Dr. _Lower_, and Mr. _Cowper_, deliver their Opinions of the Cause of the Dilatation of the Heart so very short, and without any Arguments to support them, that by exposing them naked, they seem rather to discourse of it transiently, as Men oblig'd by the Nature of their Subjects to say something of it, than solicitous to give any full or satisfactory Account; and therefore I shall proceed no farther upon them here. 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.