The great architechtonical use of this principal part, is to convey the blood to every part of the body, for the vivifying and nourishing of the whole, which you shall be best able to understand, when we shall have declared the manner of this conveyance, and the nature of that which is convey'd.
We shall therefore first begin, with shewing the certainty of the truth of the circulation of the blood, wrought by the motion of the heart, then shall declare what the nature of blood is, of what parts its mass doth consist, how it warms and nourisheth the whole, and consequently in what life is properly said to consist: the action and function of every part being best understood by its natural figures, frame and constitution.
That therefore we may demonstrate the proper operation of the heart, we must curiously observe the parts and vessels belonging to it, and not only of the heart, but of the lungs also, which for this purpose are fastened to it, and therefore ought to be considered with it: neither do I think there hath been any greater reason of the long concealment of the circulation, from the discovery and knowledge of man, than the looking upon the heart and lungs as parts of distinct concernments; for altho' the circulation be as true, and sometimes more manifest in such animals as have no proper lungs fastened to the heart, as in fishes; yet in those animals with lungs, there can be no accurate consideration of the heart, without them; because there is no communication of one ventricle of the heart with the other, but by and through them: let us therefore first consider the frame and structure of the heart, together with the vessels disseminated through the lungs, which are affixed and implanted into it.
The heart of its self is a firm muscular; that is, a fleshy and fibrous part, not wholly solid, but having two cavities allowed it, commonly called the right and left ventricles; again, being the orifice of the vena arteriosa is immediately fastened to the same right ventricle, and the valves planted in the orifice of this vessel, are properly framed and adapted for the admission of any thing out of this ventricle, and hindring the return of any thing into it: therefore is it most evident, that the blood which passed out of the cava into the right ventricle, should pass out of this ventricle into the vena arteriosa.
Thirdly, being the orifice of the arteria venosa, is fastened by the left auricle to the left ventricle of the heart, and the valves planted in the ventricle, are framed for the admission of any thing that comes that way, and hindring all regurgitation backwards: it is likewise here evident, that out of the very frame and contexture of the part, that what blood passeth out of the right ventricle, through the vena arterosa into the lungs, should also pass out of the lungs, through the arteria venosa, into the left ventricle of the heart. Lastly, being the orifice of the arteria aorta, or great artery is immediately affixed to the left ventricle, and the valves implanted in it, are so framed on purpose to admit any thing out of the ventricle into the artery, and to hinder all passage out of the artery into the said ventricle, it is likewise most evident, that what blood passeth out of the arteria venosa into the left ventricle, be conveyed out of the left ventricle into the aorta, or great artery: and thus have we clearly brought the blood out of the vena cava, thro' most manifest open channels, framed by nature her self, through the ventricles of the heart, and the vessels of the lungs, into the great artery or aorta.
That the blood may thus march, and that nature design'd it should do so, is hence sufficiently shewn even from the construction of the parts; and that it must necessarily do so, and that it actually goes this round, is next further to be demonstrated.
At every pulse of the heart, there is a small quantity of blood forced out of the left ventricle into the aorta; and this is manifest by most certain experience, now the pulses of the heart are so many, and the quantity of blood so considerable that is expelled, that it cannot be denied; but there is in less space that an hour, more blood sent out of the left ventricle into the aorta, than the quantity of the whole mass of blood in the body amounts to; but whatsoever comet hint the left ventricle, must come out of the arteria venosa, and whatsoever come out of the arteria venosa, must first come through the vena arteriosa, whatsoever passeth through that, must first come out of the right ventricle; and whatsoever comes thence, must have its passage from the vena cava, as we have before demonstrated out of the frame of the parts: Therefore, a primo ad ultimum, whatsoever quantity of blood cometh into the aorta, must consequently come out of the cave, but a quantity exceeding the whole mass of blood cometh into the aorta in the space of an hour, therefore the same blood must return out of the great artery into the vena cava, which is the circulation we mentioned, and which we contend for. And thus far for the truth and certainty of it: now follows the manner of this circulation.
