But having judiciously and solidly explain'd the _Systole_, he contents himself to ascribe the _Diastole_ to a motion of _Restitution_, which account gives me no Satisfaction: Because the _Systole_ being the proper, and (as himself confesses) the only motion of the Heart, a State of _Contraction_ seems to be the natural State, and consequently without External Violence, it shou'd have no _Diastole_ at all. This will appear more plain, if we consider the Circumstances of it, and its Motion, as a Muscle, with respect to other Muscles. That Contraction is the proper Action, and State of all Muscles, is evident from Experience of Fact, as well as Reason. For, if any Muscle be freed from the power of its _Antagonist_, it is immediately _contracted_, and is not by any Action of the Will, or Spirits, to be reduced to a State of _Dilatation_. Thus, if the _Musculi Flexores_ of any Joint be divided, the _Extensores_ of that Joint being by that means free'd from the contrary Action of their _Antagonists_, that Joint is immediately extended without any consent of the Will, and in that State it remains; and so _Vice versa_, if the _Extensores_ be divided. From whence it is plain, that the Muscles have no restitutive Motion, but what they derive from the Action of their _Antagonists_, by which they are balanc'd. Thus likewise the _Sphincters_ of the _Gula_, _Anus_ and _Vesica_, having no proper _Antagonists_, are always in a State of Contraction, and suffer nothing to pass them, but what is forced through them by the contrary Action of some stronger Muscles, which, though not properly to be call'd _Antagonists_, yet on all necessary Occasions perform the Office of such. That the Heart is a Muscle, furnish'd and instructed for Motion like other Muscles, is (in my Opinion at least) demonstrated beyond Contradiction by Dr. _Lower_ and others. And, as it is a _Solitary_ Muscle without any proper _Antagonist_, and not directly under the power of the Will, nor exercising _Voluntary_ Motion, it approaches nearest to the _Sphincter_ kind, which only has these Conditions in common with it. But in constant and regular Alternations of Contraction and Dilatation, it differs exceedingly from all the Muscles of the Body. This _reciprocal Æstus_ of the Heart has given the Learned abundance of trouble; who, finding nothing peculiar in the Structure, which shou'd necessarily occasion it, nor any _Antagonist_, whose re-action should produce it, have been extreamly perplex'd to find out the cause of it. But passing over the various Opinions of Authors, to avoid being tedious, I shall take notice here only of the very Learned Dr. _Lower_'s, in whose Account of the _Systole_, however solid and ingenious, I observe something deficient, and whose _Hypothesis_ of the _Diastole_ I think to be precarious and false.