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: [Sidenote: _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.