And altho' the manner and nature of circulation, (as the circle its self) admist of no beginning; yet for doctrin's sake, we must begin somewhere, and for perspicuity's sake, we shall begin where motion doth last appear at the left auricle. The blood in the vena cava, is by the right auricle forced into the right ventricle of the heart; the heart by its systole or contraction forceth the blood out of the right ventricle, into the vena arterosa; by virtue of which stroke, it passeth through the branches all over through the body of the lungs, and so into the branches of the arteria venosa, through which it is conveyed to the left auricle, each of these ventricles having two large vessels annext to it; one by which it receives, the other by which it dischargeth the blood.
The right ventricle hath immediately fastned to it, the right auricle, which is as it were the extremity of the vena cava; by which the blood is constantly conveyed into it, besides which, it hath a large orifice of the vena arteriosa annext to it, by which it dischargeth the blood into the lungs, which it received from the above said auricle: in like manner the left ventricle hath annext to it the left auricle, which is as it were the extremity of the arteria venosa, thro' which it receives the blood cut of the lungs; besides which, it hath a large orifice of the aorta annext to it, by which it dispenseth and dischargeth into the arteries all the blood which is received from the lungs; but the use of all these vessels will more clearly appear, if we consider the strange artifice of certain valves, or little floodgates planted at these their orifices.
These valves are of two sorts, tricuspidal, and sigmoidal; the tricuspidal valves being planted in the ventricles for the admission of blood into the heart, and hindring its reflux into the veins. The sigmoidal are planted in the arteries for the admittance of blood out of the ventricles into the arteries, and preventing its return out of the arteries into the ventricles.
Having thus considered the natural frames structure of ventricles, vessels and valves; we shall now more easily demonstrate the circulation of the blood, and how it is naturally performed. The first way being the vena cava by the right auricle to the right ventricle of the heart, and the valves planted in the ventricle, are tramed for the admission of any thing into it, and preventing all regurgitation back. Therefore it is most evident, even from the very frame of the part, that the blood passeth out of the vena cava into the right ventricle of the heart, and is thence dispersed and dispatch into the left ventricle, from whence by the heart contraction, it is forced into the trunk of the great artery, and by the branches of that artery, into the whole habit, and all the parts of the body; in all which parts there being extremities of veins, answering to the extremities of the arteries in the same manner, as the extremities of the arteria venosa does answer the extremities of the vena arteriosa in the lungs.
The blood is conveyed out of the capillary branches to the arteries into the capillary branches of the veins; and through these into the larger vessels, till its arrives at the trunk of the vena cava, whence it is sent back again into the right ventricle of the heart; from thence thro' the lungs into the left ventricle, and so into the aorta, and so about perpetually; and this is the manner of the circulation: and thus have we absolved the sirst part of our discourse.
In the second part, we are to consider the nature of that which is thus circulated, and the end of circulation, which is the life of the whole.
That which is thus circulated, we commonly call the mass of blood, which I shall not distinguish with the ancient physitians into the four humours; but rather content my self with that division nature her self maketh, when the blood is out of the body. In the body of a sound man, take what quantity you please ways by phlebotomy, and let it stand some few hours, and in it you will find two distinct substances, of different colours, tasts, qualities and operations; the one a dark, and turning towards black; the other of a darkish white, or watery colour. These two in the blood, were blended and mingled together, insomuch that all the whitish liquor was in a manner absorbed with the tincture of a deep red; of these, one is said and allowed to be the proper substance of the blood; the other the chyle, preserved in the blood; prepared by it, and circulated with it, for the nourishment of every part: and that it is of this nature, is evident; because, set it upon the fire, and it will not evaporate, as does the serum, or the urine, but will rather coagulate, and grow to the substance of the same consistence, smell, and tast, with the white of a roasted egg; which is the true connatural nourishment of the parts, whose colour and constitution are the same.
