"The arteries are never depleted except into the veins or the porosities of the parts, but are continually stuffed full by the pulse of the heart; but in the vena cava and the circulatory vessels, into which the blood glides at a quick pace and hastens toward the heart, there would be the greatest scarcity of blood, did not all the parts incessantly pour out again the blood poured into them. Add, also, that the impetus of the blood which is urged and driven at every pulsation into all parts of the second and third regions, forces the blood contained therein from the porosities into the little veins and from the branches into the larger vessels; this being effected also by the motion and compression of the surrounding parts; for contents are squeezed out of whatever contains them, when it is compressed and narrowed. So by the movements of the muscles and limbs the venous branches which creep on between are pressed upon and narrowed, and push on the blood from the lesser toward the greater."[266] A similar touch of vagueness is perceptible when the venous flow is dealt with by Harvey in that very same résumé of the circulation which seats the underlying cause of the pulse in the hot blood of the vena cava close to the auricle. In that résumé he says to Riolanus:-- "I assert, further, that the blood in the veins courses always and everywhere from the lesser into the greater and hastens from all parts toward the heart; whence I gather that the amount, continuously sent into the arteries, which the arteries have received is transferred through the veins, and at length returns and flows back whence it first was impelled; and that in this wise the blood is moved in a circle in flux and reflux by the heart, by an impulsion the impetus of which forces the blood through all the arterial filaments; and that afterward in a continuous flow from all parts it goes back through the veins, one after another, by which it is absorbed, drained away, and transported."[267] As to the flow in the lungs Harvey says in the treatise of 1628:-- "It being the will of nature that the blood itself be strained through the lungs, she was obliged to superadd the right ventricle, in order that by the beat thereof the blood might be driven through the lungs themselves, out of the vena cava into the cavity of the left ventricle."[268] We have already found Harvey saying to Riolanus, in regard to the pulmonary transit, that the blood within the branches of the arterial vein "cannot now go back in opposition to the sigmoid valve, while at the same time the lungs are widened and enlarged and then narrowed, by inspiration and expiration, and with the lungs their vessels also, and offer to the blood aforesaid a path and transit into the venous artery."[269]