The _Pulmonary_ Artery rises from the _right_ Ventricle of the Heart, and runs in one Trunk, till it comes to the _Aspera Arteria_, where it is divided, and sends a Branch along with each Division of the _Aspera Arteria_, according to all the minutest Subdivisions, of which it is likewise subdivided, accompanying all the _Bronchi_, in their whole progress through the Lungs. The _Pulmonary_ Vein, which empties itself into the _Left_ Ventricle of the Heart, spreads it self on the _Aspera Arteria_ and _Bronchi_, in the same manner that the Artery does. The necessary confluence of this Disposition if, that this Artery and Vein being co-extended with, and fasten'd to the _Bronchi_, must needs suffer such alteration of _Superficial_ Dimensions, as the _Bronchi_ do in the _Elevation_ or _Depression_ of the _Costæ_. While the Ribs are in a State of _Depression_ (whether before Commerce with the External Air or after) the _Annular Cartilages_ of the _Bronchi_ shrink one into another, and by that means their _Dimensions_ are exceedingly contracted. In conformity to this condition of the _Bronchi_, the _Pulmonary_ Artery and Vein must likewise, either by means of their _Muscular_ Coats, contract themselves to the same _Dimensions_, or lye in _Folds_ or _Corrugations_, which is less probable. On the other hand, when the Ribs are elevated, and the _Diaphragm_ bears downward, the Air rushing into the Lungs, shoots out the _Cartilaginous_ Rings, and _divaricates_ the Branches of the _Trachea_, and by them extends and divaricates the several Divisions of the _Pulmonary_ Artery and Veins, and thereby lengthens and enlarges their Cavities. This enlargement of their Cavities is very considerable, not only upon the score of the addition, which they receive in length thereby, but also upon the account of their _Divarication_. For whereas, when the Ribs are depress'd, and the Lungs subside, the Blood-vessels are not only contracted, (as I have already observ'd) but their Branches, which are exceeding numerous, approach one another, and lie in _juxta-position_, by which their Cavities are very much compress'd and streighten'd: When the Ribs are elevated, and the Lungs turgid with Air, not only the Fibres, by which their Coats in the opposite state were contracted, are extended; but those innumerable Vessels, which lying before in lines almost parallel upon one another, compress'd one another, making an _acute_ Angle at their Junctures, are divaricated and separated from each other, and make an _obtuse_, whereby their Channels are widened. Thus a passage is open'd to the Blood, from the _Right_ Ventricle of the Heart to the _Left_, through the Lungs, to which it could not otherwise pass; and the opposition, which the Blood contain'd in that Ventricle, must otherwise necessarily have made to its Constriction, is taken off, and the _Systole_ thereby facilitated.