The Accurate Borellus, in his Oeconomia Animalis, computes the Motive Power of the Machine of the Heart to be equal to, or to surmount that of a Weight of 3000l. The Obstacles to the Motion of the Blood thro' the Arteries he esteems equivalent to 180,000l. which is 60 times as much as he rates the Force of the Heart at. Then deducting 45,000l. for the adventitious Help of the Muscular Elastic Coat of the Arteries, he leaves the Heart with a Force of 3,000l. to overcome a resistance of 135,000l. that is, with 1, to remove 45.
This stupendous Effect he contents himself to ascribe to the Energy of Percussion. But, had he proceeded in his Calculation to the Veins, which he allows to contain constantly a quantity of Blood, quadruple to the Contents of the Arteries, and to which this Energy of Percussion does either not reach at all, or but very languidly, he might probably have seen a necessity for some other Expedient to remove so insuperable a Difficulty.
But not to insist rigorously on the Exactness of this Calculation, (though the great Abilities of the Author in this way, and his Ingenuity and Modesty, are a sufficient Warrant for the Accuracy of his Computations, and the Fidelity of his Accounts) we may allow a much greater Deduction, than would be justifiable, without lessening the Difficulty. But this Account I have taken notice of purely for the sake of the Calculation, which may be of use in the Sequel; the account it self being in other respects more defective than Dr. Lower's, to which we will return.
The Doctor, notwithstanding his great Sagacity, appears (to me) to have overlook'd something of very great moment, and importance in the explication of the Action of the Heart. For, tho' it should be granted, that the Muscular Fibres of the Heart acted by the Nerves, are the immediate Instruments of its Constriction or Systole, yet it must not be denied, that the Intercostal Muscles and Diaphragm are of great service to aid and facilitate this Contraction, by opening a Passage for the Blood through the Lungs, which denied would be an invincible Obstacle.
Neither do they promote it that way only. The manner how they farther assist the Heart in its Contraction, will appear manifestly, if we consider the different Posture, Situation, and Capacity of the Blood-Vessels of the Lungs in the several times of Elevation and Depression of the Costæ.
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.