The formation of the solid parts

These veins and arteries in mammals, together with the epigastrics, seem to be the last of the internal parts of the seed to be formed before the external parts, and because of this the blood from the womb comes through the navel to the heart. For the agitation of the spirits causes the parts of the seed which are at the places where they pass, rather than the others, to go to the heart.

And because they pass from the brain through the spinal column to several sides at the same time, they finally come together again in the same place, which is that where the navel will be formed. But before I pause to describe this, I shall explain how the heart, the brain, the flesh of the muscles, and the majority of skins or membranes come to be formed, because this does not depend on the nourishment that the animal being formed receives from the womb.

When the arteries and the veins begin to be formed, they still have no membrane covering them, and are just tiny channels of blood spreading this way and that in the seed.

There are 2 kinds of blood:

1. The blood that decompression in the heart separates from one another.

This is first areso fluid that they do not seem to be able to enter into the composition of those parts of the body which harden; but except for the spirits that go to the brain, and which are formed and made up from the finest, all the others should just be considered as vapours or serosities of the blood, from which they are continually issued, via all the pores they find along the arteries and the veins through which they pass.

2. The blood that is joined by the same action of the heart

This appears red and nourishes the solid parts.

nonetheless, they do not serve this role while they are severally joined together, but only when they have come apart from one another, for in going backwards and forwards several times through the heart, their branches gradually break, and finally are separated by the same action that had joined them.

Then, because they are less readily moved than the other particles of blood, and because some branches usually remain, they come to a halt against the surface of the passages through which they pass, and thus they begin to form their skins.

Then, those that come after these membranes have begun to form are joined to the first, not indiscriminately in every direction, but only from the side where, without preventing the flow of serosities, there can be vapours, and also other finer matter, namely the first two elements that I described in my Principles of Philosophy, which run incessantly through the pores of these membranes; and gradually joining themselves to each other, they form the tiny filaments of which I said above all solid parts were made.

And it is notable that all the filaments have their roots along the arteries, and not along the veins. Because of this, I even doubt whether the membranes of the veins form immediately from the blood that they contain, or whether they are formed rather from the tiny filaments that come from neighbouring arteries; for what contributes most to the formation of these tiny filaments is, first, the action by which the blood comes from the heart towards the arteries, which inflates their membranes, and dilates or contracts their pores at intervals, which does not happen in the veins. Second, there is the flow of liquid matter which leaves the arteries through the pores in their skins, in order to enter all the other places in the body, where it causes these tiny filaments gradually to advance; and flowing from all sides around them, it also causes their tiny parts to adjust to one another, join together, and refine themselves. But although some fluid parts can leave the veins in the same way, I believe  nonetheless that often it enters the other places in the body from those fluid parts which, leaving the arteries, do not take their course towards the surface of the body, but towards the veins, where they mix a second time with the blood.

And the only thing that leads me to believe that the blood of the veins contributes anything to the production of their covering membranes is that these skins are browner, or less white, than those of the arteries. For what causes the whiteness of the latter is the force with which the fluid matter flows around their small filaments, which breaks all the small branches of the particles of which they are composed, which I said above was the reason why the blood appeared red. And because this force is not so great in the veins, where the blood does not come in such an impulsive way, so that it does not make them inflate in jolts, as it does in the arteries, the tiny parts of this blood, which attach themselves to their covering membranes, still retain some of the tiny branches that make them red. But they make these skins blackish, not red, because the action of the fire that has agitated them has ceased: just as one observes that soot is always black, and that coals, which are red when they are alight, become black when they have been extinguished.

Now since the solid parts of the tiny filaments are composed, turned, folded, and intertwined in various ways, following the various routes of fluid and fine matters which surround them, and following the shapes of  the places where they encounter one another, if one had a good knowl- edge of all the parts of the seed of some species of a particular animal, man for example, one could deduce from this alone, by entirely certain and mathematical arguments, every shape and structure of each of its bodily parts. And reciprocally, if one knows all the peculiarities of this structure, one can deduce what it is the seed of.

But because I am considering only the production of the animal in general here, and to the extent that there is a need to explain how all its parts are formed, grow, and are nourished, I shall continue just to explain the formation of the principal bodily parts.

