VOr.. XXIir.3 PHILOSOPHICAL TRANSACTIONS. 703 



must be the instrument of constriction, and dilatation must be the natural 

 state, or spontaneous motion, to which it would, when under no violence, 

 return ; the contrary of which will hereafter appear. 



But if he means, that the blood in its reflux, by gravitating on the auricles 

 and ventricles, dilates and expands them, acting therein as a counterpoise to its 

 contraction as a muscle, I could wish his design had not bound him up to so 

 narrow a compass, and that he had given us an explication at large of so abstruse 

 and so important a phenomenon. Because the specific gravity of the blood 

 seems to me a cause by no means alone adequate to the effect, which it is here 

 supposed to produce. For, if the blood acts only as a weight, by mere gravi- 

 tation, then that part of it only which descends from the parts above the heart 

 can be employed in that action. This, at the largest computation, cannot amount 

 to 5 lb. weight, and must, according to the computation of Borelli, force a 

 machine that is able to overcome a resistance of 135,0001b. 



But neither does the refluent blood gravitate in any such proportion as I have 

 here assigned : for, to make a true estimate of its gravitation, we must consider 

 the circumstances of the liquor supposed to gravitate ; in which, it very much 

 resembles water inclosed in a recurve tube, of which, if the length of the two 

 legs be equal, it may be suspended in the air full of water, with the extremities 

 downwards, without losing a drop, although the diameter of those legs should 

 be very unequal. The case of the arteries and veins is pretty nearly similar to a 

 tube so filled and inverted ; for if the arteries and veins be continued tubes, as 

 they appear by the microscope, then supposing their contents to have no other 

 determination of motion than their own weight would give them, the contained 

 fluids must be counterpoises to each other. For the veins and arteries being 

 joined at the smaller extremities, and the larger of both terminating in the same 

 parallel line, it is impossible, according to the laws of hydrostatics, that the 

 contents of either should overbalance the other. How far then must it fall 

 short of forcing the natural power and resistance of so strong a muscle as the 

 heart, by mere gravitation. 



The blood indeed has a progressive motion through its vessels, wherein it 

 differs from water in a recurve tube, in the experiment above stated. But if 

 the natural gravitation of the blood contributes nothing to the dilatation of the 

 heart, this progressive motion will be found more insufficient ; for, as this mo- 

 tion is derived entirely from the heart's constriction, could the blood be sup- 

 posed to react upon the heart, with all the force first impressed on it by the 

 heart, it would be insufficient, unless we will suppose the force communicated 

 to be superior to the power communicant, which is absurd. 



But when the just and necessary deductions for the impediments, which the 



