The Movement of the Blood

The violent agitation of the heat makes the blood:

• expand and move apart and become separated
• gather and press and bump against one another and divide into many extremely tiny branches.

These branches remain so close to one another.

• Only the fire-aether can occupy the spaces remaining around them.

The blood particles that leave the heart join together in this way.

They never leave the circuit by which they return to it.

This is in contrast to the many other particles that penetrate the mass of seed more easily.

From the seed, new particles continue to move towards the heart, until it is all used up.

This is why the blood of all animals is red, as explained in:

• Dioptrics and Principles of Philosophy on the nature of light
• Meteors on the nature of colours

We see light through the pressure exerted by the air-aether.

• These are made up of many little corpuscles all touching one another.

These corpuscles have 2 motions:

1. They follow a straight line towards our eyes

This gives us the sensation of light.

2. They rotate

If their rotation speed is much less than that of their linear motion, the body which they come from appears blue.

If they rotate faster than their linear motion, the body looks red to us.

Blood particles have branches so delicate and so close to one another. Only the fire-aether can go around them.

On the surface of the blood, the air-aether meets the fire-aether.

The fire-aether continually:

• passes very quickly obliquely from one blood-pore to the next
• moves in the opposite direction to the air-aether

This forces the air-aether to rotate faster, creating a red color.

This is why hot iron and burning coals appear red – their pores are filled only with the fire-aether.

• Their pores are not as small as those of blood.
• This is why their shade of red is different from that of blood.

As soon as the heart begins to form in this way, the decompressed blood which leaves it takes its course in a straight line in the direction in which it is freest to move, which is the region where the brain will later be formed.

The path taken by the blood begins to form the upper part of the aorta.

Because of the resistance offered by the parts of the seed that it encounters, the blood does not travel very far in a straight line. It is pushed back towards the heart along the same path by which it came.

But it cannot go back down this path because the way is blocked by the new blood that the heart is producing.

This forces it to return to the side opposite to that of the new material entering the heart.

It is on this side, where the spine will later develop, that it makes its way towards the region where the parts that will serve for generation will be formed, and the path that it takes in its descent is the lower part of the aorta.

But because parts of the seed also press on it from this side, they resist the movement of the blood.

The heart continually sends new blood to the top and bottom of this artery. And so this blood is forced to take a circular path back towards the heart, via the side furthest from the spine, where the chest will later develop.

The path that the blood takes in returning thus to the heart is the vena cava.