CIRCULATION OF THE BLOOD 109 



arterial pulse enters and is impelled in a continuous, equable, 

 and incessant stream through every part and member of 

 the body, in much larger quantity than were sufficient for 

 nutrition, or than the whole mass of fluids could supply; 

 third, the veins in like manner return this blood incessantly 

 to the heart from parts and members of the body. These 

 points proved, I conceive it will be manifest that the 

 blood circulates, revolves, propelled and then returning, 

 from the heart to the extremities, from the extremities to the 

 heart, and thus that it performs a kind of circular motion. 



Let us assume, either arbitrarily or from experiment, 

 the quantity of blood which the left ventricle of the heart 

 will contain when distended, to be, say, two ounces, three 

 ounces, or one ounce and a half in the dead body I have 

 found it to hold upwards of two ounces. Let us assume 

 further how much less the heart will hold in the contracted 

 than in the dilated state; and how much blood it will pro- 

 ject into the aorta upon each contraction; and all the world 

 allows that with the systole something is always projected, 

 a necessary consequence demonstrated in the third chapter, 

 and obvious from the structure of the valves; and let us 

 suppose as approaching the truth that the fourth, or fifth, 

 or sixth, or even but the eighth part of its charge is thrown 

 into the artery at each contraction ; this would give either 

 half an ounce, or three drachms, or one drachm of blood 

 as propelled by the heart at each pulse into the aorta; 

 which quantity, by reason of the valves at the root of 

 the vessel, can by no means return into the ventricle. 

 Now, in the course of half an hour, the heart will have 

 made more than one thousand beats, in some as many as 

 two, three, and even four thousand. Multiplying the num- 

 ber of drachms propelled by the number of pulses, we 

 shall have either one thousand half ounces, or one thou- 

 sand times three drachms, or a like proportional quantity 

 of blood, according to the amount which we assume as 

 propelled with each stroke of the heart, sent from this 

 organ into the artery a larger quantity in every case than 

 is contained in the whole body! In the same way, in 

 the sheep or dog, say but a single scruple of blood passes 

 with each stroke of the heart, in one half-hour we should 



