This enlargement of their Cavities is very considerable, not only upon the score of the addition, which they receive in length thereby, but also upon the account of their Divarication. For whereas, when the Ribs are depress'd, and the Lungs subside, the Blood-vessels are not only contracted, (as I have already observ'd) but their Branches, which are exceeding numerous, approach one another, and lie in juxta-position, by which their Cavities are very much compress'd and streighten'd: When the Ribs are elevated, and the Lungs turgid with Air, not only the Fibres, by which their Coats in the opposite state were contracted, are extended; but those innumerable Vessels, which lying before in lines almost parallel upon one another, compress'd one another, making an acute Angle at their Junctures, are divaricated and separated from each other, and make an obtuse, whereby their Channels are widened.
Thus a passage is open'd to the Blood, from the Right Ventricle of the Heart to the Left, through the Lungs, to which it could not otherwise pass; and the opposition, which the Blood contain'd in that Ventricle, must otherwise necessarily have made to its Constriction, is taken off, and the Systole thereby facilitated.
Nor is that all. For the Diastole being caus'd (as I shall in the Sequel shew) by the force of the Blood rushing into the Ventricles, this Ampliation and Extension of the Pulmonary Artery is a sort of Check or Counterpoise to it, and prevents an endeavour towards two contrary Actions at once, which must necessarily frustrate both. For the Heart being a Springy, Compressible Body, whose proper Action, which is Contraction, depends on the influx of certain Fluids into its Fibers, or Substance; and containing besides a Fluid in its Ventricles, or great Cavities, in one of which is the Mouth of this Artery, the action of this Vessel must in great measure resemble that of a Syringe, whose extremity is immers'd in Water, the Enlargement or Expansion of the Chanels of the Artery answering the drawing of the Embolum, as the constrictive motion of the Muscle of the Heart does the pressure of the Atmosphere upon the Surface of the Water, the one making way for the fluid, and the other forcing it to follow, where the resistance is least. In this Sense we may allow a sort of Attraction to the Pulmonary-Artery, depending wholly upon the Action of the Intercostal Muscles and Diaphragm, which we must therefore confess to be very serviceable and instrumental in promoting the Systole of the Heart.
But if the Learned Author be deficient in his Account of the Systole; that is, if he has not observ'd all the Mechanism and Contrivance of Nature for the Contraction of the Heart; much less sufficiently has he accounted for the Diastole, or Dilatation of it, which he ascribes to a motion of Restitution of the over-strain'd Fibres, which yet he confesses are made for Constriction only. 'Tis true, he immediately after joins the Influx of the Blood as a concurrent Cause; but from the slight notice that he takes of it, 'tis plain, that he did not so much as dream of any great share it had in that Action. His Words are these:
De Corde,
Pag. 75.
Quin & (ut obiter hoc moneam) omnis motus contractione perficiatur, & Cordis Fibræ ad constrictionem solum factæ sint, apparet quoque Cordis motum totum in Systole positum esse; cumque Fibræ ultra tonum suum in omni constrictione eius tendantur, idcirco ubi nixus iste absolvitur, motu quasi restitutionis Cor iterum relaxatur, & sanguine à Venis influente rursus distenditur; à nullo enim cordis motu, nisi tensionem suam remittente, & ab irruente sanguine Diastole ejus libratis adeo viribus succedit.
I have transcrib'd the intire Paragraph, because it contains his whole Hypothesis of the Diastole, and all the notice that he takes of it through his whole Work. But how slender soever this may prove, it is the most substantial that I have any where met with, except a late one of Mr. Cowper, which is properly an Improvement of this, and shall be consider'd in the Sequel.
But if Contraction be the sole Action of these Fibres (as this Great Man confesses it to be) and as indeed it is of all Muscular Fibres, I wonder how so judicious a Writer came to slip into such an Absurdity, as to call their Distention (vulgarly but improperly call'd Relaxation) a Motion of Restitution. For from the Nature of those Fibres, and their disposition in the Structure of the Heart, the natural State of the Heart appears manifestly to be Tonical, and its Dilatation a State of Violence; and consequently, the Constriction is the true motion of Restitution, and the State to which it will spontaneously return, when the Force is taken off, which is the work of the Intercostal Muscles and Diaphragm.
Thus we are left still to seek for the true Cause of the Diastole, which seems to me to be the main and most difficult Phænomenon, relating to the Heart and the Circulation of the Blood. But in Mr. Cowper's ingenious Introduction to his Anatomy of Humane Bodies, I find the Share which Dr. Lower hints the Blood to have in that Action, further prosecuted, and improved into the main Instrument of the Dilatation of the Heart, wherein I agree intirely with him. But as to the manner, and reasons of its being so very instrumental, I can't be so perfectly of his mind.
The Heart (says this accurate Anatomist) of an Animal bears a great Analogy to the Pendulums of those Artificial Automata, Clocks and Watches, whilst its motion is performed like that of other Muscles, the Blood doing the Office of a Pondus.