But though the Hypothesis of Borellus may, in this Case, be found precarious or insufficient (a Misfortune that has befallen him in divers other Particulars) his Theory holds still good. At least it ought to be allow'd, in justice to his great Abilities and Exactness, till some Body convicts him of some material Error in his Calculations, which has not as yet been done by any Body, that I know of.
Supposing then the force of the Heart, and of the Muscular Coat of the Arteries, as likewise of the resistance, which they must overcome, to be computed with any degree of accuracy, there remains yet such a prodigious disproportion to be accounted for, as requires some more powerful Agent, than any yet assign'd, to make up the deficiency.
What assistance the Heart receives from the action of the Thorax towards the facilitating its Contraction, without which assistance there cou'd have been no Systole, has been already shewn. But neither the Intercostal Muscles, or Diaphragm, which are so instrumental in that part of its action, can contribute any thing to the Diastole; because they serve only to enlarge the Cavity of the Thorax, and thereby to open a passage to the Blood from the Heart, and promote its Constriction.
Whatever therefore the force is, that dilates the Heart, and is the cause of the Diastole, it must be equal to that of the Heart, the Intercostal Muscles and Diaphragm; to all which it acts as an Antagonist. I take no notice of the Serratus Major Anticus, and other Muscles; which have an obscure share in the Elevation of the Costæ, because as much may reasonably be deducted upon the account of the Obliquus externus Abdominis, and other Muscles; which having their Insertions on some of the lower Ribs are as instrumental towards the Depression of them, and so balance the Account. But the chief use of these is in violent Respiration: In ordinary Respiration their share is small.
Such a real Power (which may in the least be suspected of any share in this Action) is hard, perhaps impossible to be found in the Machine of any Animal Body; and yet without some such Antagonist, it is as impossible the Circulation of the Blood should be maintain'd. All the Engines yet discover'd within the Body, conspire towards the Constriction of the Heart, which is the State of Quiescence, to which it naturally tends. Yet we find it alternately in a State of Violence, that is, of Dilatation; and this upon necessity, because upon this Alternation depends all Animal Life.
Some sufficient Cause External must therefore be found, to produce this great Phænomenon; which Cause must be either in the Air, or Atmosphere, because we have no constant and immediate Commerce with any other Mediums.
Some great Physicians observing this, and that depriv'd by whatsoever means of Communication with the external Air, we became instantly extinct, have imagin'd, that in the Act of Inspiration certain purer parts of the Air, mixed with the Blood in the Lungs, and was convey'd with it to the Heart, where it nourish'd a sort of Vital Flame, which was the Cause of this reciprocal Æstus of the Heart. Others not quite so gross, rejecting an Actual Flame, have fancied, that these fine Parts of Air mixing with the Blood in the Ventricles of the Heart, produc'd an Effervescence which dilated it. But these Fancies have been long since exploded and condemn'd upon ample Conviction; and 'tis a Point yet undetermin'd, whether any Air does mix with the Blood at all in the Lungs, or not.
But supposing, that some Air may insinuate it self into the Pulmonary Vein, it can no other way dilate the Heart than by an Effervescence in the Left Ventricle, which wou'd not dilate the Right. But this Opinion is contradicted by Autopsie, and too laboriously confuted by others, to be brought upon the Stage again here.
There remains therefore only the gross Body of the Atmosphere to be considered, which is undoubtedly the true Antagonist to all those Muscles, which serve for ordinary Inspiration, and the Constriction of the Heart. This will appear more evidently, if we consider not only the Power, but the Necessity of its Action upon Animal Bodies, as well as the want of other sufficient Agents.
The Heart is a Solitary Muscle of very great strength, and the Intercostal Muscles and Diaphragm, which likewise have no Antagonists, are a vast additional Force, which must be balanc'd by the contrary Action of some equivalent Power or other. For, tho' the Action of the Intercostal Muscles be voluntary, that does not exempt them from the condition of all other Muscles serving for voluntary motion, which wou'd be in a State of perpetual Contraction, notwithstanding any Influence of the Will, were it not for the Libration of Antagonist Muscles. This Libration between other Muscles, is answer'd by the Weight of the incumbent Atmosphere, which presses upon the Thorax and other parts of the Body. And, as in all other voluntary Motions the influence of the Will only gives a prevalence to one of its two Powers before equilibrated, so here it serves to enable those Muscles to lift up a weight too ponderous for their strength not so assisted; and therefore as soon as that assistance is withdrawn, the Costæ are again depress'd by the meer Gravitation of the Atmosphere, which wou'd otherwise remain elevated through the natural Tendency of those Muscles to Contraction.