This Excellent Author, having by sound Arguments drawn from the Structure and Mechanism of the Heart, establish'd the Certainty of its _Muscular Motion_, rests satisfied, without taking notice of any Assistance, that the Heart receives from any other Part, except from the Brain, by the means of the eight pair of Nerves. [Sidenote: _Part_ 2_d_. _Prop._ 67. _Prop._ 73.] [Sidenote: _Prop._ 76.] 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 3000_l._ The _Obstacles_ to the Motion of the Blood thro' the _Arteries_ he esteems equivalent to 180,000_l._ which is 60 times as much as he rates the Force of the Heart at. Then deducting 45,000_l._ for the adventitious Help of the _Muscular Elastic Coat_ of the _Arteries_, he leaves the Heart with a Force of 3,000_l._ to overcome a resistance of 135,000_l._ 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_.