But that you might see how I treated this matter, I shall here present you with the explication of the motion of the heart, and of the arteries, which being the first and most general (which is observed in animals) we may thereby easily judge what we ought to think of all the rest. And that we may have the less difficulty to understand what I shall say thereof, I wish those who are not versed in Anatomy, would take the pains, before they read this, to cause the heart of some great animal which hath lungs, to be dissected; for in all of them its very like that of a Man: and that they may have shewn them the two cels or concavities which are there: First that on the right side, whereto two large conduits answer, to wit, the _vena cava_, which is the principal receptacle of bloud, and as the body of a tree, whereof all the other veins of the body are branches; and the arterious vein, which was so mis-call'd, because that in effect its an artery, which taking its _origine_ from the heart, divides it self after being come forth, into divers branches, which every way spred themselves through the lungs. Then the other which is on the left side, whereunto in the same manner two pipes answer, which are as large, or larger then the former; to wit, the veinous artery, which was also il named, forasmuch as its nothing else but a vein which comes from the lungs, where its divided into several branches interlaid with those of the arterious vein, and those of that pipe which is called the Whistle, by which the breath enters. And the great artery, which proceeding from the heart, disperseth its branches thorow all the body. I would also that they would carefully observe the eleven little skins, which, as so many little doors, open and shut the four openings which are in these two concavities; to wit, three at the entry of the _vena cava_, where they are so disposed, that they can no wayes hinder the bloud which it contains from running into the right concavity of the heart; and yet altogether hinder it from coming out. Three at the entry of the arterious vein; which being disposed quite contrary, permit only the bloud which is in that concavity to pass to the lungs; but not that which is in the lungs to return thither. And then two others at the entry of the veinous artery, which permits the bloud to run to the left concavity of the heart, but opposeth its return. And three at the entry of the great artery, which permit it to go from the heart, but hinder its return thither. Neither need we seek any other reason for the number of these skins, save only that the opening of the veinous artery, being oval-wise, by reason of its situation, may be fitly shut with two; whereas the other, being round, may the better be clos'd with three. Besides, I would have them consider, that the great artery and the arterious vein are of a composition much stronger then the veinous artery or the _vena cava_. And that these two later grow larger before they enter into the heart,