The Pulse or Beat of the Arteries

5. Some animals entirely lack lungs and have only one ventricle in their heart.

If they have multiple ventricles, these are all consecutive to each other.

6. The pulse, or beat of the arteries, depends on the 11 little membranes.

These function as valves, opening and closing the entrances of the 4 vessels connected to the 2 ventricles of the heart.

When one of these beats ceases, another is about to begin. At that moment:

• the little doors which are at the entrances of the 2 arteries are exactly closed
• those which are at the entrances of the 2 veins are opened

In this way, 2 drops of blood fall through these 2 veins, one into each cavity of the heart.

These drops of blood then become rarefied, and suddenly spread out into a larger space to occupy it. They push and close those valves at the entrances of the 2 veins.

This prevents any more excess blood from going down into the heart. They then push and open the valves of the 2 arteries, through which they promptly and forcefully rush from the heart.

This makes the heart and all the arteries of the body swell at the same time.

But immediately afterwards, this rarefied blood condenses again, or enters the other parts.

And so the heart and arteries swell up:

• the valves at the 2 entrances of the arteries close again
• those at the entrances of the 2 veins open again.

This gives passage to two other drops of blood, which again swell up the heart and arteries, just as the previous ones did.

7. The blood restores whatever is expelled by the continuous movement of surrounding bodies.

The 2 kinds of blood are:

• The blood in the veins is freshly transferred from the liver.
• The blood in the arteries and already perfected in the heart.

The cause of the arterial pulse explains how both of these 2 kinds of blood adheres to other parts of the blood and restores whatever was expelled from the continuous actions and movement of these parts.

The blood contained in the veins flows back slowly from their extremities to the heart, according to the arrangement of certain valves embedded in various locations within the veins.

Conversely, the blood in the arteries is propelled through multiple pulses from the heart to the extremities of the arteries.

• In this way, the blood easily unites with all its parts and thus is retained in them.
• This nourishes and even increases them.