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to lift up a weight too ponderous for their strength not so assisted; and therefore as soon as that assistance is withdrawn, the Costæ are again depress'd by the meer Gravitation of the Atmosphere, which wou'd otherwise remain elevated through the natural Tendency of those Muscles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.
This is evidently prov'd from the Torricellian Experiments, and those made upon Animals in Mr. Boyle's Engine; where, as soon as the Air is withdrawn, and the pressure thereby