The formation of the solid parts

These veins and arteries in mammals, together with the epigastrics, are the last of the internal parts of the seed to be formed before the external parts because the blood from the womb comes through the navel to the heart.

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.

They pass from the brain through the spinal column to several sides at the same time. So they finally come together again in the same place, which is that where the navel will be formed.

How are the heart, the brain, the flesh of the muscles, and the majority of skins or membranes formed?

• This formation 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. They 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.

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

They stop against the surface of the passages through which they pass because:

• they are less readily moved than the other particles of blood
• some branches usually remain

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 other fire-aether and air-aether which run incessantly through the pores of these membranes.

These gradually join themselves to each other, forming the tiny filaments which make up the above solid parts.

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,

1. 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.

2. 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.

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.

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.

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.