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 _Boyle_'s Engine; where, as soon as the Air is withdrawn, and the _pressure_ thereby taken off, the Intercostal Muscles and Diaphragm are contracted, and the Ribs elevated in an instant, and can't by any Power of the Will be made to subside, till the Air is again let in to bear them forcibly down. [Sidenote: _De Respirationis Organis & Usu._] It were scarce worth while to take notice here of a Mistake of the Learned Dr. _Willis_, were it not for the great Authority of the Man, which is almost sufficient to keep Error in countenance. The Doctor having observ'd, that the Fibres of the _External_ and _Internal Intercostal_ Muscles ran in a contrary order, as it were, decussating each other, takes occasion from thence to fansie, that there was an opposition in their Office; and that as the _External_ serv'd to raise up the Ribs, the _Internal_ drew them down again, forgetting at that time, That, when a contractile Body is fasten'd at the several ends to Points unequally moveable, let the Contraction happen in what part or manner soever, the more moveable Point must be drawn towards the less moveable: By which Rule, whether _External_ or _Internal Intercostals_ be contracted, the lower Ribs will be forc'd to approach the upper, that is, be rais'd up. As in the Elevation of the _Costæ_, the Blood, by the passage that is open'd for it, is in a manner solicited into the Lungs; so in the Depression of them, by the subsidence of the Lungs, and the Contraction of the Blood-Vessels, both which are consequent thereof, the Blood is forcibly driven, as it were with an _Embolum_, through the Pulmonary Vein into the Left Ventricle of the Heart. And this, together with the _general Compression_ of the _Body_ by the _weight_ of the _Atmosphere_, which surrounds and presses upon the whole Surface of it, is that Power which causes the Blood to mount in the Veins, after the force impress'd upon it by the Heart is broken and spent, and which is sufficient to force the Heart from its natural State to Dilatation.