For,after these things,it is not necessary for me to say anything more with a view to explain the motion of the heart,except that when its cavities are not full of blood,into these the blood of necessity flows,--from the hollow vein into the right,and from the venous artery into the left;because these two vessels are always full of blood,and their orifices,which are turned towards the heart,cannot then be closed.But as soon as two drops of blood have thus passed,one into each of the cavities,these drops which cannot but be very large,because the orifices through which they pass are wide,and the vessels from which they come full of blood,are immediately rarefied,and dilated by the heat they meet with.In this way they cause the whole heart to expand,and at the same time press home and shut the five small valves that are at the entrances of the two vessels from which they flow,and thus prevent any more blood from coming down into the heart,and becoming more and more rarefied,they push open the six small valves that are in the orifices of the other two vessels,through which they pass out,causing in this way all the branches of the arterial vein and of the grand artery to expand almost simultaneously with the heart which immediately thereafter begins to contract,as do also the arteries,because the blood that has entered them has cooled,and the six small valves close,and the five of the hollow vein and of the venous artery open anew and allow a passage to other two drops of blood,which cause the heart and the arteries again to expand as before.And,because the blood which thus enters into the heart passes through these two pouches called auricles,it thence happens that their motion is the contrary of that of the heart,and that when it expands they contract.But lest those who are ignorant of the force of mathematical demonstrations and who are not accustomed to distinguish true reasons from mere verisimilitudes,should venture.without examination,to deny what has been said,I wish it to be considered that the motion which I have now explained follows as necessarily from the very arrangement of the parts,which may be observed in the heart by the eye alone,and from the heat which may be felt with the fingers,and from the nature of the blood as learned from experience,as does the motion of a clock from the power,the situation,and shape of its counterweights and wheels.
But if it be asked how it happens that the blood in the veins,flowing in this way continually into the heart,is not exhausted,and why the arteries do not become too full,since all the blood which passes through the heart flows into them,I need only mention in reply what has been written by a physician 1of England,who has the honor of having broken the ice on this subject,and of having been the first to teach that there are many small passages at the extremities of the arteries,through which the blood received by them from the heart passes into the small branches of the veins,whence it again returns to the heart;so that its course amounts precisely to a perpetual circulation.Of this we have abundant proof in the ordinary experience of surgeons,who,by binding the arm with a tie of moderate straitness above the part where they open the vein,cause the blood to flow more copiously than it would have done without any ligature;whereas quite the contrary would happen were they to bind it below;that is,between the hand and the opening,or were to make the ligature above the opening very tight.For it is manifest that the tie,moderately straightened,while adequate to hinder the blood already in the arm from returning towards the heart by the veins,cannot on that account prevent new blood from coming forward through the arteries,because these are situated below the veins,and their coverings,from their greater consistency,are more difficult to compress;and also that the blood which comes from the heart tends to pass through them to the hand with greater force than it does to return from the hand to the heart through the veins.
And since the latter current escapes from the arm by the opening made in one of the veins,there must of necessity be certain passages below the ligature,that is,towards the extremities of the arm through which it can come thither from the arteries.This physician likewise abundantly establishes what he has advanced respecting the motion of the blood,from the existence of certain pellicles,so disposed in various places along the course of the veins,in the manner of small valves,as not to permit the blood to pass from the middle of the body towards the extremities,but only to return from the extremities to the heart;and farther,from experience which shows that all the blood which is in the body may flow out of it in a very short time through a single artery that has been cut,even although this had been closely tied in the immediate neighborhood of the heart and cut between the heart and the ligature,so as to prevent the supposition that the blood flowing out of it could come from any other quarter than the heart.
