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_._ This Explication, being but a Simile without a distinct application to Particulars, is beside so very short, that I can at best but give a conjecture at the meaning; which if I mistake, I shall deserve to be excused, and expect to be better inform'd. By the _Bloods_ doing the Office of a _Pondus_, I suppose he means, that the Blood contributes in the same manner to the motion of the Heart, as the _Weights_ do to that of the _Pendulum_ of a _Clock_. If so, the Blood, according to him, must be the Instrument of _Constriction_; and _Dilatation_ must be the _Natural_ State, or _Spontaneous_ Motion, to which it wou'd, when under no violence, return; the contrary of which, I presume, will appear e're I have done. But if he means, that the _Blood_ in its reflux, by _gravitating_ on the _Auricles_ and _Ventricles_, dilates and expands 'em, acting therein as a _Counterpoise_ to its contractions as a Muscle, I cou'd wish his Design had not bound him up to so narrow a compass, and that he had given us an explication at large of so abstruse and so important a _Phænomenon_: Because the _Specifick Gravity_ of the Blood seems to me a cause by no means alone adequate to the effect, which it is here suppos'd to produce.