I said above that the heart began to form from some of the tiny parts of the seed being squeezed by others that had expanded due to heat. But to know how it enlarges and becomes perfected, it must be borne in mind that the blood that produces this first dilation returns a second time to expand in the same place and contains in it some particles composed of several of those of the seed that are joined together and, consequently, are much larger, but it also has several that are finer, just as I said, and some of these finest ones penetrate the pores of the compressed seed that has begun to form the heart, and some of the larger ones are brought to a halt against it, and gradually drive it out of there, beginning to form  tiny filaments there, similar to those that I have said form along all the arteries, except only that they are harder and stronger there than else- where because the force of the expansion of the blood in the heart is very much stronger. Nevertheless, it is not noticeably stronger than in the first branches of the artery called the ‘coronary’ branches, because they surround the whole heart. This is why the small filaments that form along these coronaries blend easily with those that have their roots in the ventricles of the heart. And as these latter make up the internal parts, those that carry the nourishment from the coronaries make up the external ones, while the branches of the veins that accompany them carry back to the heart the particles of blood which are not suitable for nourishing them.

There are still many different things to consider here, the first being the way in which certain very bulky fibres are formed, taking the form of cords, which are of the same substance as the rest of its flesh. To achieve this result we must consider its ventricles to have had, from the begin- ning, very irregular shapes, because, the parts of the blood that they contain being unequal, they took different paths in expanding; as a result of this, they made a number of holes in the parts of the seed that they compressed, and all of these holes gradually widened, finally making a single ventricle; and the parts of the seed that separated them, having been gradually driven out from their places by the tiny filaments that make up the flesh of the heart, also made up these fibres in the form of columns.

The same thing is responsible for the production of the valvules, the  little flaps of skin that close the entrances to the vena cava and the pulmonary vein. For since the blood descends into the heart through these two entrances, stretching them as it returns and causing them to expand, the other blood that follows it through these same entrances prevents it returning via these; this is why its parts spread all around the seed that makes up the heart and makes a number of small holes there. Then the tiny filaments of the flesh of the heart drive out the parts of seed that are all around these holes and put themselves there instead, arranging themselves in such a way as to compose the valvules and the fibres attached to them. For in considering the action of the blood that descends into the heart by means of these entrances, and that which tends to leave them via neighbouring ones, one sees that, following the rules of mechanics, the fibres of the heart, which are found between these two actions, must have spread out in the form of flaps of skin and thus taken the shape that these valvules have.

But those at the entrances of the pulmonary artery and the aorta are not produced in the same way, for they are outside the heart, and only make up the skin of the arteries, which has been folded and moved along from the inside, on the one hand by the action of the blood leaving the heart, and on the other by the resistance of the blood that is already contained in these arteries and which withdraws towards their circumference, finally making a passage through it.

This holds generally for the production of the valvules in the rest of the body. Because of this, passages must be formed everywhere, through which flows matter which encounters other matter that resists it in some places, but which cannot for all that interrupt its flow; for this resistance makes the skin of the passage fold in, by these means forming a valvule. This can be observed in the intestines, at the spot where the excrement already collected is in the habit of resisting the flow of that coming down; it can also be seen in the passages of the gall, and still more evidently in the veins, at the spots where the weight of the blood that carries it to the extremities of the legs, arms, and other parts, often resists its ordinary course, which carries it from these extremities to the heart. Consequently, one cannot find it strange hereafter if I say that the spirits also form valvules in the nerves, and in the entries and exits of these muscles, even though their small size prevents them from being observed by our senses.

Another thing which it seems to me must be considered here is what the heat of the heart consists in, and how its movement is produced, for particularly since it does not cease to beat throughout its life, it seems that all its fibres must be made so flexible by this movement that this flexibility could easily be returned to them by an external force when it is dead and cooled. Yet on the contrary we observe that it remains rigid in this state, in the shape that it had previously in systole – that is, between two of its beats – without it being easy to give it back the shape it had in diastole – that is, the time when it beats against the chest.

The reason for this is that this movement of the diastole has from the beginning been  caused by heat, or the action of fire, which, following what I explained in my Principles of Philosophy, could not have consisted in anything else but the matter of the first element driving out that of the second element from around some parts of the seed, having communicated its agitation to them. And by these means, these parts of the seed, in expanding, squeezed the others that have begun to form in the heart. And at the same time some also entered forcefully into the pores between the others that were forming the heart, by means of which they changed their position slightly and began the motion of the diastole and after that the systole, when this position was resumed, and these parts of seed which had been agitated by fire, went out again from the pores in the flesh of the heart and returned to its ventricles.

Encountering other particles of seed, and on account of the blood descending there, they were mixed in with this blood, and drove out the second element from around many of these particles, and by these means passed their agitation on to them, all this blood expanding, and in expanding it sent once more some of its particles, surrounded exclusively with first-element matter, into the pores of the flesh of the heart, that is to say, between its fibres, which causes for a second time the motion of the diastole.

I do not know of any other fire or any other heat in the heart other than the agitation of the particles of blood, nor of any other cause which can serve to maintain this fire except only that, when most of the blood leaves the heart at the time  of diastole, those of its particles which remain there enter into the flesh, where they find pores arranged in such a way, and fibres agitated in such a way, that there is only matter of the first element surrounding them; and at systole these pores change shape because the heart lengthens, which makes the particles of blood, which remained there as if they were to serve as yeast, leave there with a great speed, and in this way entering easily into the new blood coming into the heart, they make its particles separate from one another, and in separating thus they acquire the form of fire.

