How are certain very bulky fibres formed?

These take the form of cords, which are of the same substance as the rest of its flesh.

Its ventricles have had, from the beginning, 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.

All of these holes gradually widened, finally making a single ventricle.

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.

The blood descends into the heart through these 2 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.

What does the heat of the heart consists in? How is its movement produced?

The heart beats throughout its life. All its fibres are 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, the heart remains rigid after death, 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.

This is because this movement of the diastole has from the beginning been caused by heat from the action of fire-aether driving out the air-aether from around some parts of the seed, having communicated its agitation to them.

This allowed these parts of the seed, in expanding, to squeeze the others that have begun to form in the heart.

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.

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.

The agitation of the blood particles maintains this fire.

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;

At systole, these pores change shape because the heart lengthens. This makes the blood particles which remained there as if they were to serve as yeast, leave there with a great speed. They enter easily into the new blood coming into the heart, making its particles separate from one another.

In separating thus, they acquire the form of fire.

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 2 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.

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.