The succus nutritius, mingled, warmed, and subtilized by the blood, so much as is necessary for preservation of the blood, is sanguified (that is) is conveyed into the substance of blood, the rest is conveyed with the blood to the extremities of the arteries; and so past to the habit of the body in every part, which taking into it so much as is to be assimilated to the part, sends the rest with the blood into the capillary veins, and so thro' the great vessels to the heart, to be conveyed in the same manner for the further nourishment of the parts. The other part of the mass, is the blood its self, the fountain and original of life, the primum vivens, and the ultimum moriens. Theis from its beginning having life in its self, by the additions of this nutrimental juyce; and without which, the artificer can do nothing becoming the architext, of his own house and frame; every part being fitted for its own reception and habitation: now as this hath a local motion by way of circulation, by which it provides for the circulation of the parts, so hath it also a vital one, by which it preserves its self.
This vital motion is a constant fermentation or working of the blood, by which all the most minute parts are secretly divided, for the reception of what is proper for it; and expulsion, amandation, and casting off whatever is obnoxious or injurious to it; but his secret agitation of its self, and atomical division of the minimae particulae preserves it in its usual vigour, and so long as its fluidity continues, as the proper effect of this its vitality, it becometh brisk and lively, it causing the lively part of the blood to nourish and cherish the whole.
But alas, this life is not immortal! Nor can the great architect, according to second causes, make such a house as shall never fall on its own head, since that by which he first builds, and afterwards repairs, is nothing else but the nutrimental juyce, liquor, or moist substance which is mingled, and as it were incorporated with the blood, which must intimately penetrate and enter the part which it is to nourish: now the parts of the body made and kept up by it, are so long capable of increase and nourishment, as they continue in them a consistence fit to receive such a moisture, and no longer: and whereas the bones at first were moist and supple, as were other parts, which by length of time grows to such a stability, firmness, and consequently, dryness, that they do not admit any longer nutrimental juyce into them, or their proper substance, whereby they seem to obstruct and hinder the further growth of the animal; and for the future, they stand in the body more like timber in a house, than as trees in the ground, as they formerly did; so other parts of the body after full age, do grow somewhat dryer and closer, and so consequently do make a greater resistance towards their own nourishment: for when the skin, by reason of its propinquity to the air, do first grow dry, close, and shrivell'd, as we see in decrepit old age: so we may also conceive that the membranes of all the inward parts proportionally do the same: and therefore, the blood moving about to every part, does not find an admittance for the dew of life which it carrys along with it; and yet so long as the blood does move, there is said life still to remain in it, although nothing else could be so said to live but its self.
But as the last, even the blood its self fails of the quickness of its vital motion, and not being longer able, nimbly to relieve its self by a subtile division of its parts; it at length becomes fibrous, and get into tis self a kind of dryness, which makes it unfit and uncapable of receiving it own Nourishment; and for want of its vital fermentation it formerly enjoyed, it grows more dry and more firm, it not admitting into it, its former liquidity, to resolve or bedew its parts; whereby it becomes so fibrous, as not to allow them any further capacity of making use of their own proper menstruum: so that upon the failure of this vital, the local motion must consequently cease, whereby all the parts become deprived of their vital influence: whence follows a natural death.
Thus have I humbly dispathch'd the second part of the discourse concerning the nature of the mass of blood, and that wherein vitality its self consist, by which at last we clearly understand the great life and function of the heart. And seeing all the parts do receive their vital influence from the blood, and this blood (the seat of life) serves its self by its own vital motion; and seeing this blood cannot constantly be transmitted into the parts, but by local motion, and this motion cannot be continued but by a forceable impulse, therefore nature must of necessity make some part of the body to drive it forwards, which part must necessarily have some cavities belonging to it, or allow'd it, first to contain; then, some vessels to receive, as does the cistern and pipes; and thirdly, strength to propel or drive out: and such a part as this, in every respect is the heart, which is furnished with ventricles to contain, with vessels annext to it, to convey and receive, and with a firm muscular body, to propel the blood. Thus the action of the heart, is its proper contraction by which it makes way for the propulsion or driving forward of the blood for its use, and a constant circulation, as also for the vivifying and nourishment of the whole body.