Now while the fibres of the heart are agitated by the heat of the fire, they are arranged so as to open and close their pores alternately, so as to produce the movements of diastole and systole. For even after the heart has been taken out of the body of the animal and cut into pieces, provided it is still warm, it requires only very few vapours from the blood, taking the opportunity to enter its pores, to compel the movement of diastole; but when it is already cold, the shape of its pores, which depends on the agitation of the first element, has changed, so that the vapours of the blood no longer enter them, and because its fibres are rigid and hard, they are no longer so easy to bend.

We may still consider here the causes of the shape of the heart, for they are easy to deduce from the way in which it is formed. And the first peculiarity that I note consists in the difference that exists between the  two ventricles, which clearly shows that they have been formed at different times, and this is the reason why the left ventricle is much longer and more pointed than the right. The second is that the flesh covering this left ventricle is very much thicker at the sides of the heart than at its point, the reason for this being that the action of the blood which expands in this ventricle spreads out all around, and strikes the sides with more force than at the point, because they are closer to its centre and are opposite one another. The point on the other hand is only opposite the opening of the aorta which, receiving the blood easily, prevents it offering too much hindrance to this point, and for the same reason the heart becomes shorter and rounder in its diastole than in its systole.

I see nothing else notable here, except the skin called the ‘pericardium’ which surrounds the heart. But because the cause that produces the pericardium is no different from that which forms all the other skins or membranes, and generally all the surfaces that mark out the different parts of animals, it will be easier to cover all these at the same time. There are surfaces that form first with the bodies whose boundaries they mark out, and others that form afterwards, because the body is separated from another of which it was previously a part. Of the first kind is the external surface of the skin called the ‘after-birth’, which envelops  the child before it is born; likewise, there are the surfaces of the lung, of the liver, of the spleen, the kidneys, and of all the glands. But those of the heart, the pericardium, of all the muscles, and even of all the skin of our body, are of the second kind.

What makes the first kind of skin form is this: when a body that is not liquid is produced from the joining together of the small parts of some fluid, like those I have mentioned, some of its parts will necessarily be external to others, and these exteriors must be arranged in a different way from the interiors because they touch a body of a different nature (that is to say, one whose small parts have another shape, or are arranged or move in a different way) than those of which they are composed, for if this were not the case, they would combine with one another, and there would be no surface distinguishing the two bodies.

Thus as soon as the seed begins to gather and acquire a structure, those of its parts that touch the womb, as well as some others that are very close, are forced by this contact to turn, to arrange themselves, and to join with one another in a different way from the manner in which those further away turn, arrange themselves, and join together. In this way these parts of the seed that are closer to the womb begin to form a skin which has to enclose the whole offspring; but it only achieves this some time afterwards when all the internal parts of the seed have already been driven towards the heart through the arteries and through the veins which take  their place, and finally these arteries and these veins also go towards the exteriors, which flow through the veins towards the heart, to the extent that the arteries advance and produce many small filaments from whose tissue this skin is composed.

As for those surfaces which are formed from a body dividing into two, they cannot have any cause other than this division. And in general all divisions are caused by this alone: one part of the body that is dividing is carried by its movement towards one side while the other part that is joined to it remains where it is, or is carried by its movement towards another side; for this is the only way in which they can be separated.

Thus the parts of the seed making up the heart at the beginning are joined to those that make up the pericardium and the sides, so that, all together, they form only one body. But the expansion of the blood in the cavities of the heart moved the matter that surrounded this cavity, in a way other than that which elongated it slightly, and at the same time the animal spirits descending from the brain through the spinal column towards the sides of the body also moved the matter near the sides in a different way. In this way, the matter between the two, not being able to follow the two different motions together, began gradually to come apart from the sides and from the heart and then began to form the pericardium.

Next, to the extent to which the parts of the seed which made it up flowed towards the heart, the arteries of the different places through which they passed sent tiny filaments in their place, which, joining one to the other, formed the skin of which it is made.

What then made this skin hard enough is that, on the one hand, many parts of the blood which dilated in the heart entered into the whole of its flesh, and collected there between it and the pericardium, no longer being able to proceed any further, because, on the other hand, many vapours from the blood contained in the lungs also left, to the extent to which they began to grow, and these were collected between the same pericardium and the sides. And so these vapours, compressing it from one side and another, made the fibres very hard and caused there to be always some space between it and the heart, which is filled only with these vapours, one part of which is condensed in the form of water, the other remaining in the form of air.