the Structure, which iliould neceffarily occasion it 5 nor any Antagonilt, whofe Re-action fhould produce it; have been extremely perplexed to find out the Caufe.
That, Contraction is the proper Action and State of all Mufcles, is evident both from Reafon and Experience. For if any Mufcle be freed from the Power of its Antagonist, it immediateiy contracts, and is not by any Action of the Will or Spirits, to be reduced to a State of Dilatation. ■ — ■ Thus, if the Flexors of any Joint be divided ; the Ex- tenfors of that Joint being by that Means freed from the contrary Action of their Antagoniits, the Joint is imme- diately extended, without any Content of the Will, and in that State remains ; and fo vice verfa, if the Extenfors be divided.
Hence it appears, that the ordinary Mufcles have na reftitutive Motion, but what they derive from the Action of their AntagoniHs, by which they are ballanced. — Thus the Sphincters of the Anus, Vefica, ike. having no proper Antagonists, are always in a State of Contraction, and fuffer nothing to pafs them, but what is forced through them by the contrary Action of fome ftrongcr Mufcles, which, tho' not properly called Antagonifis, yet on ail ne- ceffary Occafions, perform the Office of fuch. Sec Anus, Vesica, £?tf.
We have here, then, an adequate Caufe of the Con- traction of the Heart, viz. the natural Vis Matrix of mufculous Fibres, whofe proper Nifus, or Tendency, is to contract thcmfelves. Sec Muscular and Fibre.
It may be added, however, that tho' the mufcular Fibres of the Heart, acted by the Nerves, be the imme- diate Inftrument of its Confiriction, or Syflole, as is Shewn by Dr. Lower, yet is there another Caufe which contributes not a little thereto, and which Dr. Lower ovcr-look'd, vi&+ the intercoftal Mufcles and Diaphragm, which aid and facilitate this Contraction, by opening a Paffage for the Blood through the Lungs, which denied, would be an in- vincible Obftacle. < — Add, that the pulmonary Artery and Vein, fpreading themfclvcs throughout all the Divifions and Sub-divifions of the "Bronchia of the Lungs, and bcin<* as it were co-extended therewith, mull fuffer the like aC teration of fbperficial Dimeniions, as the Bronchia do in the Elevation and Depreffion of the Cofia. So that while the Ribs are in a State 0? Depreffion, whether before Commerce with the external Air, or after, the annular Car- tilages of the Bronchia fhrink one into another, and by that Means their Dimeniions are exceedingly contracted : In Conformity to which Condition of the Bronchia, the pul- monary Artery and Vein mull likewife, either by Means of their mufcular Coats, contract themfelves to the fame Dimeniions, or lie in Folds or Corrugations ; which is lefs probable. — On the other Hand, when the Ribs are ele- vated, and the Diaphragm bears downwards, the Air rufhing into the Lungs, Ihoots out the cartilaginous Rings, and divaricates the Branches of the Trachea^ and by them extends the feveral Divifions of the pul- monary Artery and Vein, and thereby lengthens and en- larges their Cavities : And this their alternate Action, will be continued, and imparted to the Hearty from which they arife.
Thus is a Paffage open'd to the Blood from the light Ventricle of the Heart, to the left, through the Lungs, to which it could not otherwife pals 5 and the Opposition which the Blood, contained in that Ventricle, muff other- wife neceffarily have made to its Confiriction, is taken off$ and the Syftole thereby facilitated. See Systole.
For the Diaftole, or Dilatation of the Heart, Dr. Lower contents himfelf to afcribe it to a Motion or Rcftimtion of the over-ftrained Fibres. — His Words are, * Since all ( its Motion confifts in its Contraction ; and the Fibres ' of the Heart are made for Confiriction only 5 'tis evi-
- dent, all the Motion of the Heart is in its Syitoie ; But,
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