Some sufficient Cause External must therefore be found, to produce this great _Phænomenon_; which Cause must be either in the _Air_, or _Atmosphere_, because we have no constant and immediate Commerce with any other _Mediums_. Some great Physicians observing this, and that depriv'd by whatsoever means of Communication with the _external_ Air, we became instantly extinct, have imagin'd, that in the Act of Inspiration certain purer parts of the Air, mixed with the Blood in the Lungs, and was convey'd with it to the Heart, where it nourish'd a sort of _Vital Flame_, which was the Cause of this reciprocal _Æstus_ of the Heart. Others not quite so gross, rejecting an _Actual Flame_, have fancied, that these fine Parts of Air mixing with the Blood in the Ventricles of the Heart, produc'd an _Effervescence_ which dilated it. But these Fancies have been long since exploded and condemn'd upon ample Conviction; and 'tis a Point yet undetermin'd, whether any Air does mix with the Blood at all in the Lungs, or not. But supposing, that some Air may insinuate it self into the _Pulmonary_ Vein, it can no other way dilate the Heart than by an Effervescence in the Left Ventricle, which wou'd not dilate the Right. But this Opinion is contradicted by _Autopsie_, and too laboriously confuted by others, to be brought upon the Stage again here. There remains therefore only the _gross Body_ of the _Atmosphere_ to be considered, which is undoubtedly the _true Antagonist_ to all those Muscles, which serve for ordinary Inspiration, and the Constriction of the Heart. This will appear more evidently, if we consider not only the Power, but the Necessity of its Action upon _Animal_ Bodies, as well as the want of other sufficient Agents. The Heart is a _Solitary_ Muscle of very great strength, and the _Intercostal_ Muscles and Diaphragm, which likewise have no _Antagonists_, are a vast additional Force, which must be balanc'd by the contrary Action of some equivalent Power or other. For, tho' the Action of the _Intercostal_ Muscles be voluntary, that does not exempt them from the condition of all other Muscles serving for _voluntary_ motion, which wou'd be in a State of perpetual Contraction, notwithstanding any Influence of the Will, were it not for the Libration of _Antagonist_ Muscles. This Libration between other Muscles, is answer'd by the _Weight_ of the incumbent _Atmosphere_, which presses upon the _Thorax_ and other parts of the Body. And, as in all other voluntary Motions the influence of the Will only gives a prevalence to one of its two Powers before equilibrated, so here it serves to enable those Muscles to lift up a weight too ponderous for their strength not so assisted; and therefore as soon as that assistance is withdrawn, the _Costæ_ are again depress'd by the meer _Gravitation_ of the _Atmosphere_, which wou'd otherwise remain elevated through the natural Tendency of those Muscles to Contraction.